Transfer cases are used in part-time and full-time four-wheel drive vehicles to transfer power from the transmission to the front and rear axles.

At CRP Automotive, we’ve heard from our Innovation Council members that they are seeing a higher number of transfer case issues in all-wheel drive cars like the BMW xDrive models.

One challenge with the transfer case we’re hearing about is that car owners are replacing only one set of tires at a time. This causes a disparity in RPMs between the front and rear drive shafts due to a difference in tire radius.

Since one of the primary jobs of a transfer case is to balance the RPMs between the front and rear drive shafts, the transfer case compensates for this by working overtime and essentially cooking the transfer case fluid — remember, these transfer cases only hold about one liter of fluid. With no gear oil to keep things lubricated, the mechanisms inside the transfer case become damaged.

To prevent this from happening, and potentially costing the car owner an expensive repair, we recommend the following:

1. Always advise owners to change tires on an all-wheel drive car in sets of four, not two.
2. Check the transfer case fluid each time the vehicle is in for service and top off if needed.
3. Service the transfer case and change the fluid regularly.

If you’re looking for transfer case fluid with a BMW approval, the Rein brand TF0870 is a smart option. It also meets specifications for select Porsche applications. Part #: FLDF002.