GMC 4000 Series Service Manual page 453

Sec. 7B
Page 442
TRANSMISSION ON-VEHICLE SERVICE
1. Growl and Humming, or more serious, a
grinding noise. These noises are caused by worn,
chipped, rough, or cracked gears. As gears con­
tinue to wear, the grinding noise will be notice­
able, particularly in the gear position that throws
the greatest load on the worn gear.
2. Hissing, or more serious, a thumping or
bumping-type noise. Hissing noises
caused by bad bearings.
retainers start to break up, etc., the noise could
change to a thumping or bumping.
3. Metallic Rattles within the transm ission
usually result from a variety of conditions. En­
gine torsional vibrations are transmitted to the
transm ission through the clutch. A characteristic
of the two-plate clutch is a rattling noise, due to
oscillation of the intermediate plate drive lugs
within the flywheel openings when the clutch is
released. In general, engine speeds could be 600
rpm or above, to eliminate objectionable rattles
and vibration during the idle. A defective or faulty
injector would cause a rough or lower idle speed
and a rattle in the transm ission. Rattle could also
be caused by excessive backlash in power take-off
unit mounting.
4. Improper Lubricants, or lack of lubricant
can produce noises. Transm issions with low oil
levels som etim es run hotter than normal, as
there is insufficient lubricant to cool and cover
the gears.
5. Squealing, particularly when the trans­
m ission is operating at higher speeds, could be
caused by one of the free-running gears seizing
on the thrust face or fluted diameter temporarily
and then letting go. In general, a mild seizure
will clear itself up and the transm ission will
continue to operate very satisfactorily without
this defect being known. Refer to Step 7 following:
6. Gear Seizure at high speed, usually ac­
companied with loud squealing noise. This type
of seizure is readily apparent to the driver since
the truck will suddenly slow down as if the brakes
were being applied. If the truck continues to move
ahead, even though the gearshift lever is placed
in neutral, it would indicate the floating gear on
the mainshaft had seized. Depressing the clutch
should interrupt the driving torque. The seized
gear could be checked quite readily by depressing
clutch and checking the action with the gearshift
lever progressively in all shift positions. If re­
leasing the clutch tends to kill the engine, then
this gear position has not seized. In other words,
the transm ission would be in two gears at the
same time. By a process of elimination, the gear
at fault can be readily identified. Refer to Step 7
following:
7. Vibration: Gear seizures on thrust faces
or fluted diameters are usually caused by vibra­
tions in the power train; this could be engine,
G M C S E R V I C E M A N U A L
could be
As bearings wear and
propeller shafts, joint angle, rear axle, differen­
tials, etc.
a. Improved highways permit sustained high
speeds. The fact that engines and entire power
trains can now cruise at a high rpm can introduce
vibration frequencies, that were not critical in
the past. At slower speeds these item s would
get by or only pass through critical periods while
accelerating or decelerating through the gears.
b. In the past, drive line vibrations such as
bent tubes, joints out of phase or alignment, bad
angles due to short couples,
balance, gears and shafts in transm ission out of
balance, were fairly obvious. These item s will
become more critical in vehicles
sustained high speeds.
c. Critical vibrations associated with higher
speeds are not the old thumping or bumping type,
but are high frequency vibrations. This type of
vibration will cause gear seizures, damaged
synchronizers, bearing failure due to retainer
bolt failures, promote brinelling, fretting cor­
rosion, etc.
8. Gear Whine is usually caused by lack of
backlash between mating gears or improper shim­
ming of power take-off units.
Noise In Neutral
1. Misalignment of transm ission.
2. Worn or scored main drive gear and/or
countershaft bearings.
3. Scuffed gear tooth contact surfaces on
gears.
4. Unmatched constant mesh gears.
5. Worn, rough reverse idler gear.
6. Eccentric countershaft gear assembly.
7. Sprung or worn countershaft.
8. Excessive backlash in constant mesh gear.
9. E xcessive end play in countershaft, or
reverse idler pinion.
10. Worn mainshaft pilot bearing.
11. Scuffed gear tooth contact surface insuf­
ficient lubrication.
12. Incorrect grade of lubricant.
13. Incorrect clutch linkage adjustment.
Noise In Gear
1. Worn, or rough mainshaft rear bearing.
2. Rough, chipped or tapered sliding gear
teeth.
3. Noisy speedometer gears.
4. E xcessive end play of mainshaft gears.
5. Refer to conditions listed under "Noise
in Neutral.''
WALKING OR SLIPPING OUT OF GEAR
1. If the units are walking out of gear it
could be caused by:
a. Interference or resistance in the shift
clutches out of
running at