Circuit Diagnosis; Diagnostic Tools - GMC G Service Manual

8A-4 CAB ELECTRICAL
GROUNDED CIRCUIT (Figure 5)
A grounded circuit is like a short circuit but the current
flows directly into a ground circuit that is not part of the
original circuit. This may be caused by a wire rubbing
against the frame or body. Sometimes a wire will break and
fall against metal that is connected electrically to the ground
side of the power supply. A ground circuit may also be
caused by deposits of oil, dirt and moisture around connec­
tions or terminals, which provide a good path to ground.

CIRCUIT DIAGNOSIS

A clear understanding of the circuit and a wiring diagram
are needed for effective diagnosis. Use a logical sequence
of testing to find the trouble. Use the diagnostic tools. After
the trouble is fixed, make sure the circuit works correctly.

DIAGNOSTIC TOOLS

UNPOWERED TEST LAMP
This tool consists of a 12 volt lamp with leads. The ends
of the leads usually have alligator clamps, but various kinds
of probes, terminal spades, and special connectors are
used also.
The unpowered test lamp is used on an open circuit.
One lead of the test lamp is grounded and the other lead is
moved around the circuit to find the open. Depending on
the physical layout of the circuit, sometimes it will be easier
to start at the power supply and other times it is easier to
start at the circuit load or ground circuit.
POWER TEST LAMP
This lamp is a pencil shaped unit with a self contained
battery, a 1.5 volt lamp bulb, a sharp probe and a ground
lead fitted with an alligator clip.
This test lamp is used mainly for testing components that
are disconnected from the vehicle power supply. The power
test lamp is also useful for testing suspected high resis­
tance points in a circuit such as connectors and ground
circuits that are corroded or loose.
JUMPER
The jumper is usually a long wire with alligator clamps. A
version of the jumper has a fuse holder in it with a 10 Amp
fuse. This will prevent damaging the circuit if the jumper is
connected in the wrong way.
The jumper is used to locate opens in a circuit. One end
of the jumper is attached to a power source and then the
other end is attached to the load in the circuit (i.e.; lamp,
motor). If the load works, try "jumping" to circuit points that
are progressively closer to the power supply. When the
circuit load stops working, the open has been located.
The jumper is also used to test components in the circuit
such as connectors, switches, and suspected high resis­
tance points.
NOTICE: The following instruments: Ammeter,
Voltmeter, and Ohmmeter, each have a particu­
lar application for troubleshooting electrical
circuits.
When using an ammeter or voltmeter, and the
value being tested is unknown, always use the
highest scale first and work downward to a
midscale reading whenever possible. This will
avoid damage to the instrument.
Never use an ohmmeter in a power circuit, or as
a substitute for a voltmeter or ammeter, as dam­
age to the instrument will result.
AMMETER (Figure 6)
Disconnect the circuit from the power source before con­
necting the ammeter. The ammeter measures the amount
of electrical current, amperes, moving through a conductor.
The ammeter must be placed in series with the circuit being
tested. Be sure that the ammeter's positive terminal is con­
nected to the positive (battery) side of the circuit and the
ammeter's negative terminal is connected to the negative
(ground) side of the circuit.