This manual discusses troubleshooting vehicle electrical systems. Automobiles, trucks, power generation equipment, and construction equipment are as much electrical and electronic as they are mechanical. The era of the backyard mechanic is gone; you must be as well trained in electricity and electronic aspects along with the mechanical workings of CESE. Knowledge of specialized diagnostic equipment, such as the use of on-board diagnostics, is also essential for today's mechanic to troubleshoot equipment systems.

The first topic discussed is alternators, rectifiers, and voltage regulators, their nomenclature, and their function. The next manual will cover troubleshooting the charging system and its various tests.

The subsequent topics will cover troubleshooting the alternator with emphasis on diodes and windings, the cranking system and its various tests, the ignition system and its components, lighting systems, and electrical accessories.

The senior mechanic is going to have to take on the responsibility of keeping up with the rapid changes in technology for automotive, truck, power generation, and construction equipment electricity and electronic systems, along with their diagnostic procedures.

When you have completed this manual, you will:

  1. Understand and identify the components of AC charging systems.
  2. Understand how to use a volt-ampere tester to troubleshoot a charging system.
  3. Understand how to use the engine analyzer to troubleshoot an alternator.
  4. Understand how to troubleshoot the cranking system.
  5. Understand and identify the ignition system.
  6. Understand how to troubleshoot ignition systems.
  7. Understand how to troubleshoot lighting systems and accessory systems.


1.0.0 AC Charging Systems

2.0.0 Troubleshooting the Charging System with a Volt-ampere Tester

3.0.0 Troubleshooting the Alternator Using the Engine Analyzer

4.0.0 Troubleshooting the Cranking System Using the Battery Starter Test

5.0.0 Crank Voltage Test

6.0.0 Starting Motor Current Draw Test

7.0.0 Starter Insulated Circuit Resistance Test (Cable and Switches)

8.0.0 Starter Ground Circuit Resistance Test

9.0.0 Ignition Systems

10.0.0 Troubleshooting

11.0.0 Troubleshooting Lighting Systems and Electrical Accessories

Review Questions


The starting system draws a tremendous amount of current from the battery. This runs down, or discharges, the battery. The system to recharge the battery is the charging system. Figure 1 shows a basic charging circuit.

Figure 1 - Schematic of a basic charging circuit.

The charging system forces current back through the battery for recharging and also provides electricity for all the electrical devices when the engine is running. This system serves as the electrical power supply under normal operating conditions. The charging system comprises of the battery, the alternator, the voltage regulator, ignition switch, and indicator light.

The alternator, sometimes called generator, produces the current output. This current is fed to the battery and to other electric- electronic systems.

The voltage regulator maintains an alternator output of approximately 13 to 15 volts. This is higher than battery voltage, which is 12.6 volts (12 volt battery). This higher voltage is needed to recharge the battery. The charging system can affect the operation of the rest of the vehicle. Undercharging will allow the battery to become weak, making the vehicle hard to start. Overcharging can damage the battery and other vehicle components. Overcharging can affect the operation of the ECM by creating voltages that are beyond the normal range of ECM operation.

1.1.0 Alternators

The alternator is a compact electrical component that changes rotational movement into electricity by using magnetism. The alternator consists of a set of rotating windings, called a rotor, and a set of stationary windings, called a stator. The alternator rotor is turned by the engine crankshaft through a drive belt and is magnetized by battery current delivered through slip rings and brushes, as illustrated in Figure 2. The typical components of an alternator include:

Figure 2 - Exploded view of an alternator.

  1. Rotor assembly (field windings, claw poles, rotor shaft, and slip rings)
  2. Stator assembly (three stator windings or coils, stator core, and output wires)
  3. Brush assembly (brush housing, brushes, brush springs, and brush wires)
  4. Rectifier assembly (diodes, heat sink or diode plate, and electrical terminals)
  5. Housing (drive end frame, slip ring end frame, and frame bolts)
  6. Fan and pulley assembly (fan, spacer, pulley, lock washer, and pulley nut).
  7. Integral regulator (electronic voltage regulator mounted in or on rear of alternator).

Rotor and stator operation. The rotor is a spinning magnetic field. It fits in the center of the alternator housing. The fan belt turns the rotor, making the field spin.

The stator is a stationary set of windings in the alternator. The stator surrounds the rotor. The stator serves as the output winding of the alternator. Figure 3 shows an illustration of a rotor and stator.


Figure 3 - The rotor mounts inside the stator.

Figure 4 - The rotor has claw poles that surround its windings.

When the rotor spins, its strong magnetic field cuts across the stator windings as depicted in Figure 4. This spinning induces current in the stator windings. If the stator windings are connected to the load, such as a light bulb, the load will operate.

Rotor. An alternator rotor consists of field coil windings mounted on a shaft. Figure 5 shows the components of a rotor. Two claw-shaped pole pieces surround the field windings to increase magnetic field strength. The rotor is mounted on roller or needle bearings so the rotor can turn freely, as illustrated in Figure 6.

Figure 5 - Components of a rotor.

Figure 6 - Rotor shaft and bearings.

The claw on one of the pole pieces produces S (south) poles. The claws on the other pole form N (north) poles. As the rotor spins inside the alternator, an alternating N-S-N- S polarity and AC current is produced. This pulls one way and then the other to produce AC.

Slip rings. Slip rings are mounted on the rotor shaft to feed a small current into the rotor windings, as illustrated in Figure 7. Each end of the field coil connects to one of the slip rings. An internal source of electricity is needed to excite the field and produce a magnetic field.

Figure 7 - Brushes and slip rings allow current to be fed into rotor windings.

Alternator brushes. Alternator brushes ride on the slip rings to make a sliding electrical connection. The brushes feed current into the slip rings and rotor windings.

Small brush springs push the brushes out and into contact with the slip rings. Since current flow into the rotor windings is low, the brushes are small compared to motor brushes.

A brush holder encloses the brush springs and brushes. It holds the brushes in alignment with the rotor slip rings. The brush holder is made of insulating material to prevent brush grounding; refer to Figure 8.

Figure 8 - Brushes and brush holder.

Stator. Previously mentioned, the stator is a stationary set of windings mounted between the end frames. The stator usually consists of three coils wrapped around an iron core. The iron core increases the field strength so that more current can be induced into the stator by the rotor field. The output of the stator is AC and is fed into the diodes that convert to DC.

A Y-type stator has the wire ends from the stator windings connected to a neutral junction. The circuit looks like the letter "Y". A Y-type stator provides good current output at low engine speeds; see Figure 9.

Figure 9 - Y-type stator windings.

A Delta type stator has the stator wires connected end to end. With no neutral junction, two circuit paths are formed between the diodes during each phase. A delta wound stator is used in high output alternators; see Figure  10.

Figure 10 - Delta-type stator windings.

Bearings. Needle or ball-type alternator bearings are commonly used to produce a low friction surface for the rotor shaft. These bearings support the rotor and shaft as they spin inside the stator.

The alternator bearings are normally packed with grease. The front bearing is frequently held in place with a small plate and screws. The rear bearing is usually press-fit into place.

Cooling Fan. Behind the drive pulley on most alternators is a cooling fan that rotates with the rotor. This cooling fan draws air into the housing through the openings at the rear of the housing. The air leaves through the openings behind the cooling fan, as illustrated in Figure 11. The moving air pulls heat from the diodes, and their heat decreases.

Figure 11 -Cooling fan draws air through to cool the diodes.

Cooling the diodes is important for the efficiency and durability of the alternator. Several different alternator designs have been introduced that increases the cooling efficiency of an alternator. One of these alternators is lighter in weight and capable of very high output and contains two internal fans rather an external fan.

Liquid cooled. A recent design uses liquid cooling. Using water or coolant to cool an alternator is a very efficient way to keep diode temperature down. One other reason to for eliminating the fan and using liquid to cool the alternator is to reduce noise. The rotating fan is a source of underhood noise that was essential to eliminate. These new alternators have water jackets cast into the housing. Hoses connect the housing to the engine's cooling system. Not only do these alternators make less noise, they have higher output and should last longer in the high temperature environment.

Pulley and belt. An alternator pulley is secured to the front of the rotor shaft by a large nut. It provides a means of spinning the rotor through the use of a belt.

An alternator belt, running off the crank pulley, turns the alternator pulley and rotor. One of three types of belts may be used; V-belt, cogged V-belt, and ribbed belt, as shown in Figure 12.

Figure 12 -Alternator belts.

1.2.0 Rectifiers

An alternator rectifier assembly, also known as a diode assembly, commonly uses six diodes to convert stator output (alternating current) to direct current. The diodes are usually wired as shown in Figure 13. The rectifier provides full-wave rectification (changes both positive and negative outputs into direct current) as the different polarity rotor claws pass the stator windings.

Figure 13 - Wiring diagram showing the relationship between stator windings, rotor windings, diodes, and electrical connections.

A diode trio may be used to supply current to the rotor field windings. In a diode trio, three diodes are connected to the field through a connection in the voltage regulator. The stator output feeds the diode trio.

The rectifier diodes are mounted in a diode frame or heat sink (metal mount for removing excess heat from electronic parts). Three positive diodes are press-fit in an insulated frame, and three negative diodes are mounted in an uninsulated or grounded frame.

AC output. AC flows one way and then the other. As the rotor turns into one stator winding, current is induced in one direction. Then, when the same rotor pole moves into the other stator winding, current reverses and flows out in the other direction.

Rectified AC current. An electrical system is designed to use DC or direct current that only flows in one direction. It could not use alternating current as it comes out of the alternator stator. Alternator current must be rectified (changed) into DC current before entering the electrical system, see Figure  14.

Figure 14 - Converting AC to DC.

A diode is a semi-conductor that allows current to flow in only one direction. Diodes are one-way check valves for electricity, becoming a conductor or insulator, depending on which way the current tries to flow. In Figure 15, View A, when polarity is connected one way, current flows. Figure 15, View B shows when polarity is reversed, current is blocked.

Figure 15 - Cutaway view showing diode operation.

When a diode is connected to a voltage source in where the current passes through the diode, the diode is said to be forward biased. A forward- biased diode acts as a conductor.

When reverse biased, the diode is connected to a voltage source in such a way that current does not pass through. A reverse- biased diode acts like an insulator.

If a diode were placed on the stator output of a simple alternator, current would only flow out through the circuit in one direction.

A single diode would not use the entire alternator's output. It would result in pulsing direct current, not smooth current flow. Therefore, an alternator uses several diodes connected into a rectifier circuit. This produces more efficient alternator output, as demonstrated in Figure 16.

Figure 16 - Several diodes are needed to convert AC into DC.

1.3.0 Voltage Regulators

A voltage regulator controls alternator output by changing the amount of current flowing through the rotor field windings. Any change in rotor winding current changes the field strength acting on the stator or output windings. In this way, the voltage regulator can maintain a preset charging voltage.

The voltage regulator keeps alternator output at a preset charging voltage of approximately 13 to 15 volts. Since this is higher than the battery voltage (12.6 volts), current flows back into the battery and recharges.

Current also flows to the ignition system, electronic fuel system, on-board computer, radio, or any other device using electricity.

There are four basic types of voltage regulators:

  1. Electric voltage regulator mounted inside the alternator
  2. Electronic regulator mounted away from alternator in engine compartment
  3. Electronic voltage regulator mounted on rear of the alternator
  4. Contact point type regulator mounted in engine compartment.

