7.7 Automotive Wiring

Electrical power and control signals must be delivered to electrical devices reliably and safely so that the electrical system functions are not impaired or converted to hazards. To fulfill power distribution, military vehicles use one- and two-wire circuits, wiring harnesses, and terminal connections.

Among your many duties will be the job of maintaining and repairing automotive electrical systems. All vehicles are not wired in exactly the same manner; however, once you understand the circuit of one vehicle, you should be able to trace an electrical circuit of any vehicle using wiring diagrams and color codes.

One-Wire Circuit

Tracing wiring circuits, particularly those connecting lights or warning and signal devices, is no simple task. Branch circuits making up the individual systems have one wire to conduct electricity from the battery to the unit requiring it, and ground connections at the battery and the unit to complete the circuit. These are called one- wire circuits or branches of a ground return system. In automotive electrical systems with branch circuits that lead to all parts of the equipment, the ground return system saves installation time and eliminates the need for an additional wiring to complete the circuit. The all-metal construction of the automotive equipment makes it possible to use this system.

Two-Wire Circuits

The two-wire circuit requires two wires to complete the electrical circuit—one wire from the source of electrical energy to the unit it will operate, and another wire to complete the circuit from the unit back to the source of the electrical power.

Two-wire circuits provide positive connection for light and electrical brakes on some trailers. The coupling between the trailer and the equipment, although made of metal and a conductor of electricity, has to be jointed to move freely. The rather loose joint or coupling does not provide the positive and continuous connection required to use a ground return system between two vehicles. The two-wire circuit is commonly used on equipment subject to frequent or heavy vibrations. Tracked equipment, off-road vehicles (tactical), and many types of construction equipment are wired in this manner.

Shielded Wiring

Shielded wire has a center conductor that is surrounded by an outer metal shield. Insulation is used to separate the shield and the conductor. This construction keeps magnetic pulses from being inducted into the center conductor causing unwanted voltage pulses.

This type of wire is mostly used for the automotive antenna. The lead must be protected from the magnetic fields from the engine’s ignition system to prevent static from being heard over the radio.

There is also twisted shield wire. This type of wire uses multiple insulated conductors wrapped around each other. This design still provides the protection from the magnetic fields and is used to connect the computer to various sensors, particularly those near the ignition system. Twisted shield wire helps keep high voltage pulses from interfering with the tiny voltage signals going between the computer and other sensors in the vehicle.

Unshielded Wiring

Unshielded wire is the most common type of wire found in automotive manufacturing. There is no shield on the wire except for the insulation wrapped around the wire to prevent accidental grounding. There is no special shield to protect the wire from electromagnetic force.

Wiring Assemblies

Wiring assemblies consist of wires and cables of definitely prescribed length, assembled together to form a subassembly that interconnect specific electrical components and/or equipment. The two basic types of wiring assemblies are as follows:

The cable assembly consists of a stranded conductor with insulation or a combination of insulated conductors enclosed in a covering or jacket from end to end. Terminating connections seal around the outer jacket so that the inner conductors are isolated completely from the environment. Cable assemblies may have two or more ends.

Figure 7-57— Wiring harness.

Wiring harness assemblies serve two purposes (Figure 7-57). They prevent chafing and loosening of terminals and connections caused by vibration and road shock while keeping the wires in a neat condition away from moving parts of the vehicle. Wiring harnesses contain two or more individual conductors laid parallel or twisted together and wrapped with binding material, such as tape, lacing cord, and wire ties. The binding materials do not isolate the conductors from the environment completely, and conductor terminations may or may not be sealed.

Wiring harnesses also may have two or more ends.

Wiring Identification

Wires in the electrical system should be identified by a number, color, or code to facilitate tracing circuits during assembly, troubleshooting, or rewiring operations. This identification should appear on wiring schematics and diagrams and whenever practical on the individual wire. The assigned identification for a continuous electrical connection should be retained on a schematic diagram until the circuit characteristic is altered by a switching point or active component.

Wiring color codes are used by manufacturers to assist the mechanics in identifying the wires used in many circuits and making repairs in a minimum of time. No color code is common to all manufacturers. For this reason, the manufacturer’s service manual is a must for speedy troubleshooting and repairs.

Wiring found on tactical equipment (M-series) has no color. All the wires used on these vehicles are black. Small metal tags stamped with numbers or codes are used to identify the wiring illustrated by diagrams in the technical manuals (Figure 7-58). These tags are securely fastened near the end of individual wires.

Figure 7-58 — Metal tag wire identification.

Wiring Diagrams

Wiring diagrams are drawings that show the relationship of the electrical components and wires in a circuit (Figure 7-59). They seldom show the routing of the wires within the electrical system of the vehicle.

Figure 7-59 — Wiring diagram.

Often you will find electrical symbols used in wiring diagrams to simulate individual components. Figure 7-60 shows some of the symbols you may encounter when tracing individual circuits in a wiring diagram.

Figure 7-60 — Wiring diagram symbols.

Wire Terminal Ends

Wire terminals are divided into two major classes—the solder type and the solder-less type, which is also known as the pressure or crimp type. The solder type has a cup in which the wire is held by solder permanently. The solder-less type is connected to the wire by special tools that deform the barrel of the terminal and exert pressure on the wire to form a strong mechanical bond and electrical connection. Solder-less type terminals are gradually replacing solder type terminals in military equipment.

Wire Support and Protection

Wire in the electrical system should be supported by clamps or fastened by wire ties at various points about the vehicle. When installing new wiring, be sure to keep it away from any heat-producing component that would scorch or bum the insulation.

Wire passing through holes in the metal members of the frame or body should be protected by rubber grommets. If rubber grommets are not available, use a piece of rubber hose the size of the hole to protect the wiring from chafing or cutting on sharp edges.

Test your Knowledge

17. What type of wire circuit is commonly used on equipment that is subject to heavy vibrations?

A. One-wire
B. Shielded wire
C. Two-wire
D. Unshielded wire

18. How many different types of wiring assemblies are there?

A. 4
B. 3
C. 2
D. 1


In this chapter we discussed the different automotive electrical systems, their functions, and associated troubleshooting methods. Because there are so many different components and designs, always check with the manufacturer’s specifications when working on an unfamiliar circuit. Almost everything you work on will have an electrical circuit of some sort, and you need to be familiar with how the components operate. To be a good construction mechanic you will need to study these systems and stay up to date with current systems to keep them operating in peak condition.