Wiring and Harnesses

INTRODUCTION

Now that we have discussed current flow, voltage, sources, electrical loads, and series and parallel circuits, in this chapter we start to build some automotive circuits. To build a complete circuit, we must have conductors to carry the current from the voltage source to the electrical loads. The conductors are the thousands of feet of wire and cable used in the complete electrical system. The vehicle chassis is also a conductor for the ground side of the circuits, as we will see later. We will begin our study by looking at the wiring harnesses, connectors, and terminals of the system.

The preceding chapters used symbols to show some of the components in an automotive electrical system. After studying the basic parts of the system (voltage source, conductors, and loads), it is time to put them together into complete circuits.

In real-world cases, diagrams of much greater complexity are used. Technicians must be able toidentify each component by its symbol and determine how current travels from the power source to ground. Technicians use electrical circuit diagrams to locate and identify components on the vehicle and trace the wiring in order to make an accurate diagnosis of any malfunctions in the system.

WIRING AND HARNESSES

An automobile may contain as much as half a mile of wiring, in as many as 50 harnesses, with more than 500 individual connections (Figure 6-1). This wiring must perform under very poor working conditions. Engine heat, vibration, water, road dirt, and oil can damage the wiring and its connections. If the wiring or connections break down, the circuits will fail.

To protect the many wires from damage and to keep them from becoming a confusing tangle, the automotive electrical system is organized into bundles of wire known as wiring harnesses that serve various areas of the automobile. The wires are generally wrapped with tape or plastic covering, or they may be enclosed in insulated tubing. Simple harnesses are designed to connect two components; complex harnesses are collections of simple harnesses bound together (Figure 6-2).

Main wiring harnesses are located behind the instrument panel (Figure 6-3), in the engine compartment (Figure 6-4and Figure 6-5), and along the body floor. Branch harnesses are routed from the main harness to other parts of the system. Items 1, 2, and 3 in Figure 6-4are ground connections. The colored insulation used on individual wires makes it easier to trace them through

Figure 6-1. The wiring harness in this vehicle is typical of those in most late-model cars. (GM Service and Parts Operations)

Figure 6-2. Wiring harnesses range from the simple to the complex. (DaimlerChrysler Corporation)

Figure 6-3. This instrument panel wiring harness has 41 different connectors. (GM Service and Parts Operations)