Electronic voltage regulator. An electronic voltage regulator uses an electronic circuit to control rotor field strength and alternator output. A circuit diagram for an alternator and regulator is shown in Figure 17.

Figure 17 - Schematic with electronic voltage regulator.

An electronic voltage regulator is a sealed unit and is not repairable. The electronic circuit must be sealed because it can be damaged by moisture, excessive heat, and vibration. Usually the circuit is surrounded by a rubber-like gel for protection.

An integral voltage regulator is an electronic regulator mounted inside or on the rear of the alternator. This is the most common type used today. It is very small, efficient, and dependable. Most use an integrated circuit to provide alternator regulation.

Electronic voltage regulator operation. To increase alternator output, the electronic voltage regulator allows more current into the rotor windings. This strengthens the magnetic field around the rotor. More current is then induced into the stator windings and out of the alternator.

To reduce alternator output, the electronic regulator places more resistance between the battery and the rotor windings. Field strength drops, and less current is induced in the stator windings.

Alternator rpm and electrical load determine whether the regulator increases or decreases charging output. If load is high or rotor speed is low, such as engine idling, the regulator will sense a drop in system voltage. The regulator then increases rotor current until a preset output voltage is obtained. If load drops or rotor speed increases, the opposite occurs.

An electronic voltage regulator must be replaced when it is not operating properly.

Temperature compensation. Some newer electronic regulators are temperature compensating regulators. This means the regulator changes alternator output as the outside temperature changes. In cold weather, alternator output voltage is increased. This helps recharge the battery more quickly. Cranking loads are higher in cold weather and battery drain is more severe. A temperature compensating regulator will decrease alternator output voltage in warm weather.

Diode trio. The diode trio consists, as the name suggests, of three diodes, one per phase. A diode can be used to feed current to the rotor field through the electronic regulator. As diagramed in Figure 18, the stator coils are connected to each diode in the trio. This rectifies the current entering the voltage regulator and field or rotor windings.

Figure 18 - Wiring diagram of a charging circuit with a diode trio.

Field circuit modulation. Field circuit modulation refers to how the voltage regulator can cycle the rotor field current on and off to control charging system output.

For example, if the battery is discharged, the regulator may cycle the field current on 90% of the time. This will increase output. If the electrical load is low, the regulator may cycle the field current off 90% of the time to decrease output. By controlling what percent of the time current is flowing through the rotor field, the modulating-type regulator can control the alternator.

Alternator capacitor. An alternator capacitor or condenser can be used to prevent radio noise. It absorbs alternating current inside the alternator. The capacitor can also protect the rectifying diodes from high voltage spikes and damage.

Contact point voltage regulator. A contact point voltage regulator uses coils, set of points, and resistors to control alternator output. This is an older type of regulator that has been replaced by electronic or solid state regulators.

When alternator output is too high, high current flow through the voltage regulator coil pulls the moveable contact down against the ground contact. This bypasses the current flowing to the alternator field, which causes the field current and the alternator output to decrease.

When charging output is too low, less current flows through the regular coil. The spring on the moveable contact then pulls the contact up. This connects the alternator field circuit to battery voltage. Current enters the alternator field and output increases. The coil points cycle at about 100 times per second to provide smooth regulation.

The field relay coil simply disconnects power from the charging system when the ignition switch is OFF.

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Charging system precautions. Observe the following rules when working on a charging system. They will prevent possible damage to electrical-electronic systems:

2.1.0 Alternator Test

Charging System Tests. Charging system tests should be done when symptoms point to low alternator voltage and current. These tests will quickly determine the operating condition of the charging system.

Charging system tests are performed in three ways: using a load tester, which is the same tester used to check the battery and starting system; a scope tool; or a volt-ohm-milliammeter (VOM).

A load tester provides the most accurate method of checking a charging system, as shown in Figure 19.

Figure 19 - Load tester.

Before testing the charging system, it is common practice to check the condition of the battery. Although charging system problems often show up as a dead battery, do not forget that the battery itself may be bad. Measure the battery's state-of-charge and perform a battery load test. Then, you will be sure that the battery is not affecting your charging system tests.

2.2.0 Excessive Output Test

To conduct an excessive output test, set the voltmeter to the correct voltage range and the volt lead selector to the required position. Connect the black external volts lead to the generator armature terminal and the red external volts lead to the generator frame for a good ground. While observing the voltmeter scale for the highest voltmeter reading, start the engine and slowly increase its speed. If the voltmeter reads less than 16 volts (12-volt system) or 8 volts (6-volt system), the current limiter relay of the regulator is the reason for the high output. If the voltmeter reads more than 16 volts (12- volt system) or 8 volts (6-volt system), remove the field wire at the regulator and observe the ammeter scale. When the ammeter reading shows no output, you have a defective regulator, which should be repaired or replaced. When the ammeter reading indicates a current flow, remove the field wire at the generator and observe the ammeter. If the ammeter reading then shows no output, you have a shorted field wire. Replace the field wire and connect the generator to the regulator. On the other hand, if the ammeter shows that current is flowing, then the generator has a grounded field.

Another component of the vehicle charging system you should test is the voltage regulator. If the results of the test indicate the voltage is too high or too low, a faulty regulator voltage limiter or a high-series resistance in the charging system could be causing the trouble. Erratic or unstable voltage indicates poor circuit electrical connections, faulty regulator contacts (burned or oxidized), or damaged regulator resistors. In any case, you should proceed with a charging system circuit resistance test.

2.3.0 Charging System Circuit Resistance Test

Circuit resistance tests are used to locate wiring problems in a charging system: loose connections, corroded terminals, partially burned wires, and similar troubles. Resistance tests should be performed when symptoms point to problems other than the alternator or regulator. Two common circuit resistance tests are the insulated-circuit resistance test and ground-circuit resistance test.

In a ground-circuit resistance test, the voltmeter is placed across the negative battery terminal and alternator housing.

The voltmeter should not read over 0.1 volt per electrical connection. If the voltmeter reading is higher, look for loose connections, a burned plug socket, or similar problems.

To do an insulated-circuit resistance test on a charging system, connect the tester as described by the manufacturer. The voltmeter leads are connected across the alternator output terminal and positive battery terminal.

With the vehicle running at a fast idle, turn the load control to obtain a 20-amp current flow. All lights and accessories should be off. Read the meter.

If the circuit is in good condition, the voltmeter should not read over about 0.7 volts (0.1 volt per electrical connection). If the voltage drop is higher than 0.7 volts, circuit resistance is high. A poor connection exists in that section of the charging circuit.

2.4.0 Charging System Ground Circuit Test

This test measures voltage drop within the system wiring. It helps pinpoint corroded connections or loose or damaged wirings.

Circuit resistance is checked by connecting a voltmeter to the positive battery terminal and the output, or battery terminal of the alternator. The positive lead of the meter should be connected to the alternator output terminal and the negative lead to the positive battery terminal. To check the voltage drops across the ground circuit, connect the positive lead to the alternator housing and the negative meter lead to the battery negative terminal. When measuring the voltage drop in these circuits, a sufficient amount of current must be flowing through the circuit. Therefore, turn on the headlights and other accessories to ensure the alternator is putting out at least 20 amps. If a voltage drop of more than 0.5 volt is measured in either circuit, there is a high resistance problem in that circuit.

2.5.0 Regulator Ground Circuit Resistance Test

To conduct this test, set the volt lead selector to the INT VOLTS position. Then, observing polarity, connect the external volts lead to the generator or alternator ground terminal and to the regulator ground terminal, as shown in Figure 20. Adjust the load increase knob until the ammeter scale indicates a current of 10 amperes.

Figure 20 - Regulator ground circuit resistance test.

Also observe the reading on the (3-volt) voltmeter scale and compare it with the specifications. If the voltmeter reading exceeds 0.01 volt, excessive resistance is in the ground circuit between the regulator and the generator or alternator. Check the regulator ground system for loose mounting bolts or a damaged ground strap.

2.6.0 Battery Drain Test

A battery drain test will check for an abnormal current draw with the ignition key off. When a battery goes dead without being used, you may need to check for a current drain. It is possible that there is a short or other problem constantly discharging the battery.

A battery can be discharged if an electrical accessory remains on when the ignition is shut off. For example, a short in a switch could cause a glove box light to always stay on. This could slowly drain the battery and cause a starting problem.

To perform the ammeter current drain test:

  1. Turn off all accessories and close the door.
  2. Remove the under-hood lamp, if equipped.
  3. Disconnect the negative battery cable.
  4. Attach the ammeter, as shown in Figure 21.

Figure 21 - Battery drain test.

  1. Zero the ammeter.
  2. Read the ammeter.



To prevent damage, do not operate starting motor or any high-current-draw device with a meter connected in series for measuring current drain. High current draw will blow the meter fuse or damage the meter.

If everything is off (good condition), the ammeter should read almost zero or only a few milliamps (10 mA, typically). However, an ammeter reading above this would point to a drain problem. To pinpoint a drain, pull fuses one at a time. When the ammeter reads zero, the problem is in the circuit on that fuse.

Remember that normal parasitic current drain for the clock and computers can discharge a battery if the vehicle sits unused for an exceeded period of time. Also account for this small current draw when checking for a battery drain.

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Normally, when an engine analyzer is available for use, it is in the electrical shop. The following information explains how to use the analyzer to test alternators. In considering this information, remember the following points:

  1. The example shown is one of several manufactured.
  2. The analyzer will do much more than just test alternators.
  3. Always refer to the manufacturer's manual of the analyzer and the unit being tested before making any connections.

3.1.0 Charging Circuit Diodes

Bad alternator diodes reduce alternator output current and voltage and may also cause voltage ripple that can upset computer system operation. Faulty diodes are a frequent cause for alternator failure. It is important to check the condition of the diodes when rebuilding an alternator.

There are various methods used to test alternator diodes: ohmmeter, test light, diode tester, and scope test. The ohmmeter is the most common testing tool used when the alternator is disconnected.

When using an ohmmeter or a test light, the diodes must be unsoldered and isolated from each other. Some special diode testers, however, will check the condition of the diodes with all the diodes still connected to each other.

When an alternator fully produces, each of its diodes conducts an equal share of the current. This condition is indicated by a ripple pattern that appears on the screen of the engine analyzer. But a single non-conducting diode places a strain on the charging circuit, which causes a decrease in the output of the alternator. Whereas an ammeter or voltmeter may not detect this strain, the analyzer can do so easily. The strain brought on by an open field condition, for example, will stop the alternator output ripple entirely.

A likely result of decreased alternator output is an undercharged battery, and without a fully charged battery, there may not be enough current available to start the engine or meet the demands of the electrical circuits. When a good battery cannot be fully charged, the fault is usually in the alternator or voltage regulator. The engine analyzer can help you determine which is at fault. However, the regulator has to be bypassed altogether, and battery voltage must be applied to the field terminal of the alternator. Not all alternators can be full fielded. Refer to the manufacturer's field test procedure.

3.2.0 Open and Shorted Diodes

To use an ohmmeter to test the diodes, connect the meter to each diode in one direction and then the other, as indicated in Figure 22. The meter should read high resistance in one direction and low resistance in the other. This will show you that the diode is functioning as an "electrical check valve." The test should be performed on each diode.

Figure 22 - Testing diodes.

