7.5 Lighting Circuit

The lighting circuit includes the battery, vehicle frame, all the lights, and various switches that control their use. The lighting circuit is known as a single-wire system since it uses the vehicle frame for the return.

The complete lighting circuit of a vehicle can be broken down into individual circuits, each having one or more lights and switches. In each separate circuit, the lights are connected in parallel, and the controlling switch is in series between the group of lights and the battery.

The marker lights, for example, are connected in parallel and are controlled by a single switch. In some installations, one switch controls the connections to the battery, while a selector switch determines which of two circuits is energized. The headlights, with their high and low beams, are an example of this type of circuit.

In some instances, such as the courtesy lights, several switches may be connected in parallel so that any switch may be used to turn on the light.

When a wiring diagram is being studied, all light circuits can be traced from the battery through the ammeter to the switch (or switches) to the individual light.


The headlights are sealed beam lamps that illuminate the road during nighttime operation (Figure 7-32). Headlights consist of a lens, one or two elements, and an integral reflector. When current flows through the element, the element gets white hot and glows. The reflector and lens direct the light forward. Many modern passenger vehicles use a halogen or HID headlights.

Figure 7-32 — Sealed beam headlight assembly.

Headlight Switch

The headlight switch is an on/off switch and rheostat (variable resistor) in the dash panel or on the steering column. The headlight switch controls current flow to the lamps of the headlight system. The rheostat is for adjusting the brightness of the instrument panel lights.


Small gas-filled incandescent lamps with tungsten filaments are used on automotive and construction equipment (Figure 7-33). The filaments supply the light when sufficient current is flowing through them. They are designed to operate on a low voltage current of 12 or 24 volts, depending upon the voltage of the electrical system used.

Figure 7-33 — Different types of lamps.

Lamps are rated as to size by the candlepower (luminous intensity) they produce. They range from small 1/2-candlepower bulbs to large 50-candlepower bulbs. The greater the candlepower of the lamp, the more current it requires when lighted. Lamps are identified by a number on the base.

When you replace a lamp in a vehicle, be sure the new lamp is of the proper rating. The lamps within the vehicle will be of the single- or double-contact types with nibs to fit bayonet sockets (Figure 7-34).

Figure 7-34 — Double-contact bulb and bayonet socket.


Most vehicles made today use a halogen headlamp bulb insert (Figure 8-35, View A). These are small heat-resistant quartz bulbs filled with halogen gas to protect the filament from damage. They are inserted to a headlight lens assembly. This assembly will protect the light bulb and disperse the light given from the halogen bulb.

Figure 8-35 — Halogen and HID headlight



Never touch the glass surface of a halogen or HID light. The oil in your skin and the high operating temperature can shorten the life of the bulb or cause the glass to shatter.

The white halogen bulb increases visibility and increases output by about 25% while drawing the same amount of current. A typical low beam bulb is 45 watts and a high beam bulb is 65 watts.

High Intensity Discharge (HID)

A high intensity discharge lamp does not use a filament (Figure 8-35, View B). Instead, a high voltage electric arc flows between two electrodes in the bulb. This arc excites xenon vapor contained in the bulb, producing a bright blue-white light.

An external ballast is used to convert battery voltage into high-voltage AC to create and maintain the arc. When it is first turned on, an igniter works with the ballast to provide several thousand volts to establish the arc. The ballast then provides as many as 450 volts to maintain the arc. As the bulb warms up, the voltage needed to maintain the lamp can be as low as 50 volts.

HID lights produce more light than a standard halogen bulb while consuming less power, and they last longer.


HID bulbs require a large amount of voltage for startup: beware of a shock hazard. Also, HID bulbs are under pressure when hot and may lead to an explosion hazard.

Light Emitting Diode (LED)

A light emitting diode is a semiconductor that will emit light when electrically energized. The LED converts electricity directly into light; this makes it much more efficient than a normal filament bulb.

The LED is an N-P junction with special doped semiconductors. When energized, photons (electrons) are emitted from the semiconductor substance. We then see these photons as light.

Turn-Signal Systems

Vehicles that operate on any public road must be equipped with turn signals. These signals indicate a left or right turn by providing a flashing light signal at the rear and front of the vehicle.

The turn-signal switch is located on the steering column (Figure 7-40). It is designed to shut off automatically after the turn is completed by the action of the canceling cam.

Figure 7-40 — Turn signal switch

A wiring diagram for a typical turn-signal system is shown in Figure 7-41. A common design for a turn signal system is to use the same rear light for both the stop and turn signals. This somewhat complicates the design of the switch in that the stoplight circuit must pass through the turn-signal switch. When the turn signal switch is turned off, it must pass stoplight current to the rear lights. As a left or right turn signal is selected, the stoplight circuit is open and the turn signal circuit is closed to the respective rear light.

Figure 7-41 — Turn signal wiring diagram.

The turn signal flasher unit creates the flashing of the turn signal lights (Figure 7-42). It consists basically of a bimetallic (two dissimilar metals bonded together) strip wrapped in a wire coil. The bimetallic strip serves as one of the contact points.
Figure 7-42 — Flasher unit.

When the turn signals are actuated, current flows into the flasher—first through the heating coil to the bimetallic strip, then through the contact points, then out of the flasher, where the circuit is completed through the turn-signal light. This sequence of events will repeat a few times a second, causing a steady flashing of the turn signals.

