Manual transmissions usually provide reliable service while requiring only basic maintenance. A gear box simply needs the proper type and amount of gear oil to be kept in good repair. Parts do wear over time, however, and the input shaft is in constant motion any time the engine is running. The input shaft bearing is prone to suffer oil starvation at slightly deficient oil levels, while other components may not be affected as seriously.
A worn or defective input shaft bearing can cause noisy operation in neutral with the engine running. The pitch or tone of the noise changes with engine speed and a slight vibration might be felt through the shifter. This symptom can also be caused by deficient gear oil levels or quality. Check and replenish the gear oil to eliminate this possibility as the origin of the noise, before condemning the input shaft bearing.
Excessive and constant noise in all gears can also be caused by an input shaft bearing that is defective. The tone of the noise may change with engine speeds or torque demands. This symptom can also be produced by a defective output shaft bearing, except that the noise stops when the vehicle stops. The input shaft bearing noise continues as long as the engine runs, and the difference helps determine which bearing is at fault.
A severely worn input shaft bearing can cause the transmission to slip out of gear while moving. This symptom usually occurs just as the clutch is applied and is sometimes accompanied by a popping noise or sensation. The excess movement of the input shaft affects the alignment of the gear shafts, preventing complete engagement. The shifter seems to move by itself as forces from the misaligned transmission components act upon it.
Many causes can duplicate the symptoms of a bad input shaft bearing. Fresh gear oil might eliminate some symptoms, and shifter linkage adjustments may reduce or remedy others. Drive line noise can travel through the many components involved and frustrate attempts to pinpoint the problem. Intricate diagnosis techniques may be required to ascertain exact faults. Should doubts persist, consult a professional in order to prevent ineffective and costly repair attempts.