A bad diode can either be shorted or opened. An open diode will have a high (infinite) resistance in both directions. A shorted diode will have a low (zero) resistance in both directions. In either case, the diode must be replaced. Press a new diode or obtain a new diode pack.

If diodes were unsoldered for testing or replacement, they must be resoldered. Use a soldering gin and rosin-core solder to attach the diode leads as demonstrated in Figure 23.

Figure 23 - Soldering diode leads or stator to rectifier wires.

A shorted diode or shorted winding will usually burn itself open. The pattern on the screen will show a shorted diode or open diode. Notice the similarity in the patterns. At any rate, the alternator will require service or replacement even though both output current and voltage regulation appear to be acceptable. As a general rule, a shorted diode affects the output more than an open diode does. It not only reduces the output, but it also opposes the next pulse by allowing the current to flow back through the winding containing the shorted diode.

3.3.0 Weak Diodes

As you can see from the screen pattern in Figure 24, there is no interruption in the rectification of the diodes. However, there is a high and low peak every sixth pulse, indicating that the output of one diode is low and that it may be deteriorating (high resistance). This pattern may also occur due to a shorted winding since the number of windings determines the amount of output as well as the condition (resistance) of the diodes.

Figure 24 - Poor diode pattern.

3.4.0 Shorted Windings

A bad alternator stator can have shorted or open windings. Inspect the stator windings for signs of burning (darkened windings with a burned insulation smell). An open winding is usually detected using an ohmmeter.

To test a stator for open or grounded windings, connect an ohmmeter to the stator leads, as shown in Figure 25. Connections A and B will check for stator opens. They shall produce a low ohmmeter reading. If the reading is high (infinite), the windings are broken and the stator is defective.

Figure 25 - Stator test.

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A dead or discharged battery is one of the most common reasons the starting system fails to crank the engine properly. The starting motor draws much more current (over 200 amps) than any other electrical component. A discharged or poorly connected battery can operate the lights, but it may not have enough power to operate the starter motor.

If needed, load-test the battery and make sure the battery is in good condition and is fully charged. A starter motor will not function without a fully charged and well- connected battery.


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The cranking voltage test measures the available voltage to the starter during cranking. To perform the test, disable the ignition or use a remote starter switch to bypass the ignition switch. Normally, the remote starter switch leads are connected to the positive terminal of the battery and the starter terminal of the solenoid or relay, as illustrated in Figure 26. Refer to the service manual for specific instructions on the model of vehicle being tested. Connect the voltmeter's negative lead to a good chassis ground. Connect the positive lead to the starter motor feed at the starter relay or solenoid. Activate the starter motor and observe the voltage reading.

Figure 26 - Using a remote starter switch to bypass the control circuit and ignition system.

Compare the reading to the specifications given in the service manual. The normal voltmeter readings should be as follows:

4.8 volts or more for a 6-volt system

9.6 volts or more for a 12-volt system

18 volts or more for a 24-volt system

If the reading is above specifications but the starter motor still cranks poorly, the starter motor is faulty. If the voltage reading is lower than specifications, a cranking current test and circuit resistance test should be performed to determine if the problem is caused by high resistance in the starter circuit or an engine problem.

To check an excessive starting motor current, you can perform a starting motor current draw test of the 6-, 12-, or 24-volt series system.

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A current draw test measures the current used by the starting system. It will quickly tell you about the starting motor and other system parts. If current draw is higher or lower than specifications, there is a problem.

To perform a current draw test, connect meters to measure the battery voltage and the current flow out of the battery. A load tester may also be used. Two testing methods are shown in Figures 6-27. In Figure 27, View A, a voltmeter and an ammeter are being used to measure starter current flow. A voltmeter reading is needed to compare different battery conditions. If current draw is not within specifications, there are starting system troubles.

Figure 27, View B is showing a battery load tester being used to check the starter current draw. Crank the engine and note the voltage reading. Then, load the battery to obtain the same voltage. This will equal the current draw of starting motor.

Figure 27 - Starter current draw test.

To keep the engine from starting during the test, disconnect the coil primary supply wire or ground the coil wire. You can also pull the fuse for the electric fuel pump if this is easier (direct ignition systems).

With a diesel engine, you must disable the injection system. You may have to unhook the fuel shutoff solenoid. Check the shop manual for specific details.



Do not crank the engine for more than 15-30 seconds or starter damage may result. Allow the starter to cool off for a few minutes if more cranking time is needed.

Crank the engine and note the voltage and current readings. If they are not within specifications, something is wrong in the starting system or engine. Further tests are needed.

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The complete starter circuit is made up of the insulated circuit and the ground circuit. The insulated circuit includes all of the high current cables and connections from the battery to the starter motor.

To test the insulated circuit for high resistance, disable the ignition or bypass the ignition switch with a remote starter switch. Connect the positive (+) lead of the volt meter to the battery's (+) terminal post or nut. By connecting the lead to the cable, the point of high resistance (cable-to-post connection) may be bypassed. Connect the (-) lead of the voltmeter to the starter terminal at the solenoid or relay. Crank the engine and record the voltmeter reading. If the reading is within specifications (usually 0.2.and 0.6 voltage drop), the insulated circuit does not have excessive resistance. Proceed to the ground circuit test. If the reading indicates a voltage loss above specifications, move the negative lead of the tester progressively closer to the battery, cranking the engine at each test point. Normally, a voltage drop of 0.1 volt is the maximum allowed across a length of cable.

When excessive voltage drop is observed, the trouble is located between that point and the preceding point tested. It is a damaged cable or poor connection, an undersized wire, or possibly a bad contact assembly within the solenoid. Repair or replace any damaged wiring or faulty connections.

Refer to Figure 28. When you test a 6-volt system, the completed circuit shown in View A allows a 0.2-volt loss and that of view B allows a 0.3-volt loss. When you test a 12-volt system, the completed circuit, as shown in Figure 28, View A, allows a 0.4-volt loss and that of View B, a 0.3-volt loss, and that of view C, a 0.1 volt loss. If testing a 24- or 32-volt system, refer to the manufacturer's specifications. If the voltmeter reading is more than specified for the type of system being tested, high resistance is indicated in the cables, switches, or connections. Repeat the test with the voltmeter connected to each cable, switch, and connector of the circuit. The maximum readings taken across these parts should not exceed the values listed below.

  6-Volt System 2-Volt System
Each cable 0.1 volt 0.2 volt
Each switch 0.1 volt 0.1 volt
Each connector 0.0 volt 0.0 volt

Figure 28 - Starter insulated circuit resistance test.

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The ground circuit provides the return path to the battery for the current supplied to the starter by the insulated circuit. The circuit includes the starter-to-engine, engine-to-chassis, and chassis-to- battery ground terminal connections.

To test the ground circuit for high resistance, disable the ignition or bypass the ignition switch with a remote starter switch. Refer to Figure 29 for the proper test connection. Crank the engine and record the voltmeter reading.

Figure 29 - Starter ground circuit resistance test.

Good results would be less than 0.2-volt drop for a 12-volt system. A voltage drop in excess of this indicates the presence of a poor ground circuit connection resulting from a loose starter motor bolt, a poor battery ground terminal post connector, or a damaged undersized ground system wire from the battery to the engine block. Isolate the cause of excessive voltage drop in the same manner as recommended in the insulated circuit resistance test by moving the positive (+) voltmeter lead progressively closer to the battery. If the ground circuit tests out satisfactorily and a starter problem exists, move on to the control circuit test.

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The ignition system of a spark-ignition engine produces the high voltage needed to ignite the fuel charges in the cylinders. The system must create an electric arc across the gaps at the spark plugs. These arcs must be timed so they happen exactly as each piston nears the top of its compression stroke. The heat of the arcs starts combustion and produces the engine's power stroke.

More recently, ignition systems have been developed to reduce emissions and improve engine performance, fuel economy, and dependability.

An ignition system performs the following six functions:

  1. Provide a method of turning a spark ignition on and off
  2. Operate on various supply voltages (battery or alternator voltage)
  3. Produce high-voltage arcs at the spark plug electrodes to start combustion
  4. Distribute high-voltage pulses to each spark plug in the correct sequence
  5. Time the spark so that it occurs as the piston nears TDC on the compression stroke
  6. Vary spark timing as engine speed, load, and other conditions change

The ignition system changes battery voltage to a very high voltage and then sends the high voltage to the spark plugs. The parts needed to do this include the following:

  1. Battery-provides power for the system.
  2. Ignition switch-allows driver to turn ignition on and off.
  3. Ignition coil-changes battery voltage into 30,000 volts or more during normal operation. It has the potential to produce up to 60,000 volts.
  4. Switching device-uses high voltage from the ignition coil to produce an arc, igniting the fuel charge.
  5. Ignition system wires-connect ignition system components.

When the ignition switch is on and the switching device is closed (conducting current), current flows through the ignition coil. When the piston is near TDC on its compression stroke, the switching device opens to stop current flow through the ignition coil. This causes the coil to produce a high-voltage surge, which flows through the spark plug wire and arcs across the spark plug's electrodes.

The electric arc, or spark, at the spark plug ignites the fuel mixture. The mixture begins to burn, forming pressure in the cylinder for the engine's power stroke.

When the ignition key is turned off, an arc cannot be produced at the spark plugs and the engine stops running.

An actual ignition system is much more complex than the one just discussed. Vehicles have multiple-cylinder engines, and the timing of the spark must vary with operating conditions.

9.1.0 Computerized Ignition System

The computer ignition system is known as an electronic spark advance system or also as computer-controlled spark advance system that uses engine sensors, an ignition control module, and/or a computer (engine control module or power train control module) to adjust ignition timing. A distributor may or may not be used in this type of system. If a distributor is used, it will not contain centrifugal or vacuum advance mechanisms.

The engine sensors check various operating conditions and send data representing these conditions to the computer. The computer can then analyze the data and change ignition timing for maximum engine efficiency.

Sensors that can affect the ignition system include:

The computer receives input signals (different current or voltage levels) from these sensors. It is programmed (preset) to adjust ignition timing to meet different engine conditions. The computer may be mounted on the air cleaner, on the fender inner panel, under the dash, or under a seat.

The computer can also measure battery voltage to compensate for voltage variations due to battery state of charge, accessory loads, etc.

Electronic Spark Advance Operation. Let us discuss a sample situation of an electronic advance operation. A vehicle is travelling down the highway at 55 mph. The crankshaft sensor detects moderate engine rpm. The throttle position sensor detects part throttle. The intake air and coolant temperature sensors report normal operating temperatures. The manifold absolute pressure sensor sends high vacuum signals to the computer.

The computer could then calculate that the engine needs almost maximum spark advance. The timing occurs several degrees before TDC on the compression stroke. This ensures that the engine attained high fuel economy on the highway. There is enough time for all the fuel to burn and produce maximum pressure on the downward motion of the pistons.

If the operator began to pass a vehicle, engine intake manifold vacuum will drop to a very low level. The manifold absolute pressure sensor signal is fed to the computer. The throttle position sensor signal will detect wide open throttle. Other sensor outputs will stay about the same. Based on the signals from the manifold absolute pressure sensor and throttle position sensor, the computer could then retard ignition timing to prevent spark knock or ping. Since computer systems vary, refer to a service manual for specific procedures on the operation of your particular model.

9.2.0 Electronic Ignition System

An electronic ignition system, also called a solid state or transistor ignition system, uses an electronic control circuit and a distributor pickup coil to operate the ignition coil.

