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 tooth 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 gear.
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 trouble.
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 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 reverse.
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 gear.
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 changes. 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.
Occasionally, a transmission slips out of gear because the driver does not fully engage
the gear when moving the lever. However, when a transmission slips out of gear fairly
often, it should be replaced.
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NAVEDTRA
14050A