The front output shaft seal is an oil seal that is installed on the front of transfer cases. As its name implies, the front output shaft seal is responsible for sealing the front output shaft of the transfer case, keeping the gear oil or transmission fluid inside of the unit. The seal is usually round in shape, and made of rubber and sometimes metal, not unlike many other automotive engine and transmission seals. Over time the rubber can dry up and the seal can wear out, which can lead to leaks and other issues. Usually a bad or failing front output shaft seal will produce a few symptoms that can alert the driver of a potential issue.
Fluid leaking from the transfer case
The most absolute most common symptom of a problem with a front output shaft seal is fluid leaking from the front of the transfer case. If the rubber transfer case seals dry out or crack they can leak gear oil or transmission fluid. A fluid leak can put the transfer case at risk of suffering internal damage due to low lubrication. Any sort of puddles, or drips of fluid found underneath the vehicle should be addressed as soon as possible to prevent the possibility of damage occurring.
Hum, whine, or growling noise coming from the transfer case
Another symptom of a problem with a front output shaft seal is a noisy transfer case. This problem usually occurs after the seal has been leaking for a while and the fluid has run low. If the fluid runs low the vehicle may produce a hum, whine, or growling sound from the transfer case, that may be especially pronounced when the four wheel drive or all wheel drive modes are engaged. A noisy transfer case can also be caused by a number of other issues, so a proper diagnosis is highly recommended.
Like most rubber automotive seals, the front output shaft seals will eventually dry or wear out as time goes on. If you find that your front transfer case seal is leaking, or perhaps having another issue, have the vehicle inspected by a professional technician, such as one from YourMechanic, to determine if the seal should be replaced.