Figure 30 shows a typical circuit for an electronic ignition.

Figure 30 - Electronic ignition system.

An electronic ignition is more dependable than a contact point type. There are no mechanical breakers to wear or burn. This helps avoid trouble with ignition timing and dwell.

An electronic ignition is also capable of producing much higher secondary voltages than a breaker point ignition. This is an advantage because wider spark plug gaps and higher voltages are needed to ignite lean air-exhaust emissions and fuel consumption.

Ignition control module. The ignition control module (ICM) is an "electronic switch" that turns the ignition coil primary current on and off. The ICM does the same thing as contact points but more efficiently.

An ignition control module is a network of transistors, resistors, capacitors, and other electronic components. The circuit is sealed in a plastic or metal housing and can be located a couple of locations such as:

Trigger Wheel. The trigger wheel, also called the reluctor or pole piece, is fastened to the upper end of the distributor shaft, as shown in Figure 31. The trigger wheel replaces the distributor cam used in a contact point distributor. One tooth is normally provided on the wheel for each engine cylinder.

Figure 31 - Electronic ignition system trigger wheel and pickup coil.

Pickup coil. The pickup coil, also called the sensor assembly or sensor coil, produces tiny voltage pulses that are sent to the ignition control module, as illustrated in Figure  31. The pickup coil is a small set of windings that forms a coil.

As a trigger wheel tooth passes the pickup coil, it strengthens the magnetic field around the coil. This causes a change in the current flow through the coil. As a result, an electrical impulse (voltage or current change) is sent to the ignition control module as each trigger wheel tooth passes the pickup unit.

Hall-effect pickup. A hall-effect pickup is a solid-state chip or module that produces an electrical signal when triggered by a slotted wheel. A constant amount of current is sent through the device. A permanent magnet is located next to the Hall-effect chip, as shown in Figure 32.

Figure 32 - Hall-effect pickup.

When the slotted wheel's tab passes between the permanent magnet and the Hall- effect chip, the magnetic field is blocked, decreasing the chip's output voltage (sensor or switch off). When the slotted wheel's tab moves out from between the magnet and chip, magnetic field action increases the chip's voltage output (sensor or switch on). This on/off action operates the ignition control module.

Optical pickup. An optical pickup uses light-emitting diodes (LED) and photo diodes (light sensors) to produce an engine speed signal for the ignition system. The rotor plate rotates between the light-emitting diodes and the photo diodes. When a slot, or window, passes between the two diodes, light from the LEDs strikes the photo diodes and an electrical signal is generated.

An optical pickup is seldom used because its operation is adversely affected by a dirt buildup on the LEDs and photo diodes.

Electronic ignition system operation. With the engine running, the trigger wheel spins inside the distributor. As the teeth pass the pickup, a change in the magnetic field causes a change in output voltage, or current. This output voltage, which represents engine rpm, is sent to the ignition control module.

The ignition control module increases these tiny pulses into on/off current cycles for the ignition coil. When the module is on, current flows through the primary windings of the ignition coil, developing a magnetic field. Then, when the trigger wheel and pickup turn off the module, the ignition coil field collapses and fires a spark plug.

Dwell time (number of degrees of camshaft rotation that the circuit conducts current to the ignition coils) is designed into the ignition control module's electronic circuit. It is not adjustable.

9.2.1 Distributorless Ignition System

A distributorless ignition, also referred to as a computer-coil ignition, uses multiple ignition coils, a coil control unit, engine sensors, and a computer (engine control module) to operate the spark plugs. A distributor is not needed in this type, as depicted in the diagram Figure 33.

Figure 33 - Schematic of a distributorless ignition system.

An electronic coil module consists of two or more ignition coils and a coil control unit (electronic circuit) that operates the coils. Each coil in the coil module serves two cylinders. Therefore, a four-cylinder engine has a coil module with two ignition coils. A six-cylinder engine, on the other hand, needs a coil module with three ignition coils.

The coil control unit performs about the same function as the ignition module in an electronic ignition. It is more complex, however, because it must analyze data from the engine sensors and the engine control module.

The coils in the distributorless ignition system are wired so they fire two spark plugs at once. One spark plug fires during its cylinder's exhaust stroke. The other plug fires during its cylinder's power stroke, so its spark has no effect on engine operation.

A camshaft position sensor sends electrical pulses to the coil control unit, providing data on camshaft and valve position. The crankshaft position sensor feeds pulses to the control unit that show engine speed and piston position. A knock sensor may be used to allow the system to retard timing if the engine begins to ping or knock.

9.2.2 Distributorless Ignition System Operation

Operation. The on-board computer monitors engine operating conditions and controls ignition timing. Some sensor data is also fed to the electronic coil module, illustrated in Figure 34.

Figure 34 - Illustration of a distributorless ignition operation.

When the computer and sensors send correct electrical pulses to the coil module, the module fires one of the ignition coils.

Since each coil secondary output is wired to two spark plugs, both spark plugs fire. One produces the power stroke. The other spark plug arc does nothing because that cylinder is on the exhaust stroke. Burned gases are simply being pushed out of the cylinder.

When the next pulse ring tooth aligns with the crank sensor, the next ignition coil fires. Another two spark plugs arc for one more power stroke. This process is repeated over and over as the engine runs.


A distributorless ignition system has several possible advantages over ignition systems with a distributor. Some of these include:

  1. No rotor or distributor cap to burn, crack, or fail
  2. All computer controlled advance - no mechanical weights to stick or wear, no vacuum advance diaphragm to rupture or leak
  3. Play in timing chain and distributor drive gear eliminated as a problem that could upset ignition timing; crank sensor unaffected by slack in timing chain or gears
  4. More dependable due to fewer moving parts to wear and malfunction
  5. Requires less maintenance; ignition timing is usually not adjustable.

9.3.0 Integrated Direct Ignition

In a direct ignition system, one coil assembly is mounted directly above each spark plug. This eliminates the need for spark plug wires. It also allows the use of smaller ignition coils. A four-cylinder engine uses four ignition coils. A direct ignition is very similar to a disributorless ignition, except for the lack of spark plug wires and the increased number of coils. The other components in a direct ignition system (computer, sensors, etc.) are the same as those used in a distributorless system.

The direct ignition coils fire only on the power strokes. They do not fire on the exhaust strokes like wasted spark ignition systems. Figure 35 shows a cutaway of the direct ignition coils for a four-cylinder engine.

Figure 35 - Cutaway of an ignition coil for a direct ignition system for a four cylinder engine.

Sensor inputs allow the electronic control module to alter ignition timing with changes in operating conditions. The control unit can then make and break primary current into the correct ignition coil to make it produce high voltage.

On occasion there is "confusion" in the shops and even in the service manuals over the terms distributorless ignition and direct ignition, and they are misused or get switched. Just keep this in mind when reading the manuals or discussing the systems:

Some mechanics mistakenly call a distributorless ignition a direct ignition system when high-voltage output is not directly fed to each spark plug.

In some multi-coil ignition systems, the engine control computer measures the time needed to charge each ignition coil. Charge time varies with each coil's resistance/inductance and with the output voltage from the alternator. The computer uses this charge time to determine the ignition coil on-time for optimum spark plug arc duration and spark timing.

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An ignition system is one of the most important systems on a gasoline-powered engine. If a problem exists in the ignition, engine performance and emissions will suffer. As a mechanic, you must be able to quickly and accurately correct ignition system troubles.

10.1.0 Computerized Ignition System

Many of the components of a computer controlled ignition system are similar to those of electronic or contact point ignition systems. This makes testing about the same for many parts (spark plugs, secondary wires, ignition coil). However, the computerized ignition has engine sensors and a computer, which add to the complexity of the system.

Computer self-diagnosis mode. Most computerized systems have a check engine light in the dash that glows when a problem exists. The computer can be activated to produce a number code. The code can be compared to information in the vehicle's service manual to pinpoint the source of a problem. This makes testing and repairing a computerized system much easier.


A computerized ignition system can be seriously damaged if the wrong wire is shorted to ground or if a meter is connected improperly. Always follow manufacturer's testing procedures.

Computer ignition testers. Most auto makers provide specialized testing equipment for their computerized ignition systems. Like an ignition control module tester, the computer system tester plugs into the wiring harness. It will then measure internal resistances and voltages in the system to determine where a problem is located.

Ignition control module. A faulty ignition control module will produce a wide range of problems: engine stalls when hot, engine cranks but fails to start, engine misses at high or low speeds, etc. Quite often, an ignition control module problem will show up after a period of engine operation. Engine heat will soak into the module, raising its temperature. The heat will upset the operation of the electronic components in the unit.

Testing and ignition control module. You will generally find that service manuals list the ignition control module as one of the last components to test when troubleshooting an ignition system. If all the other components are in good working order, then the problem might be in the ignition control module.

If a specialized tester is available, it may be used to quickly determine the condition of the ignition control module. The wires going to the module are unplugged and the tester is connected to the module. The tester will then indicate whether an ignition control module fault exists.

Heating an ignition control module. The microscopic components (transistors, diodes, capacitors, resistors) inside the ignition control module are very sensitive to high temperatures and vibration.

When testing the control module, many mechanics use a heat gun or light bulb to warm the unit. This will simulate the temperature in the engine compartment after the engine has been running. The heat may make the control module act up and allow you to find an intermittent problem.

Do not apply too much heat to an ignition control module or it may be ruined. Only heat the unit to a temperature equal to its normal operating temperature.

10.2.0 Electronic Ignition System

Electronic ignition system service. As you have just learned, most electronic ignition systems use a pickup coil to sense trigger wheel (distributor shaft) rotation. The pickup coil sends small electrical impulses to the ignition control module.

If the distributor fails to operate properly, the complete ignition system can stop functioning. It is important to know how to make several basic tests on an electronic ignition system.

Pickup coil service. A bad pickup coil can produce a wide range of engine problems:stalling, missing, no-start troubles, and loss of power at specific speeds. If the tiny windings in the pickup coil break, they can cause problems that only occur under certain conditions. Also, because of vibration and movement, the thin wire leads going to the pickup coil can break. Though the insulation may look fine, the conductor could be damaged inside the insulation. When this happens, the engine may lope, miss, or not run.

Testing a magnetic pickup coil. A magnetic pickup coil or a speed sensor can be located in the distributor or on the engine block (crankshaft speed or position sensor). Tests for either type are similar.

Your scan tool may show a readout of primary circuit problems with a bad pickup coil or crankshaft sensor. Refer to your service manual for in-depth details.

A magnetic pickup coil test compares actual sensor resistance or voltage output with specifications. If resistance or voltage output is too high or low, the unit is bad.

To perform a pickup coil test:

  1. Connect an ohmmeter or low-reading AC voltmeter across the pickup coil output leads, as shown in Figure 36.

Figure 36 - Testing magnetic pickup coils.

  1. Observe the meter reading. Ohmmeter readings will usually vary between 250 and 1500 ohms. If an AC voltmeter is used, a small AC voltage (3-8 volts) should be produced by the magnetic pickup coil when the engine is cranked.

Check a service manual for the exact specifications.

  1. Wiggle the wires to pickup coil while watching the meter. This will help locate a break in the leads to the pickup. Also, lightly tap on the coil with the handle of a screwdriver. This could uncover any break in the coil windings.
  2. If the meter reading is not within specifications, or if the reading changes when the leads are moved or the coil is tapped, replace the pickup coil.

