BMW 335xi Transfer Case Output Shaft Oil Seal Replacement Cost

Transfer cases are driven gear units that accept power input from the transmission output shaft, convert that to two separate outputs, and sometimes changes the output to a different speed than the transmission is spinning. Just like the shaft from the transmission, the two shafts that the transfer case uses to send power to the front and rear wheels are called output shafts. These are covered with oil inside the transfer case, but must have a means of keeping that oil from draining out of the output shaft housing. This is accomplished using a round rubberized seal known as the transfer case output shaft seal.

Like other shaft seals, the transfer case output shaft seal is a continuous circle made of a synthetic rubberized material that conforms to the surface of the shaft and allows the shaft to rotate freely inside the seal. The outer casing of the output shaft seal is metallic, and is pressed into the output shaft housing, sealing it externally as well. To keep tension between the rubber, watertight seal and the output shaft, there is a spring inside the seal that constantly applies light pressure inward towards the transfer case output shaft. This ensures that no oil or dust can exit or enter through the seal.

A failing transfer case output shaft seal will first show signs of seepage, that is, the drive shaft and output shaft housing will begin to show signs of contact with oil, though droplets of oil may not be forming underneath the seal. Over time, droplets will begin to form at the base of the transfer case output shaft housing, and possibly trickle down the drive shaft. At this point, there will be clearly visible fluid marking the storage location of the vehicle, and there may be a slightly foul or oily smell around the vehicle after operation. Finally, if a severe leak has occured while driving, the likely symptom will be a check engine light, in some models, but others will only present a clattering noise from the transfer case, or the transfer case may fail.

Since the transfer case is a power distribution unit to all four wheels, failure of the transfer case will render the vehicle useless, so an obviously leaking transfer case output shaft seal should be repaired immediately to protect the expensive unit. However, if only seepage is noticed, or in other words, the area near the seal seems a little glossy, the repair is not extremely urgent. In this case, the transfer case fluid level should be checked, and if the transfer case is full, the manufacturer may even consider slight seepage normal.

According to a comparison of real-world data, transfer case output shaft seals typically fail any time after 85,000 miles, with the majority of them failing long afterwards. Some failures are reported before the 85,000 mile mark, but that is typically not the case. Risk factors that could cause the output shaft seal to fail prematurely are lifted suspensions that alter the driveline angle, a worn drive shaft yoke, worn transfer case output shaft bearing, or lack of maintenance.

Diagnosing a leaking transfer case output shaft seal is very simple and straightforward. If the seal is allowing a droplet to form, it must be changed. Also, if the transfer case output shaft bearing is being replaced, or if the driveshaft is removed for any reason, the seal should typically be replaced. Lastly, if there is play in the driveshaft, the seal will be changed to prevent future leaks due to uneven wear of the rubberized lips on the seal.

Before access to the transfer case output shaft seal is possible, the driveshaft must be removed from the transfer case, and some fluid may have to be drained from the transfer case to prevent spillage. Once the driveshaft is out of the way, a seal puller is used to pull or bend the seal out of the transfer case output shaft housing. The output shaft is cleaned, as is the housing where the new seal will be installed, and depending on the recommendations of the manufacturer, a sealant will be applied between the new output shaft seal and the transfer case housing. When the drive shaft is re-installed, if it directly contacts the seal, the seal will be lubricated and protected while inserting the driveshaft yoke onto the output shaft of the transfer case. The driveshaft and transfer case bushings will be inspected for excessive play before the transfer case is filled and tested.

There are many choices when selecting new seals for the transfer case output shaft, and the type of seal should be selected based on wear patterns on the shaft and newest revisions. If there are wear patterns visible on the driveshaft yoke or output shaft, there are typically aftermarket seals available which allow the seal to ride on a different portion of the shaft, often eliminating the need to replace the shaft.

Transfer case mounts do fail over time, and require replacement to ensure the longevity of driveshaft u-joints, input shaft seals, and output shaft seals. Also, proper maintenance of the transfer cases, i.e. replacing the transfer case fluid or gear oil at the specified intervals, and inspection of the entire driveline assembly periodically can greatly reduce the need for later repairs. Especially for suspension lifted vehicles, the free-play in the drive shaft and output shafts should be checked periodically to prevent damage to the shaft in the event of bearing failure. Waiting to inspect the shaft until there is a bearing failure will typically require the transfer case output shaft to be replaced.

This is a great DIY for the moderately skilled technician. As long as proper safety precautions are taken, such as proper lifting and supporting practices, changing the transfer case output shaft seal should not be a problem at all. However, if the transfer case is making grinding, crunching, knocking, or rattling noises while driving, it is best to have it inspected by a professional technician who can recommend repair.