Noisy Generator; Electrical Tests - GMC G Service Manual

6D-10 ENGINE ELECTRICAL
DIAGNOSIS OF CHARGING SYSTEM

NOISY GENERATOR

Noise from a generator may be caused by a loose drive
pulley, loose mounting bolts, worn or dirty bearings, worn
diode or stator. If the pulley and mounting bolts are snug
and the noise continues, remove the generator for inspec­
tion and repair. Refer to "Generator Replacement" in this
section.

ELECTRICAL TESTS

Before performing the diagnosis procedures on the vehi­
cle, be certain that the system wiring is good and the gener­
ator belt is not slipping. Also, the battery must be fully
charged for a valid test of the charging system.
NOTICE: To avoid damage to the vehicle electri­
cal system, observe the following precautions:
Do not polarize the generator.
•
• Do not short across or ground any of the termi­
nals in the charging circuit except as specifically
instructed herein.
• NEVER operate the generator with the output
terminal open-circuited.
• Make sure the generator and battery have the
same ground polarity.
• When connecting a charger or booster battery
to the vehicle battery, connect negative to nega­
tive and positive to positive.
Trouble in the charging system will show up as
one or more of the following conditions:
1. On vehicles without gages, unusual operation of the
charging indicator lamp.
2. An undercharged battery, indicated by slow cranking
or a dark battery hydrometer.
3. An overcharged battery, indicated by spewing of elec­
trolyte from the vents.
SI CHARGING SYSTEM
CHARGING SYSTEM INDICATOR LAMP OPERATION
Check the indicator lamp for normal operation in the
"Charging System Diagnosis," (figure 13). If lamp operation
is normal, proceed to "Undercharged Battery."
UNDERCHARGED BATTERY
Slow cranking or a dark battery hydrometer can be
caused by one or more of the following conditions even
though the indicator lamp may be operating normally. The
following procedures also apply to vehicles equipped with a
voltmeter:
1. Check that the undercharged condition has not been
caused by accessories having been left on for
extended periods.
2. Check the drive belt for proper tension. Refer to
EN G IN E C O O LIN G (S E C . 6B1) fo r b e lt
specifications.
3. If you suspect the battery is bad, perform a load test.
Refer to "Battery" earlier in this section.
4. Inspect the wiring for cracks or breaks. Check all cir­
cuit connections, cable clamps and battery terminals
for tightness and cleanliness.
5. With the engine control switch "ON" and all wiring
harness leads connected, use a voltmeter for the fol­
lowing checks:
H Install or Connect (Figures 7 and 8)
1. Voltmeter from the generator battery terminal to
ground.
• Should read 12 volts.
2. Voltmeter from the number 1 terminal to
ground.
• Should read one volt or more.
3. Voltmeter from the number 2 terminal to
ground.
• Should read 12 volts.
A zero reading on any of the above indicates an open
between the voltmeter connection and the battery. If checks
1 through 5 are normal, perform the generator output test.
OVERCHARGED BATTERY
If the battery feels hot, is spewing electrolyte, or lamps
seem too bright when turned on, an overcharged condition
may exist.
To check the voltage, install a voltmeter across the bat­
tery. Run the engine at moderate speed with all accesso­
ries off. If the voltage reads 15.5 or more, remove the
generator for repair.