Testing Hall-effect and optical sensors. Hall-effect and optical sensors are tested in much the same way as the more common magnetic sensor. You can check their output signals and compare them to specifications. However, they are often tested with a scope to analyze more accurately their output signals.

Hall-effect sensor. A Hall-effect sensor test is best done by checking the sensor's output waveform with an oscilloscope. Without disconnecting the circuit reference voltage, probe the output wire at the sensor connector. The service manual will give pin numbers for probing, as shown in Figure 37, View A.

Figure 37 - Testing a Hall-effect sensor.

A Hall-effect sensor waveform should switch rapidly, have vertical sides, and have the specified voltage output (typically about 4-5 volts peak-to-peak). The top of the square wave should reach reference voltage and the bottom should reach ground, or zero.

Signal frequency should change with engine cranking speed or engine rpm, as shown in Figure 37, View B.

Hall-effect pickups can be found in distributors and some crankshaft position sensors. Refer to the service manual for your vehicle.

Optical sensor testing. An optical sensor can also be tested with an oscilloscope. You can probe the output wires from the sensor and compare the waveform to specifications.

An optical pickup test measures the output generated by the photo diodes as they are energized by the LEDs. It is also easily done with a hand-held scope probing into the sensor's electrical connector. Again, refer to the service manual to find the connector pin numbers for the optical pickup's output wire. Optical sensors are used in a few distributor designs and are never used in crankshaft sensors.

An optical sensor's waveform should have straight sides and adequate voltage output. The upper horizontal line on the waveform should almost reach reference voltage. The bottom horizontal line should almost reach ground, or zero, as shown in Figure 38.

Figure 38 - Typical waveform generated by an optical sensor.

Remember that optical sensors are susceptible to dirt. An oil mist or a film of dirt can prevent light transfer from the LEDs to the photo diodes. Again, refer to the service manual for specifications.

10.2.1 Distributorless Ignition System

Distributorless ignition system service. Many of the components used in a computer-controlled ignition system are similar to those found in older electronic or contact point systems. Testing procedures for parts, such as spark plugs, secondary wires, ignition coils, etc., remain the same. However, the computerized ignition system contains engine sensors and a computer, which make it more difficult to troubleshoot and service these systems.

In systems containing a coil pack that fires two spark plugs at the same time, a bad ignition coil will kill two cylinders. For example, if a four-cylinder engine has two ignition coils, one bad coil will make the engine run on two cylinders, producing a very rough idle. If two dead cylinders correspond to a specific coil, test that ignition coil.

10.3.0 Integrated Direct Ignition System

Direct ignition system service. The procedures for servicing a direct ignition system are similar to the procedures described for other types of ignition systems. The main difference is that a direct ignition system has a coil for each cylinder.

A direct ignition coil is tested like other ignition coils. Measure both primary and secondary winding resistance. Also, make sure you are getting primary voltage to the coil.

There are tricks for working on a direct ignition system. Remove the coil cover and connect conventional spark plug wires between the coil output terminals and the spark plugs. This will let you connect a timing light, an inductive tachometer, a spark tester, etc., to the system.

Checking with an oscilloscope. Oscilloscopes can display distributorless and direct ignition system patterns. To read the distributorless ignition system (DIS) pattern, the oscilloscope has leads that are attached to each plug wire. Since direct systems have no plug wires, connections must be made to the primary side of the ignition system. If the scope was originally designed for distributor ignitions, an adapter must be used to sort out the patterns developed by the individual coils. Newer oscilloscopes are able to read the direction of the current flow in each wire. They use the current direction information to determine the firing order and which coils fire which plugs. Most scopes today can also measure the voltage and amperage of individual coils.

Oscilloscopes setup for distributorless ignition systems and direct systems is similar to that for a system with a distributor. To check a DIS ignition, make the primary and secondary wire connection as instructed by the scope's operating manual. Once the connections are made, set the oscilloscope controls to the DIS settings. Then start the engine and observe the secondary pattern. The scope pattern will resemble the pattern produced by a distributor ignition. Look for high or low firing lines that indicate fouled plugs or faulty secondary wires. As with a distributor system, a lack of oscillations often indicates a shorted coil winding.

Before deciding that the system is okay, check the DIS primary pattern. The primary pattern may show problems that are not visible in the secondary pattern. On a direct ignition system, only the primary pattern can be displayed. Figure 39 shows the primary voltage and amperage in a DIS coil during two firing cycles. Comparing this pattern to a known good pattern enables the mechanic to locate a problem in the coil, module, or primary side connections.

Figure 39 - An illustration showing the primary pattern of a DIS system.

One advantage of testing a DIS or direct ignition system with multiple coils is that the operation of the coils can be compared to determine if one of them is not working properly. Figure 40 shows DIS coil amperage draws for a V8 engine. All the coils are drawing about the same amount of current, indicating that all are performing at the same level. If one coil draws much more or much less current than the others, there is a problem. The problem could be in the coil itself or in the associated plugs or wires. Sometimes the ignition module driver for the coil in question has failed, but this is rare.

Figure 40 - DIS coil amperage draw.

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11.1.0 Headlights

Sealed-Beam Headlights. The standard sealed-beam headlight is an air-tight assembly with a filament, reflector, and lens fused together. The parabolic reflector is sprayed with vaporized aluminum and the inside of the lamp is typically filled with argon gas. The reflector intensifies the light produced by the filament, and the lens directs the light to form the required light beam pattern. The lens is designed to produce a broad, flat beam pattern. The light from the reflector is passed through the concave prisms in the glass lens.

Today, most commonly used sealed-beam headlight is the halogen type. A halogen lamp typically consists of a small bulb filled with iodine vapor. The bulb is made of high-temperature- resistant glass and it surrounds a tungsten filament. The halogen- filled inner bulb is then installed in a sealed glass or plastic housing, as shown in Figure 41. With the halogen added to the inner bulb, the tungsten filament is capable of withstanding higher temperatures, and it can burn more brightly.

Figure 41 - Halogen sealed-beam headlight with an iodine vapor light.

Halogen is the term used to identify a group of chemically related nonmetallic elements. These elements include chlorine, fluorine, and iodine.


Because the filament is contained in the inner bulb, cracking or breaking of the housing or lens does not prevent a halogen bulb from working. As long as the filament envelope has not been broken, the filament will continue to operate. However, a broken lens results in poor light quality and the lamp assembly should be replaced.

Low- and high-beam filaments are placed at slightly different locations within a sealed- beam bulb. The filament location, relative to the reflector, determines how light passes through the bulb's lens (Figure 42), which in turn determines the direction in which the light shines. In a dual filament lamp, the lower filament is used for the high beam and the upper filament is used for the low beam.

Figure 42 - Light beam projection is controlled by filament placement.

Various methods are used to identify sealed-beam headlights, such as 1, 2, and the "halogen" or "H" marking molded on the front of the headlight lens.

When a type 2 is switched to low beam, only one of its filaments is lit. When the high beam is selected, the second filament lights in addition to the low beam.

If a sealed-beam headlamp has condensation on the lens or inside the assembly or if it is cracked, the headlamp will not work and can only be repaired by replacing it.

Composite Headlights. Many vehicles have halogen headlight systems that use a replaceable bulb, as illustrated in Figure 43. These systems are called composite headlights. By using the composite headlight system, the manufacturers are able to produce any style of headlight lens they desire (Figure 43), which improves the aerodynamics, fuel economy, and styling of the vehicle.

Figure 43 - Typical mounting assembly of a replaceable halogen light.

Many manufacturers vent the composite headlight housing due to the intense heat developed by these bulbs. Because the housings are vented, condensation may develop inside the lens assembly. This condensation is not harmful to the bulb and does not affect headlight operation. When the headlights are turned on, the heat generated by the halogen bulb dissipates the condensation quickly. Ford uses integrated non- vented composite headlights so condensation is not normal and the assembly should be replaced.


Whenever you replace a composite lamp, be careful; do not to touch the lamp's envelope with your fingers. Staining the bulb with skin oil can substantially shorten the life of the bulb. Handle the bulb only by its base, as shown in Figure 44. Also dispose of the bulb properly.

Figure 44 - Handle a high-intensity halogen light properly.

Headlight Service. When there is a headlight failure, it is typically caused by a burned- out bulb or lamp, especially if only one lamp fails. However, it is possible that the circuit for that one lamp has an open or high resistance. Check for voltage at the bulb before replacing the bulb. If there is no voltage present, the circuit needs work and the original bulb may still be good. If more than one lamp (including the rear lights) is not working, carefully check the circuit. If the charging system is not being regulated properly, the high voltage will cause lamps to burn out prematurely.

Headlights Replacement. A burned-out bulb has the filament melted in half. Sealed- beam headlamps are usually held in place with small screws and a retaining ring. Halogen insert bulbs normally fit into the rear of the bulb housing. You must push and twist to install some headlamp bulb inserts. Sometimes a small ring screws over and secures the halogen insert. Spring clips can secure halogen fog lamp bulbs.

Most incandescent bulbs are housed in a lens. They are normally held in the socket by a spring and small dowels or a press fit. You may have to remove the lens or reach behind the housing to access the bulb.

No-Light Problem. A no-light problem is a total failure of the light circuit or bulb. First, check to see if the bulb is burned out. Close inspection of the filament will show whether it has burned. If the bulb is good, check for power to the bulb socket.

Figure 45 shows how to check for current in a bulb socket. With the light turned on, there is a socket or circuit problem if the test light does not glow. If you do not have power to the socket, trace back through the circuit to find the open preventing current flow.

Figure 45 - Check the socket with a test light.

Flickering Light Problem. Flickering lights (lights going on and off) point to a loose electrical connection or a circuit breaker that is kicking out because of a short.

If all or several of the lights flicker, the problem is in a section of the circuit common to those lights. Check to see if the lights flicker only with the light switch in one position. For example, if the lights flicker only when the headlights are on high beam, you should check the components and wiring in the high beam section of the circuit.

If only one light flickers, the problem is in that section of the circuit. Check the bulb socket for corrosion. Clean the socket. Also, make sure the bulb terminals are not worn. This could upset the electrical connection. If needed, replace the bulb socket and bulb.

Headlamp diagnosis. Figure 46 provides you a quick reference for headlamp diagnosis. As always, refer to the service manual for more detailed information for troubleshooting.

Figure 46 - Quick reference for headlamp diagnosis

11.2.0 Fuses and Circuit Breakers

The main types of circuit protection devices used in electrical systems are fuses, circuit breakers, and fusible links. Each of these devices is designed to be the weak point of a circuit and open before the wiring gets too hot. Typical symbols for protection devices are shown in Figure 47.

Figure 47 - Electrical symbols for common circuit protection devices.

Fuses. Fuses have a specific current rating. Fuses for truck electrical systems are available that are designed to open with less than 1A of current flow to more than 200A of current flow. Fuses also have a voltage rating. The voltage rating indicates the maximum voltage that the fuse is designed to interrupt. Most fuses designed for truck electrical systems have a voltage rating of 32V.

Fuses are also classified by how long it takes for the fuse to open with a given percentage of current overload. Fuses may be classified as fast-acting or time- delay fuses. Any fuse opens based on a combination of current and time. A 20A fuse does not open just as soon as current flow through the fuse reaches 21A. Most fuses are designed to maintain a 110 percent overload current indefinitely, so a 20A fuse will probably never blow if 21A of current is flowing through the fuse. At a 150 percent current overload (30A of current flow for a 20A rated fuse), a 20A fuse may blow after 2 seconds for a fuse classified as a time delay fuse. The in rush current of devices controlled by the circuit that the fuse protects determines what type of fuse is necessary.

