ker88839_ch06.qxd
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Electrical Diagrams and Wiring
number of wiring circuits must pass through a barrier such as the firewall (Figure 6-17). The bulkhead connector is installed in the firewall and multiple connectors are plugged into each side of it to connect wires from the engine and front accessories to wires in the rest of the car.
Weatherproof Connectors Special weatherproof connectors are used in the engine compartment and body harnesses of latemodel GM cars. This type of connector has a rubber seal on the wire ends of the terminals, with secondary sealing covers on the rear of each connector half. Such connectors are particularly useful in electronic systems where moisture or corrosion in the connector can cause a voltage drop. Some Japanese carmakers use a similar design (Figure 6-18).
GROUND PATHS We have spoken as if wiring carried all of the current in an automotive electrical system. In fact, wiring is only about half of each circuit. The other half is the automobile engine, frame, and body, which provide a path for current flow. This side of the circuit is called the ground (Figure 6-19). Automotive electrical systems are called singlewire or ground-return systems. The cable from one battery post or terminal is bolted to the car engine or frame. This is called the ground cable. The cable from the other battery terminal provides current for all the car’s electrical loads. This is called the insulated, or
Figure 6-18. Nissan uses this type of waterproof connector. (Courtesy of Nissan North America, Inc.)
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hot, cable. The insulated side of every circuit in the vehicle is the wiring running from the battery to the devices in the circuit. The ground side of every circuit is the vehicle chassis (Figure 6-19). The hot battery cable is always the insulated type of cable described earlier. The ground cable may be an insulated type of cable, or it may be a braided strap. On many vehicles additional grounding straps or cables are connected between the engine block and the vehicle body or frame. The battery ground cable may be connected to either the engine or the chassis, and the additional ground cable ensures a good, low-resistance ground path between the engine and the chassis. This is necessary for proper operation of the circuits on the engine and elsewhere in the vehicle. Late-model vehicles, which rely heavily on computerized components, often use additional ground straps whose sole purpose is to minimize or eliminate electromagnetic interference (EMI), as shown in Chapter 4. The resistances in the insulated sides of all the circuits in the vehicle will vary depending on the number and kinds of loads and the length of the wiring. The resistance on the ground side of all circuits, that is, between each load and its ground connection, must be virtually zero. For more information about ground paths, see the “Copper Wiring Repair” section in Chapter 6 of the Shop Manual. Early Wiring Problems Early automobiles had many problems with their electrical systems, usually the result of poor electrical insulation. For example, high-tension cable
Figure 6-19. Half of the automotive electrical system is the ground path through the vehicle chassis.
