GMC 4000 Series Service Manual page 425

Sec. 6Y
Page 414
A/C GENERATING SYSTEM
O N VEHICLE MAINTENANCE, TESTS, A N D ADJUSTMENTS
GENERATING SYSTEM M AINTENANCE
At regular intervals, inspect generating sy s­
tem to locate and correct potential causes of trouble
before generating system performance is affected.
1. Check generator drive belt tension and ad­
just if n ecessary. See procedure later under "Gen­
erator Drive Belt Tension Adjustment."
2. Check generator pulley nut, mounting and
adjusting arm bolts and tighten as necessary.
3. Check all electrical connections for tight­
ness and corrosion. Clean and tighten connections
as necessary. Be sure wiring insulation is in good
condition, and that all wiring is securely clipped
to prevent chafing the insulation.
4. With engine running, listen for noise and
check generator for vibration. If generator is noisy
or vibrates excessively, it should be removed for
inspection and repair.
5. Check battery electrolyte level and specific
gravity. Replenish electrolyte level, as necessary.
GENERATOR DRIVE BELT
TENSION ADJUSTMENT
Because of the higher inertia and load capacity
of rotor used with A.C. generators, PROPER BELT
TENSION is more critical than onD.C. generators.
All generators are pivot-base mounted with
the belt tension adjustment arm at the top. Use a
belt tension dial gauge to check tension on each
individual belt. If tension is not within 80-90 lbs.
(used belts) or 100-110 lbs. (new belts), loosen the
adjustment arm clamp bolt and move generator to
obtain recommended tension.
IMPORTANT: When adjusting belt tension,
apply pressure at center of generator, never
against either end frame.
NOTE: On a new vehicle, or after having in­
stalled new belts, check tension of belt(s) twice in
first 200 m iles of operation. When making adjust­
ment, examine belt(s) and replace if necessary.
A loose or broken drive belt will affect oper­
ation of generator. A drive belt that is too tight
will place too much strain on bearings.
IMPORTANT: When replacing dual drive belts,
it is essential that entire set be replaced at same
time. Belts are available in matched sets only.
GENERATING SYSTEM
TROUBLE SYM PTO M S
Abnormal operation of the generating system
is usually indicated by one or more of the follow­
ing symptoms:
G M C S E R V I C E M A N U A L
of electrolyte).
indicating an extremely high charging rate.
when ignition switch is on (engine not running).
running.
or control switch is turned off.
fully charged battery.
partially discharged battery.
generator defects encountered:
be checked as explained under "Generator Output
Test" later in this section. If a defect is indicated
by this test, remove generator and repair.
be checked as outlined under "Charging System
Trouble Analysis Chart" later in this section. If
this check indicates a defect in the field winding,
remove generator and repair.
sult of one or more of the following:
one of the brushes or rough or dirty slip rings. To
check for brush "Squeal," remove generator drive
belt and spin generator drive pulley by hand. Lift
brushes off slip rings and spin drive pulley again.
If noise disappears, clean and inspect slip rings
and replace brushes if worn.
electrical unbalance.
form "Generator Output Test" explained later in
this section. If a defect is indicated by this test,
remove generator and repair.
1. Battery undercharged (low specific gravity
2. Battery using an excessive amount of water,
3. Excessive generator noise or vibration.
4. Failure of indicator lamp to illuminate
5. Indicator lamp continues to glow with engine
6. Indicator lamp fails to go out when ignition
7. Ammeter shows high charging rate with a
8. Ammeter shows low or no charge with a
GENERATOR ON-VEHICLE TEST
The following is a list of the most common
1. Open or shorted generator diodes.
2. Open, shorted, or grounded stator winding.
3. Open, shorted, or grounded field winding.
4. Worn generator brushes.
5. Excessive generator noise.
Generator diodes and stator windings should
Generator field windings and brushes should
E xcessive generator noise is usually the re­
1. Brush "Squeal" caused by a hard spot on
2. Dry or rough bearings in end frame.
CAUTION: Dry or rough bearings may
be the result of over-tightening generator
drive belt(s), loose generator mountings,
or an unbalanced generator fan or pulley.
3. A defective diode or stator resulting in an
To check for a defective diode or stator, per­