Most vehicles fuses are blade-type fuses. The blades of the fuse are male terminals, as the samples shown in Figure 48). A blade fuse has colored, translucent plastic housing. This allows the fuse element to be visible for inspection. A fuse that is blown may have a visible gap in the element or the plastic housing may be blackened from heat, depending on the amount of current that caused the fuse to blow. The colored fuse housing is colored based on the fuse's current rating. The color of the housing for a particular fuse current has been standardized in the industry. Standardized fuse colors throughout the automotive industry help make sure that the correct value of fuse for each circuit is installed in the fuse box.

Figure 48 - Examples of fuses used in electrical systems.

Blade fuses are available in a variety of sizes. Most new vehicles use SAE (Society of Automotive Engineers) J2077 fuses. The J2077 designation refers to the SAE specification number that defines this fuse. These small fuses are often called by trade name Mini® or ATM® fuses. These small fuses are classified as fast-acting fuses. This means that the fuse will open relatively rapidly if current flow through the fuse exceeds the designated amperage. SAE J2077 fuses are available from 2A to 30A current ratings.

The next larger blade-type fuse is an SAE J1284 fuse. The fuse is also known as ATC®, ATP®, or auto fuse. These fuses are somewhat larger than the SAE J2077 fuses and were the standard fuse in vehicles for a long period. These fuses are also classified as fast-acting fuses. SAE J1284 fuses are available from 1A to 40A.

An even larger blade-type fuse is an SAE J1888 fuse. This fuse is also known as a Maxi® fuse. These fuses are classified as time-delay fuses, so they are often used to protect circuits connected to devices with high inrush currents, such as electric motors. SAE J1888 fuses are available from 20A to 80 A.

Standard blade fuse housing colors for each SAE classification are shown in Figure  49.

Figure 49 - Standardized blade fuse colors.

Prior to the development of blade fuses, glass cartridge fuses were the standard. A glass cartridge fuse is a small glass tube with a fuse element running the length of the tube. Each end of the glass tube has a metal cap that connects to the fuse element. The fuse current and voltage ratings are stamped into the caps. There are many types of glass cartridge fuses, but the most popular types used are SFE and AGC types.

SFE fuses all have the same physical dimension regardless of the fuse current rating. AGC-type fuses increase in length as the fuse's current rating increases. The fuse block for AGC fuse is designed so that only fuses of the correct length can be placed in the corresponding fuse block cavity. This prevents a fuse with a too high a current rating from being installed in a particular fuse block cavity because such a fuse would be too long for the cavity.

There are several other types of fuses that are found in trucks. A very large bolt-in type fuse, called a Meg® or AMG® fuse, is commonly used to protect the main battery power feed cable that supplies the cab electrical system. These bolt-in fuses have current ratings up to 500A, but a typical current rating for bolt-in fuse used to protect the cab power feed cable is 100A to 150A.

Fuse link. A fuse link, or fusible link, is a small section of wire connected in series with the larger wire. It is designed to serve as a very large fuse, (Figure 50).

Figure 50 - A typical fuse link.

Fuse links are normally located in the engine compartment where power feeds off the battery or starter solenoid. They provide circuit protection before the fuse box in the passenger compartment. If a wire is shorted to ground before the fuse panel, the fuse link can overheat and burn in half. This will protect the rest of the wiring from major damage.

The wire gauge of a fusible link is typically selected so that it is four American Wire Gauge (AWG) sizes smaller than the smallest wire that it is directly protected by the fusible link. In the event of a short to ground in a circuit protected by a fusible link, the fusible link heats up more than any wire that is protected by the fusible link. Excessive current flow through the fusible link causes the wire to melt like a large fuse element. The special insulation material used in fusible links does not melt off the fusible link nor does it support a flame, like many types of standard wire insulation. Fusible links are like fuses in that once they have melted, they must be replaced.

Fusible links are somewhat difficult to replace because unlike a fuse, they do not merely connect to the fuse block like a blade or cartridge fuse. Replacing a fusible link involves cutting the damaged fusible link out of the wire harness and splicing a new fusible link in its place. This can be a very time-consuming process because fusible links are located near the motor and are difficult to access. However, fusible links are normally limited to protecting high-current circuits, such as alternator charging circuits and cab power supply cables. These types of circuits should rarely be subjected to currents high enough to cause the fusible link to burn open. The circuits that fusible links supply are normally branched into other sub-circuits that are protected by smaller circuit protection devices, such as fuses and circuit breakers.

Unlike fuses, fusible links do not have a current rating. Instead, fusible links are rated by conductor diameter. The size of the fusible link conductor is stamped on the insulation. Often, fusible links are sold in metric sizes and converted to the closest AWG size.

A fusible link that has been burned open must be replaced with another fusible link of the same length and gauge. It is critical that the fusible link be replaced only with fusible link wiring of the correct gauge for the application. The fusible link must be the smallest diameter circuit to provide adequate wire harness protection.



A fusible link that has burned open must only be replaced with the same size of special fusible wiring. Installing a larger gauge of fusible link is like installing a larger fuse and can result in a fire. NEVER replace a fusible link with standard wire. The insulation material on standard wire can ignite, resulting in a fire.

The routing of a fusible link must also be placed so that the fusible link is not contacting anything flammable. The excessively high current flow through a fusible link causes the fusible link wire conductor to glow red hot before it melts to interrupt the circuit. The heat given off by the hot fusible ink wire is significant and can cause surrounding material to ignite.

Circuit breakers. Circuit breakers are thermal devices that use current flow to heat a thin piece of bimetallic strip, as shown in Figure 51. If the bimetal strip is sufficiently heated, the strip will bend or snap, causing a pair of contacts to open and interrupt current flow in a circuit. An open circuit breaker is referred to as being tripped. When the circuit breaker cools down, the circuit breaker can be reset, which causes the contacts to close again and restore current flow in the circuit. Because a circuit breaker can be reset, this provides an advantage over a fuse, which must be replaced if blown. Like fuses, circuit breakers have a current rating and voltage rating. Circuit breakers generally require longer to open than a comparable fuse for a given amount of excess current, so they are considered time delay.

Figure 51 - Cutaway view of a circuit breaker.

The schematic symbol for a circuit breaker is CB. The current rating of the circuit breaker is often displayed next to the schematic symbol, such as 20A, along with an identification, such as CB3.

Circuit breakers are classified by Society Automotive Engineers (SAE) into three different categories:

  1. Type I circuit breakers are automatic reset devices. Type I breakers reset automatically without operator intervention after the bimetallic strip cools down. The reset action causes the contacts to close and restore current flow to the circuit. If the circuit is still drawing too much current, the breaker will open or trip again repeatedly. Type I circuit breakers are commonly used in windshield wiper and headlamps where the automatic reset feature is desirable. However, the constant reset action of Type I circuit breakers can cause the circuit breaker contacts to become damaged from excessive making and breaking of a high-current circuit.
  2. Type II circuit breakers are modified automatic reset devices. Type II circuit breakers reset automatically after the cause of the excess current to the circuit is no longer present. Type II circuit breakers also reset when the voltage supply for the circuit breaker, such as ignition voltage, is switched off for a few minutes. Type II circuit breakers contain a heating element that causes the bimetallic strip to open. Type II circuit breakers are designed so that as the current flow through the heating element decreases, the bimetallic strip cools and automatically closes again like a Type I circuit breaker.
  3. Type III circuit breakers are manual reset devices. A trip-indicating button or lever pops up when the breaker is tripped. After the bimetallic strip cools sufficiently, the button can be depressed to reset the breaker. Depressing the reset before the bimetallic strip has cooled enough to cause the button to pop back up when released. If the cause of the excess current is still present after the bimetallic strip has cooled, the circuit breakers will trip again when the bimetallic strip heats up sufficiently.

Like blade fuses, blade-type circuit breakers are common on new trucks. Blade-type circuit breakers are designed to fit into the same fuse block cavities as blade fuses. The footprint of the terminals is the same as the blade fuses, but the circuit breaker may be wider, thicker, and taller than the blade fuse. The housing or reset button of most blade- type circuit breakers has the current rating stamped on the circuit breaker housing and has no color coding.

Besides blade-type circuit breakers, some circuit breakers have threaded stud-type connections. These are designed to accommodate wiring with ring terminals and are common on trailer and older trucks.

11.3.0 Directional (Turn) Signals

The directional (turn) signal system consists of a fuse, turn signal switch, flasher unit, turn signal bulbs, indicator bulbs, and related wiring. When the steering column- mounted switch is activated, it causes the right- or left-side turn lamps to flash. Turn indicator lights in the instrument panel or on the fenders, if equipped, also flash.

The turn signal switch may be mounted in the center of the steering column, behind the steering wheel. A multifunction switch can also be used to control turn lights, horn, and dimmer switch.

The turn signal flasher automatically opens and closes the turn signal circuit, causing the bulb to flash. The flasher unit contains a temperature-sensitive bimetallic strip and heating element. The bimetal strip is connected to a set of contact points and to the fuse panel.

When the current flows through the turn signal flasher, the bimetallic strip is heated and bends. This opens the contact points and breaks the circuit. As the bimetallic strip rapidly cools, it closes the points and again completes the circuit. This heating and cooling cycle takes place in about a second. The turn lights flash as the points open and close.

Flashers are designed to flash at a rate of about 70 to 110 times per minute. This rate may slow considerably in very cold weather because of the bimetallic strip. The flash rate is also greatly affected by electrical system voltage. The flash rate decreases as system voltage decreases because of the corresponding reduction in heating element current.

The flasher's opening and closing action also causes the familiar clicking sound to provide an audible indication that turn signals or hazard flashers are active.

There are several different classifications of bimetallic flashers, even though most have the same two-terminal footprint. Bimetallic flashers are usually designed to flash a specific number of lamps. The flasher may also provide an indication of lamp outage by changing the flash rate when a lamp circuit is open, as with a burned-out bulb.

Some bimetallic flashers may be designed for turn signals only. These types of flashers have a specific lamp rating of 2 to 5 lamps. The flasher rating indicates how many turn signal lamps the flasher is expected to flash. Other bimetallic flashers may be designed for both turn signals and hazard warning lamps. These flashers have a rating that indicates both the number of turn signals and the number of hazard lamps the flasher is designed to control. For example, a 2/4 lamp flasher is rated for two turn signal lamps and four hazard lamps.

Hybrid flashers. Hybrid flashers contain an electronic circuit used to control an electromechanical relay. The electronic circuit provides a precise flasher rate that is not dependent on temperature and system voltage. Hybrid flashers generally have a longer service life than bimetallic flashers due to the lack of a heating element. The long service life of hybrid flashers makes them ideal for truck electrical systems.

Hybrid flashers may also have lamp outage detection. The flasher monitors lamp current and causes the flash rate to increase if one of the turn signal lamps is burned out or a lamp circuit is otherwise open. This is unlike the bimetallic flasher, which causes the flasher rate to decrease or stop flashing altogether with an open lamp circuit or a burned-out lamp.

Solid-state flashers. Solid-state flashers typically contain a power metal-oxide- semiconductor field-effect transistor (MOSFET) acting as a switch and have no moving contacts. An electronic circuit controls the MOSFET and provides precise rates in all conditions. These flashers are more expensive than hybrid flashers but have a very long service life.

