Wiring & Harnesses - Chevrolet Trailblazer Service Manual

Wiring & Harnesses
The average vehicle contains many feet of wiring, with hundreds of individual connections.
To protect the many wires from damage and to keep them from becoming a confusing
tangle, they are organized into bundles, enclosed in plastic or taped together and called
wiring harnesses. Different harnesses serve different parts of the vehicle. Individual wires
are color coded to help trace them through a harness where sections are hidden from
view.
Automotive wiring or circuit conductors can be single strand wire, multi-strand wire or
printed circuitry. Single strand wire has a solid metal core and is usually used inside such
components as alternators, motors, relays and other devices. Multi-strand wire has a core
made of many small strands of wire twisted together into a single conductor. Most of the
wiring in an automotive electrical system is made up of multi-strand wire, either as a single
conductor or grouped together in a harness. All wiring is color coded on the insulator,
either as a solid color or as a colored wire with an identification stripe. A printed circuit is a
thin film of copper or other conductor that is printed on an insulator backing. Occasionally,
a printed circuit is sandwiched between two sheets of plastic for more protection and
flexibility. A complete printed circuit, consisting of conductors, insulating material and
connectors for lamps or other components is called a printed circuit board. Printed circuitry
is used in place of individual wires or harnesses in places where space is limited, such as
behind instrument panels.
Since automotive electrical systems are very sensitive to changes in resistance, the
selection of properly sized wires is critical when systems are repaired. A loose or corroded
connection or a replacement wire that is too small for the circuit will add extra resistance
and an additional voltage drop to the circuit.
The wire gauge number is an expression of the cross-section area of the conductor.
Vehicles from countries that use the metric system will typically describe the wire size as
its cross-sectional area in square millimeters. In this method, the larger the wire, the
greater the number. Another common system for expressing wire size is the American
Wire Gauge (AWG) system. As gauge number increases, area decreases and the wire
becomes smaller. An 18 gauge wire is smaller than a 4 gauge wire. A wire with a higher
gauge number will carry less current than a wire with a lower gauge number. Gauge wire
size refers to the size of the strands of the conductor, not the size of the complete wire
with insulator. It is possible, therefore, to have two wires of the same gauge with different
diameters because one may have thicker insulation than the other.
It is essential to understand how a circuit works before trying to figure out why it doesn't.
Electrical schematic shows the electrical current paths when a circuit is operating properly.
Schematics break the entire electrical system down into individual circuits. In a schematic,
usually no attempt is made to represent wiring and components as they physically appear
on the vehicle; switches and other components are shown as simply as possible. Face
views of harness connectors show the cavity or terminal locations in all multi-pin
connectors to help locate test points.