gears clash or the vehicle attempts to move with the
clutch disengaged, the trouble is in the clutch and not
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.
Loose, bent, or shifted suspension system
components will cause misalignment of the power train
components that can produce a noise that may sound like
a defective transmission.
Noises that may originate in the transmission are
difficult to describe. A noise that may sound like a howl
to you may sound like a squeal to someone else. Other
terms often used to describe gear or bearing noises may
include such words as hum, knock, grind,
whine, and thump.
If a teeth is broken off of one of the gears, a distinct
thumping noise will be heard once during a complete
revolution of the gear. The thump will be more
pronounced if torque is being delivered through that
Gears with worn, rough teeth will usually produce
a grinding noise, especially when torque is being
transmitted through them.
Bearing noise is usually described as a howl, whine,
or squeal. Actually, the type of noise made by a defective
bearing will vary, depending on the type of defect and
the load the bearing is supporting. In any event, loud
noises coming from inside the transmission mean
Some whining or grinding noise can be expected,
especially when the vehicle is being driven in first or
reverse gear. The first-and-reverse sliding gear together
with its mating countershaft gear and reverse idler gear
are spur gears, Spur gears are always noisy, but, as you
recall from a preceding lesson, they are frequently used
because they are cheaper and do not produce thrust.
In the second-, third-, and fourth-speed ranges, the
transmission should be much quieter than in first or
If, after a road test, you think the transmission is too
noisy, be sure and report it to the maintenance
supervisor. Be sure to describe the conditions under
which the noise occurs.
Another common mechanical problem with
transmissions of this type is slipping or jumping out of
gear. Actually, the transmission is much less likely to
slip or jump out of first or reverse than out of second-,
third-, or fourth-speed gear. Second-, third-, and
fourth-speed gears are all helical gears which, you
recall, produce thrust.
The most likely causes of the transmission slipping
out of gear are worn detent balls or springs in the shifter
shaft cover. These spring-loaded balls hold the shifter
shaft in position. If the spring does not have enough
tension or if the balls are worn, the transmission will
almost certainly slip or jump out of gear. Synchronizer
damage will also cause the transmission to jump out of
Slipping out of any gear is most likely to occur when
the driver suddenly takes his or her foot off the
accelerator pedal, especially when descending a steep
hill. The thrust produced by the helical gears will tend
to move all rotating gears and shafts to the rear of the
transmission, as long as the torque provided by the
engine is being delivered to the rear wheels by the
transmission. However, when the driver takes his or her
foot off of the accelerator pedal, the situation is changed.
The rear wheels now try to drive the engine through the
transmission. This reverses the direction of the torque
being delivered through the transmission gears, and the
thrust is now toward the front of the transmission. If this
thrust is not controlled by the thrust washers and bearing
retainers, it is likely to force the shifter shaft to move in
spite of the spring-loaded ball that holds it. When this
happens, the transmission slips out of gear.