Turn signal problems. If the turn signals do not flash, check first for a burned-out bulb. Even one burned-out bulb will reduce current and prevent the flasher unit from functioning. A burned-out bulb is the most common cause of turn signal problems.

If both right and left turn signals do not work, check the fuse and flasher unit. The problem may also be in the turn signal or malfunction switch. It is prone to wear and failure after prolonged service. Something common to both sides of the circuit may be at fault if no bulb is coming on.

Figure 52 provides a quick reference for turn signal diagnosis. You should refer to the service manual for any detailed information for troubleshooting procedures.

Figure 52 - Quick reference for turn signal diagnosis.

11.4.0 Brake Lights

The brake light system is commonly made up of a fuse, brake light switch, rear lamps, and related wiring. The brake light switch is normally mounted on the brake pedal, as illustrated in Figure 53. Battery power is fed to the brake light switch from the ignition switch. When the brake pedal is pressed, it closes the switch, and current flows through the wiring to the brake lights.

Figure 53 - Brake light switch action.

The brake light switch can also be located on the master cylinder with a T-fitting or connected to the brake line. In these switches, hydraulic pressure from the brake system closes the switch to turn on the brake lights.

Brake light problem. If none of the brake lights are working, something common to all the bulbs is at fault, such as the brake light switch or feed circuit. If only one bulb is not working, the bulb and its section of the circuit should be checked.

11.4.1 Trailer Lighting

The voltage source for lighting on trailers is supplied by the tractor. The electrical connection between the tractor and trailer is provided through a seven-wire trailer electrical cable with a plug on either end. The trailer cable is like a seven-wire electrical cord but has the same plug on both ends. This trailer wiring cable is usually coiled to permit movement between the truck and trailer, such as when turning.

Trailer connections. Both the tractor and trailer have a seven-terminal trailer receptacle that accepts the trailer electrical cable plug (Figure 54). Trailers used as a part of double and triple trailers have trailer receptacles at both the front and rear of the trailer to provide interconnection between trailers and dollies.

Figure 54 - Tractor-trailer connectors.

Because most tractors do not pull the same trailer all the time, the trailer receptacle and cable are standardized throughout North America. The trailer interconnection is defined by SAE recommended practice J560. Each of the seven circuits on all trucks and trailers should be wired in the prescribed manner to permit tractors and trailers throughout North America to be interchangeable.

The color of wiring insulation for each trailer circuit is also standardized, as shown in Figure 55. This wiring convention should be followed to aid in troubleshooting and identification.

Figure 55 - Trailer standard wiring colors.

Note in the schematic layout Figure 56 that the trailer running lights are broken down into two circuits: the black wire and the brown wire. The black wire should be connected to the front marker and clearance lamps, the identification lamp (three-bar light), the two lower rear-side marker lamps, and the center marker lamps (if applicable). The brown wire should be connected to the rear clearance lamps, the tail lamps, and the license plate lamp. These circuits should be kept isolated from each other because they may be protected by two different circuit protection devices in the tractor. Otherwise, a short circuit at one place in the trailer running lamp wiring would disable all trailer running lamps.

Figure 56 - Schematic layout of a trailer electrical system.

The illustration of the trailer plug and the trailer receptacle in Figure 57 is something that you should become familiar with. It shows the views of the mating end of the tractor receptacle and of the trailer plug. A test light or a special trailer light test tool is used to test for voltage at the specific terminal to determine if the problem is with the truck or the trailer. Notice the larger-diameter terminal at the top; this is the trailer ground terminal.

Figure 57 - Tractor and trailer mating ends.

Because all trailer light current must pass through the ground circuit, it uses a larger- diameter terminal than the other terminals and uses a larger-diameter wire than the other wires in the seven-way cable.

The tractor normally provides the circuit protection for trailer wiring. However, some trailers may have circuit breakers for trailer lights as well, (Figure-58). The trailer power distribution module is usually mounted on the left frame rail aft of the cab or on a rear crossmember at the end of the frame rail, as shown in Figure 59. The trailer PDM contains fuses and relays to enable high current outputs via a wiring harness and relays to enable high current outputs via a wiring harness to the trailer connector.

Figure 58 - Trailer power distribution module.

Figure 59 - Trailer PDM installation.

The length of the trailer wiring must be considered when replacing trailer wiring. Even though the rear trailer lamps may have the same ratings as the truck lamps, the long distance to the lamps can result in a substantial voltage drop on the wiring. This reduces the voltage at the trailer lamps. Wire gauge is especially important for double and triple trailers because the total circuit from the rear trailer light feed to the tractor can be more than 80 feet. Figure 60 should be used to determine the wire gauge necessary for trailer wiring repair or replacement.

Figure 60 - Recommended wire gauges for trailer wiring.

Figure 61 is a quick reference for troubleshooting electrical systems. It is always recommended to refer to the service manual for detailed information for troubleshooting and repairs.

Figure  61 - Quick reference electrical troubleshooting.

11.5.0 Horns

The horn system uses a coil-operated diaphragm to produce sound waves and an audible sound. The horn system is simple; it consists of:

  1. Fuse or circuit breaker - electrical safety device that protects horn circuit from shorts
  2. Horn wiring harness - wires that connect fuse to harness and horns to horn button switch
  3. Horns - diaphragm-operated devices that vibrate to produce sound waves
  4. Horn switch - steering wheel- or steering column-mounted switch for completing horn circuit

Figure 62 shows a basic horn system. Power is present at the horns and in the harness whenever the ignition switch is turned on. The horns switch grounds the circuit to activate the horns.

Figure 62 - Horn circuit.

When the driver presses the horn button, the wire leading from the horns is grounded. This causes current to flow through the fuse and horns. The resistance in the horn coils limits how much current flows into ground.

When the driver releases the horn button, a spring pushes the switch back open. This breaks or disconnects the ground circuit. No current can then flow through the horns and they stop sounding.

Horn nomenclature. A cutaway view of a typical horn is given in Figure 63. It is made up of the following components:

  1. Horn coil - set of windings that produce magnetic field when energized by current flow
  2. Contacts - breaker points that open and close to control current flow through horn coil
  3. Diaphragm - flexible membrane in horn that moves back and forth to produce air waves and sound
  4. Plunger - metal core that is attached by magnetic field of coil windings
  5. Wire terminal - connector for making electrical connection with wiring harness
  6. Outlet - opening of horn housing for directing sound to front of vehicle

Figure 63 - Cutaway view of a horn.

When the driver presses the horn button, current flow enters the wire terminal and horn coil. A magnetic field forms around the coil. The field attracts and pulls the plunger into the coil. Since the plunger is attached to the diaphragm, the diaphragm is flexed back toward the coil.

With enough movement, the edge of the plunger touches one of the contact point arms. This pushes the contacts open and interrupts the current flow through the coil. Without current, the magnetic field collapses, and the diaphragm snaps or flexes back into its normal position.

Once the diaphragm and plunger move back, the contacts reclose. This reenergizes the coil and the diaphragm is again pulled back toward the coil. The process is repeated rapidly and the diaphragm vibrates back and forth in the horn housing. The resulting vibration set up in the surrounding air can be heard as a "honking" sound.

A tone adjustment screw is normally provided for changing the sound of the horn. It can be turned to affect the action of the contact points and frequency of the diaphragm vibration.

Horn relay. A horn relay is sometimes used between the horn switch and the horns. It is used to reduce the amount of current flowing through the horn switch.

When the driver presses the horn switch, a small current flow enters the horn relay. This energizes the small coil in the relay to close the relay contacts. Then, a larger current flows through the closed relay points and to the horns.

Horn Service. When a horn will not sound, check the fuse and connections, and test the voltage at the horn terminal. If a horn blows continuously, the horn switch may be bad. A relay is another cause of horn problems. The contacts in the relay could be burned, or they may be stuck together.

A horn current adjusting screw is sometimes provided on the horn to set the amp draw through the horn. To adjust horn current, connect an ammeter between the feed wire and horn terminal. To prevent meter damage, be sure the ammeter can read more than 30 amps.

Get another person to sound the horn while you read the meter. If the current is not within specifications (typically 4-5 amps), turn the amps screw on the horn until the meter reads properly. Also, make sure you are getting adequate supply current/voltage and there is not a high resistance in the horn circuit. If you cannot get the horn to read within current specifications, replace it or isolate the circuit problem.

11.6.0 Small Accessory Motors

11.6.1 Windshield Wipers

A typical windshield wiper system is made up of a switch, wiper motor assembly, wiper linkage, wiper arms, and wiper blades. Either a fuse or circuit breaker protects the system.

The windshield wiper switch is a multiposition switch that sometimes contains a rheostat. Each switch position provides a different wiping speed. The rheostat operates the delay mode for slow wiping action. A relay is frequently used to complete the circuit between the battery and the wiper motor.

A wiper motor assembly consists of a permanent magnet motor and a transmission. The wiper motor transmission changes rotary motion into a back-and-forth wiping motion.

The transmission is normally a set of plastic gears, an end housing, and a crank.

On the windshield wiper assembly the drive crank on the transmission connects to the wiper linkage.

The wiper linkage is a set of arms that transfers motion from the wiper motor transmission to the wiper arms. The rubber wiper blades fit on the wiper arms.

Windshield wiper service. Windshield wiper blades should be inspected periodically. If they are hardened, cut, or split, replace them.

With electrical problems in a wiper system, refer to the service manual and its wiring diagram of the circuit. First, check the fuses and electrical connections. If they are good, use a test light to check for power to the wiper motor.

If power is being fed to the wiper, the motor or transmission may be at fault. Before replacing the motor or transmission gears, make sure the motor is properly grounded. If power is not reaching the wiper motor, check the wiper switch and circuit connections for openings.

If the windshield washer does not work, check the fuse and connections. Use a test light to check for power going to the motor. If the test light does not glow, the wiper switch may be bad.

When working on a windshield wiper system, always follow the exact recommendations given by the manufacturer, as systems and procedures vary with each vehicle.

Normally, the wiper motor must be replaced as a unit. The transmission gears are usually the only serviceable part in the assembly.

11.6.2 Windshield Washer

A windshield washer consists of a solvent reservoir, pump, rubber hoses, connections, and washer nozzles. The solvent reservoir, located in the engine compartment, holds a supply of water and solvent. When the washer switch or button is activated, the wiper motor and washer pump turn on. Solvent is forced out of the reservoir and onto the windshield.

There are two common types of pumps used with windshield washer systems: a rotary pump and a bellows (diaphragm) pump. Most new vehicles use a rotary pump mounted in the solvent reservoir. A tiny electric motor spins an impeller, which forces the washer solution onto the windshield. A bellows pump is normally mounted on and powered by the wiper motor.

Windshield washer service. Many washer problems are caused by restrictions in the fluid lines or nozzles. To check for restrictions, remove the hose from the pump and operate the system. If the pump ejects a stream of fluid, then the fault is in the delivery system. The exact location of the restriction can be found by reconnecting the fluid line to the pump and disconnecting the line at another location. If the fluid still streams out, the problem is after that new disconnect. If the fluid does not flow out, the problem is before the hose was disconnected. Repeat this process until the problem is found.