Backup Light System

The backup light system provides visibility to the rear of the vehicle at night and a warning to the pedestrians, whenever the vehicle is shifted into reverse. The backup light system has a fuse, gearshift or transmission-mounted switch, two backup lights, and wiring to connect these components.

The backup light switch closes the light circuit when the transmission is shifted into reverse. The most common backup light switch configurations are as follows:

Stop-Light System

All vehicles that are used on public highways must be equipped with a stoplight system. The stoplight system consists of a fuse, brake light switch, two rear warning lights, and related wiring (Figure 7-43).

Figure 7-43 — Brake light switch.

The brake light switch on most automotive equipment is mounted on the brake pedal. When the brake pedal is pressed, it closes the switch and turns on the rear brake lights. On construction and tactical equipment, you may find a pressure light switch. This type of switch uses either air or hydraulic pressure, depending on the equipment. It is mounted on the master cylinder of the hydraulic brake system or is attached to the brake valve on an air brake system. As the brakes are depressed, either air or hydraulic pressure builds on a diaphragm inside the switch. The diaphragm closes, allowing electrical current to turn on the rear brake lights.

Emergency Light System

The emergency light system, also termed hazard warning system, is designed to signal oncoming traffic that a vehicle has stopped, stalled, or pulled over to the side of the road. The system consists of a switch, flasher unit, four turn-signal lights, and related wiring. The switch is normally a push-pull switch mounted on the steering column.

When the switch is closed, current flows through the emergency flasher. Like a turn signal flasher, the emergency flasher opens and closes the circuit to the lights. This causes all four turn signals to flash.

Circuit Breakers and Fuses (Application or Uses)

Fuses are safety devices placed in electrical circuits to protect wires and electrical units from a heavy flow of current. Each circuit, or at least each individual electrical system, is provided with a fuse that has an ampere rating for the maximum current required to operate the units. The fuse element is made from metal with a low-melting point and forms the weakest point of the electrical circuit. In case of a short circuit or other trouble, the fuse will be burned out first and open the circuit just as a switch would do.

Examination of a burnt-out fuse usually gives an indication of the problem. A discolored sight glass indicates the circuit has a short either in the wiring or in one of its components. If the glass is clear, the problem is an overloaded circuit. Be sure when replacing a fuse that it has a rating equal to the one burned out. Ensure that the trouble of the failure has been found and repaired.

A circuit breaker performs the same function as a fuse. It disconnects the power source from the circuit when current becomes too high. The circuit breaker will remain open until the trouble is corrected. Once the trouble is corrected, a circuit breaker will automatically reset itself when current returns to normal levels. The fuses and circuit breakers can usually be found behind the instrument panel on a fuse block (Figure 7-44).

Figure 7-44 — Fuse block.

Mini Fuses

A mini fuse is a blade type fuse with two prongs that fit into sockets in the fuse block (Figure 7-45, View A). Mini fuses are color coded in accordance to the ampere rating between 1 and 40 amps. They are the smaller type of blade fuse with a dimension of 10.9x3.6x16.3mm.

Figure 7-45— Fuses.

Conventional Fuses

A conventional fuse is a blade type fuse and is a larger version of the mini fuse (Figure 7-45, View B). Conventonal fuses also are color coded in accordance to the ampere rating between 1 and 40 amps. They are the regular type of blade fuse with a dimension of 19.1x5.1x18.5mm.

Maxi Fuses

A maxi fuse is a also blade type fuse with two prongs that fit into sockets; however, they are quite a bit larger and are usually found under the hood (Figure 7-45, View C). The maxi fuse is available in current ratings from 20 to 80 amps. They are color coded in accordance to the ampere rating and their dimensions are 29.2x8.5x34.3mm.

Circuit Breakers

A circuit breaker performs the same function as a fuse. The difference is that the circuit breaker is still usable after it trips. It will sense a high current condition, disconnect the circuit temporarily, and then if the current draw returns to normal, it will reset itself.

A circuit breaker contains a bi-metal strip that remains cool and straight under normal load. Under high current load, the metal strip heats up, bends or warps, and opens the breaker.

Type 1 circuit breakers are cycling circuit breakers (Figure 7-46). This means that after the breaker cools down, the metal strip straightens out again and closes the circuit. These are sometimes seen in headlight, fog light, and windshield wiper circuits.

Figure 7-46 — Cycling circuit breaker.

Type 2 circuit breakers are noncycling breakers. This means that after the breaker heats up, the current flows through an armature on the breaker. The armature heats up and bends away from the contact points. Now the electricity can flow only through a resister, also mounted on the breaker. When a noncycling breaker trips, current can pass only through the resistor, resulting in greatly reduced current and voltage to the circuit.

To reset this circuit breaker, you must open the circuit and allow it to cool off. The cooling effect will allow the metal to straighten and make contact with the points again. Noncycling circuit breakers are used extensively in truck electrical circuits.

Test your Knowledge

11. By what percentage is light output increased when using halogen headlights?

A. 45
B. 35
C. 25
D. 15

12. What component of the headlight switch allows for adjusting the brightness of the instrument panel lights?

A. Brightness compensator
B. Illuminator
C. Dimmer dial
D. Rheostat

13. On most automotive vehicles, the brake light switch is mounted at what location?

A. At the brake light
B. In the brake line
C. In the steering column
D. At the brake pedal