* Worn output shaft: This is the most frequent cause. If the output shaft is worn or scored, it won't provide a proper sealing surface for the rear seal. The seal lip will wear out prematurely trying to compensate.
* Incorrect installation of the seal: Improper installation, including damage to the seal during installation, can lead to premature failure. This is less likely if it's happening repeatedly.
* Bent or damaged output shaft flange: A bent or damaged flange can misalign the seal and cause it to leak. This often happens due to impacts or improper installation/removal of the driveshaft.
* Excessive transmission fluid pressure: Higher-than-normal pressure can force fluid past the seal. This might be due to internal transmission problems like a failing pump or a blockage in the fluid passages.
* Incorrect fluid level: Either too much or too little transmission fluid can put undue stress on the seal.
* Transmission case damage: A cracked or damaged transmission case around the seal's mounting area will prevent a proper seal.
* Contaminated transmission fluid: Debris in the fluid can abrade the seal and the output shaft.
* Improper driveshaft installation: If the driveshaft is not properly installed or balanced, vibrations can damage the seal.
* Torque converter issues: A problem with the torque converter could lead to excessive pressure or vibrations affecting the seal.
In short: Replacing the rear seal repeatedly without addressing the root cause is a waste of time and money. A proper diagnosis is crucial. A mechanic needs to inspect the output shaft, transmission case, and the whole transmission for damage or wear before simply replacing the seal again. They may need to perform a pressure test to rule out internal transmission issues.
