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.
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 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, 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 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 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.Continue Reading