Diagnostic Tests - Hummer H1 Service Manual

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DIAGNOSTIC TOOLS
Digital Multimeter
A digital multimeter (J–39200) is required to safely test for
electrical malfunctions on the Hummer. Due to the complexity
of the electrical system, a test light should not be used to test
electrical circuits.
• Test lights do not have current limiting capabilities. The
use of a test light may actually cause an electronic com-
ponent to fail.
• Sharp test light probes may break wire strands, causing
circuit failure.
• Breaks in the insulation allow moisture and contami-
nants to enter connectors and components, increasing
the chances for corrosion. Even a small increase in resis-
tance can give false readings from a sensor to an elec-
tronic component.
A digital multimeter performs all the tests a test light can per-
form with a greater degree of accuracy. In addition, a multime-
ter can be used to test for current in a circuit.

DIAGNOSTIC TESTS

NOTE: Follow all manufacturer's recommendations when
testing for current. All multi-meters have a maximum current
rating. Not all multimeters contain a fuse that protects the mul-
timeter from excess current draw.
Amperage Test
Use caution when testing for current. Always check multimeter
owner's manual for maximum current to be tested. Most multi-
meters are fuse-protected when measuring current. However,
some meters are not protected and therefore can be damaged
by excessive current. Position multimeter leads after closed
switch and before load (Figure 12-4).
Voltage Test
Multimeters have a number of different voltage scales to
choose from. Always use the lowest scale possible to test the
05745159
Figure 12-3: Grounded Circuit
circuit. For example, if you select the 200 volt scale and you
are testing for battery voltage, most multimeters will display
12 volts. By selecting the next smallest scale, 20 volts, the dis-
play will read 12.8 volts, a more accurate measurement. Posi-
tion multimeter leads on each side of the load (Figure 12-5).
OHM Test
CAUTION: Before using a multimeter, ensure the circuit is not
energized. Even a small voltage applied to an multimeter will
damage it.
Continuity - Testing for continuity in a circuit requires the use
of the lowest ohm scale available. Position the multimeter
leads on each side of the circuit or component being tested. A
reading of less than one ohm is acceptable continuity
(Figure 12-6).
Resistance - To test for resistance, first touch the meter leads
together to ensure that the meter zeros out, then position the
leads of the multimeter on each side of the circuit or compo-
nent. Adjust the multimeter ohm setting until an acceptable
reading is observed. Verify the reading with the specification.
Electrical System
S12-163
12-5
4-1-00