Chevrolet 70 1969 Series, Shop Manual

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TRANSM ISSIO NS AND CLUTCHES 7B-12
caused by bad bearings. As bearings wear and
retainers start to break up, etc., the noise could
change to a thumping or bumping.
3. Metallic Rattles within the transmission
usually result from a variety of conditions. En­
gine torsional vibrations are transmitted to the
transmission 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 should 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 transmission. Rattle
could also be caused by excessive backlash in
power take-off unit mounting.
4. Improper Lubricants, or lack of lubricant
can produce noises. Transmissions with low oil
levels sometimes run hotter than normal, as there
is insufficient lubricant to cool and cover the
gears.
5. Squealing, particularly when the trans­
mission 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 transmission 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 transmission 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.
7. Vibration: Gear seizures on thrust faces
or fluted diameters are usually caused by vibra­
tions in the power train; this could be engine,
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 items would get by
or only pass through critical periods while ac­
celerating 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, clutches out of bal­
ance, gears and shafts in transmission out of
balance, were fairly obvious. These items will
become more critical in vehicles running at sus­
tained 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 syn­
chronizers, bearing failure due to retainer bolt
failures, promote brinelling, fretting corrosion.
8. Gear Whine is usually caused by lack of
backlash between mating gears or improper shim­
ming of power take-off units.
NOISE IN N E U T R A L
1. Misalignment of transmission.
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. Excessive end play in countershaft, or re­
verse 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 G E A R
1. Worn, or rough mainshaft rear bearing.
2 .
Rough, chipped or tapered sliding gear
teeth.
3. Noisy speedometer gears.
4. Excessive end play of mainshaft gears.
5. Refer to conditions listed under "Noise In
Neutral.”
W ALKING 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
mechanism preventing full engagement of the
sliding clutch gear, or -
b. If the gear has been shifted completely into
position some other malfunction which could move
the gear out of its proper location.
2. If remote controls are used, the mechanic
must satisfy himself that the remote units are sat­
isfactory and that transmission is actually at fault.
CHEVROLET SERIES 70-80 H EAVY DUTY TRUCK SH O P M A N U A L