temperatures, it expands to occupy a greater volume. If the breather is plugged, this can
cause seal failure.
1.1.3 Leaking Seals
With the power plant in the vehicle, you can inspect all seals except the input shaft
retainer seal. If this seal is leaking, oil will drip out through the plughole in the bottom of
the pan under the flywheel housing when the plug is removed.
If oil does drip out at the flywheel housing drain plug, examine the oil closely. It may be
engine oil leaking from the engine crankshaft rear oil seal. The engine oil is much
thinner (has less viscosity) than the transmission oil, so you should be able to tell which
seal is leaking.
An oil leak, either from the engine or transmission input shaft seals, is serious, because
the oil can ruin the clutch. An oil-soaked clutch disk will almost always slip or grab.
1.2.0 Testing Transmission for Malfunctions
In addition to the leakage problems, there are other problems that can develop in the
standard transmissions used in almost all trucks. We can classify these as mechanical
problems.
The best way to locate mechanical problems in the transmission is to road test the
vehicle. Before road testing, however, check for missing or loose bolts and be sure the
oil is at the proper level in the transmission case. Check the parking brake mechanism
for proper mounting and correct adjustment. Check all moisture seals or boots. Check
the action of the gearshift levers.
The transmission is often blamed for problems that are elsewhere. For example, with
the engine running and the vehicle standing still, disengage the clutch and move the
gearshift lever into first or reverse. You should be able to shift into either of these gear
positions without any gear clashing or without the vehicle moving. If the gears clash or
the vehicle attempts to move with the clutch disengaged, the trouble is in the clutch and
not the transmission.
Check the clutch pedal free travel and adjust it if necessary. The clutch must be
correctly adjusted before the transmission can operate properly. The clutch must fully
disengage every time the clutch pedal is pushed all the way down, and it must fully
engage every time the pedal is released.
With the transmission in neutral, the engine running, and the clutch engaged, all of the
constant-mesh gears in the transmission will be turning. There should be very little gear
or bearing noise.
If the transmission is quiet in neutral with the clutch engaged, disengage the clutch. If a
noise is now heard, the trouble is with the clutch and not the transmission. Usually, the
clutch release bearing or the clutch shaft pilot bearing is at fault if a noise is heard only
when the clutch is disengaged.
Sometimes, noises in other parts of the power train, such as U-points, propeller shafts,
and differential, sound as if they are in the transmission. The misalignment of power
train components usually produces a noise that may sound as if it is coming from the
transmission. So be sure to check all mounting bolts on the engine, transmission, and
differentials before road testing the vehicle. Also, check the propeller shafts and U-joints
for evidence of wear or looseness.
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