Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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90 Chapter Six
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
to identify 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 6-4 and 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 6-4 are 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)