If the pump does not spray out a steady stream of fluid, the problem is in the pump circuit. It should be tested in the same way as any other electrical circuit. Make sure it gets power from the switch when it should, then check the ground. If the power to the pump is good and there is a good ground, the problem is the pump. Pumps are not rebuilt or repaired; they must be replaced.

11.6.3 Power Windows

A power window uses a control switch, reversible electric motor, circuit breaker, fuse, and related wiring to operate the door windows.

A small electric power window motor is located inside each door to operate the window regulator (up-down mechanism for the glass). The motors have a gearbox, or transmission (usually worm and ring gear), that changes the rotating motion of the motor armature into a partial rotation of a larger gear. This action pushes the window open or closed.

A circuit breaker protects the window motor from overheating damage. The breaker can open if the switch is held in one position too long. The circuit breaker can be located inside the motor or elsewhere in the circuit. A simple power window circuit for a passenger-side power window is shown in Figure 64.

Figure 64 - Schematic simple power window circuit for a passenger-side window.

Some power windows will stop or reverse direction if an obstruction resists window closing. In some of these systems, a magnetic trigger wheel is mounted on the motor's armature shaft. As the trigger wheel turns, it creates a signal in a magnetic or Hall-effect sensor in the motor assembly. If the sensor outputs a slower-than-normal signal, the power windows ECM will cut voltage or reverse polarity of the voltage going to the window motor. This stops the window or lowers it to prevent damage to objects or injury to people who accidentally put their hands, head, or arms into the path of the window as it closes.

Power window service. When none of the power windows work, you first check the fuse or circuit breaker for the whole system. If only one of the windows is inoperative, use a test light to check for power to its switches.

If you hear a humming sound when a window switch is pressed, the motor gearbox may have stripped gear teeth. The plastic gears in the window motor gearbox can strip after prolonged service. The motor will spin, but movement will not be transferred to the window. If the motor or the switches are found to be bad, they should be replaced.

With hard-to-find problems, refer to the service manual wiring diagram for the power windows, which will show all the components that could affect power window operation and help with troubleshooting.

- To Table of Contents -


This manual has presented information and procedures in troubleshooting electrical systems. You covered the alternator and its components and function, along with rectifiers, voltage regulators, and troubleshooting procedures.

The succeeding topics encompassed troubleshooting the cranking system with emphasis on the various tests, the ignition system and its components and subsystems, lighting systems, and electrical accessories.

Electric and electronic devices can be found almost everywhere on an automobile, truck, power generation equipment, and construction equipment. These components have even replaced some mechanical devices. They are more efficient, compact, and lighter.

Troubleshooting equipment takes complex diagnostic procedures, much service literature, and specialized equipment. It is uncommon for the mechanic to get through even one day without referring to a service manual or technical literature.

Specialized tools, test equipment, and other diagnostic equipment, as well as service manuals and technical literature, are rapidly becoming a necessary requirement.


- To Table of Contents -

Review Questions

1. The voltage regulator maintains an alternator output of how many volts?

A. 10 to 12
B. 12 to 14
C. 13 to 15
D. 14 to 16

2. Diodes, heat sink, and electrical terminal make up what assembly?

A. rotor
B. rectifier
C. brush
D. stator

3.  The stator is a rotating set of windings mounted between the end frames.

A. True
B. False

4. There are commonly two types of bearings used in an alternator, one being ball type. What is the other type of bearings sometimes used?

A. roller thrust
B. roller
C. friction
D. needle

5. The text lists three types of alternator belts. Which of the following is not an alternator belt?

A. ribbed V-belt
B. conventional V-belt
C. ribbed belt
D. cogged V-belt

6. Which component of the alternator feeds the diode trio?

A. stator
B. rotor
C. rectifier
D. diode feeder

7. When a diode is connected to voltage source where the current passes through the diode, the diode is said to be what?

A. Reverse biased
B. Biased
C. Forward biased
D. An insulator

8. There are four basic types of voltage regulators. Where might the electronic regulator be mounted?

A. Inside the alternator
B. Away from alternator in engine compartment
C. On the rear of the alternator
D. On the front of the alternator

9. The electronic voltage regulator increases alternator output by doing what?

A. Allowing more current into the rotor windings
B. Preventing more current into the rotor windings
C. Allowing less current into the rotor windings
D. Allowing more current into the stator windings

10. Field circuit modulation refers to how the can cycle the rotor field current on and off to control charging system output.

A. stator
B. diode trio
C. voltage regulator
D. rectifier bridge

11.  It is not necessary to disconnect the battery before moving any charging system components if the ignition is off.

A. True
B. False

12. While testing a 12-volt system, the voltmeter reads 15 volts. What is the reason for this high output?

A. Blown fuse
B. Shorted field wire
C. Grounded field
D. Current limiter relay of the regulator

13. Circuit resistance tests are used to locate wiring problems in a charging system by testing voltage between which components?

A. Alternator output terminal and the negative battery post
B. Alternator output terminal and positive battery terminal or negative battery terminal and alternator housing.
C. Alternator housing and positive battery terminal
D. Alternator input terminal and negative battery terminal or positive battery terminal and alternator housing

14. When performing a regulator ground circuit resistance test, a voltmeter reading exceeding how many volts indicates a possible damaged ground strap or loose mountings?

A. 0.01
B. 0.10
C. 1.00
D. 1.10

15. When using an ohmmeter to test diodes, what must you do?

A. Connect the diodes with a piece of wire.
B. Remove the diodes from the alternator.
C. Disconnect the stator windings.
D. Unsolder the diodes and isolate them.

16. When testing a diode with an ohmmeter and it reads high resistance in one direction and low resistance in the other, what does this reading tell you?

A. The diode is good.
B. The diode is shorted.
C. The diode is open.
D. Replace the diode.

17. A weak diode will produce what type of pattern on an analyzer screen?

A. A high or low peak every sixth pulse
B. A flat signal each sixth pulse
C. A low ripple pattern
D. A steep peak every sixth pulse

18. On a vehicle equipped with a 24-volt series parallel starting system, what minimum voltmeter reading is considered normal for a cranking voltage test?

A. 12 volts
B. 14 volts
C. 16 volts
D. 18 volts

19. While conducting a starter insulated circuit test, what maximum allowed voltage drop is the norm across the length of a cable?

A. 0.01
B. 0.10
C. 1.00
D. 1.10

20. During a starter ground circuit resistance test, the measured voltage loss exceeds 0.2 volt or the loss given by the manufacturer's specifications. This loss can result from which of the following problems?

A. Loose starter bolt
B. Poor battery ground terminal post connector
C. Damaged undersized ground system wire
D. Each of the above

21. What component of the ignition system boosts battery voltage to a much as 60,000 volts?

A. Alternator coil
B. Ignition switch
C. Ignition coil
D. Battery booster

22.  An electronic ignition is more dependable than a contact point because there are no mechanical breakers to wear or burn.

A. True
B. False

23. A solid-state chip or module that produces an electrical signal when triggered by a slotted wheel is called what?

A. Hall-effect pickup
B. Trigger wheel
C. Permanent magnet
D. Pickup coil

24. In the distributorless ignition system, what component consists of two or more ignition coils?

A. power distribution module
B. distributorless coil module
C. ignition module
D. electronic coil module

25. The following are all advantages of distributorless ignition system over ignition systems with a distributor except:

A. No mechanical weights
B. No vacuum advance diaphragm
C. More moving parts to wear and malfunction
D. No rotor or distributor cap

26. What component is eliminated with a direct ignition system?

A. coils
B. spark plugs
C. ignition wires
D. spark plug wires

27. In a computerized ignition system, heat will upset the operation of the electronic components in what unit?

A. Power distribution module
B. Ignition control module
C. Electronic power module
D. Computer module

28. In the electronic ignition system, what test compares actual resistance or voltage output?

A. Magnetic pickup coil
B. Ignition coil
C. Electronic ignition resistance
D. Magnetic voltage

29. When conducting a Hall-effect sensor with an oscilloscope, how should the waveform appear?

A. Switch slowly, angled sides, top square reaching reference voltage, and bottom should reach zero
B. Switch rapidly, sloped sides, top rounded reaching reference voltage, and bottom should reach zero
C. Switch rapidly, vertical sides, top square reaching reference voltage, and bottom should reach zero
D. Switch intermittently, vertical sides, top arched reaching reference voltage, and bottom should reach zero

30.  In a direct ignition coil test, you will measure both primary and secondary winding resistance.

A. True
B. False

31. The halogen lamp typically consists of a bulb filled with what type of vapor?

A. halogen
B. oxygen
C. iodine
D. fluoride

32. How would you identify a type 1 sealed-beam headlight?

A. Both low and high beam and three terminals
B. Both low and high beam and two electrical terminals
C. "H" molded on the lens
D. High beam only and two electrical terminals

33. If one bulb is not working and you determined it is not burned out, what should you check for before replacing the bulb?

A. voltage at socket
B. ignition switch
C. headlight control module
D. amperage

34. Most fuses are designed to maintain what percent of overload current indefinitely?

A. 105
B. 110
C. 115
D. None of the above

35.  Blade fuses come in three sizes, sometimes referred to by their trade names. They are Mini-fuse, auto-fuse, and Maxi-fuse.

A. True
B. False

36. Trucks will have large, bolt-in type fuses that are commonly used to protect main battery power feed cable that supplies what?

A. The trailer electrical system
B. The engine electrical system
C. The headlights and taillights
D. The cab electrical system

37. The special insulation material used in a fusible link has what characteristics?

A. Melts off and is flammable
B. Does not melt off nor support flame
C. Does not melt off but is flammable
D. None of the above

38. A Type I circuit breaker has which characteristic?

A. Resets automatically
B. Resets automatically after cause is relinquished
C. Manually resets
D. Resets automatically with operator intervention

39. The turn signal flasher unit operates by the heating and cooling of which temperature-sensitive component?

A. Bimetallic strip
B. Metallic connector
C. Copper wire
D. Bimetallic filament

40. Which type of flasher typically contains a power metal-oxide-semiconductor field- effect transistor?

A. Hybrid
B. Solid-state
C. Bimetallic
D. Electronic

41. The brake light switch is normally mounted on the brake pedal, but can also be mounted at what other location?

A. Master cylinder
B. Brake line
C. Steering column
D. Both A and B

42. The color of wiring insulation is standardized. What color is the wire for the STOP lamps and antilock devices?

A. Red
B. Green
C. Brown
D. Blue

43. The tractor normally provides the circuit protection for trailer wiring or can have a separate component mounted on the tractor. What is the additional component with fuses and relays for trailers called?

A. Electrical relay system
B. Power delivery box
C. Power distribution module
D. Tractor/trailer power transmitter

44.  The horn system uses a coil-operated diaphragm to produce audible sound.

A. True
B. False

45. The metal core that is attached by magnetic field of coil windings is what part of a horn?

A. Outlet
B. Plunger
C. Contacts
D. Horn coil

46. What should the amp reading generally be when troubleshooting a horn?

A. 2-3
B. 3-4
C. 4-5
D. 5-6

47. After making all the required checks on the windshield washer and determining that the pump is bad, what is the next step ?

A. Rebuild the pump
B. Replace the pump
C. Repair the pump
D. Tap on the pump

48. When troubleshooting an inoperative power window, you can hear a humming noise where the motor is spinning, but there is no movement of the window. What may be the problem?

A. Stripped plastic gears
B. Stripped window cam
C. Bent tracking system
D. Stripped plastic rotor


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