The transfer case is attached to the rear of the transmission on BMW X3 E83 models. It transfers power from the transmission to the front and rear wheels. It is a full-time all-wheel drive system, no rear wheel drive is available. There is a driveshaft that runs from the rear of the transfer case to the rear differential and one that runs from the side of the transfer case to the front differential.
BMW X3 E83 models comes equipped with the ATC 400 transfer case, externally identified by the four bolt driveshaft flange. This differs from the ATC 500 on BMW X5 models.
The multi-disc clutch couples the rear axle drive shaft output flange to the front drive shaft output, and when fully engaged, the front and rear axles turn at the same speed. It allows for variable torque to be applied to each axle when needed. The servo-motor is used to regulate clutch pressure, giving the rear and front axle a stepless coupling. This is done via an adjuster lever on a disc cam. As the levers press apart, pressure is applied to the ball ramps, variably increasing pressure on the multi-disc clutch, up to a full lock. The disc cam is rotated by the servo-motor with a worm gear. The servo-motor is a permanent magnet DC motor containing a Hall sensor which detects the position and the adjusting speed of the motor shaft. The rear driveshaft input shaft splines allow for a permanent connection to the rear axle, so when the clutch is disengaged, all driving torque is distributed to the rear axle.
The amount of traction at each axle dictates the torque delivery. Accelerating from a standstill in first gear under full throttle, if both axles are experiencing identical traction, the rear axle will sustain greater driving torque as the vehicle weight shifts to the rear with the acceleration. However, if there were higher traction under the front axle, more driving torque will be transmitted to the front axle.
Limp Home Operation
If a fault is detected in the drivetrain, four wheel drive function is maintained for as long as possible by a limp home operation. In the event of individual sensor signal failures, substitute values are calculated and function is continued with increased control thresholds. This process is employed until useful four wheel drive control is no longer possible, in which case the drive is alerted by the DSC / xDrive lamp lighting up in the instrument cluster and by an acoustic warning signal. If rear wheel speed signals are faulty, wheel speed is calculated using engine speed. If the front wheel speed signals fail, the values of the rear are adopted. Wheel speeds are also substituted for a faulty steering angle signal.
Remember your car may have been serviced before and parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.
Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.
Our vehicle may vary slightly from yours as models do change and evolve as they grow older. If something seems different, let us know and share your info to help other users. Questions or want to add to the article? Leave a comment below. When leaving a comment, please include your vehicle information.
Raise and support your vehicle. See our tech article on jacking up your X3. Working at the left side of the exhaust head shield, remove the 10mm fastener (red arrow).
Jack up all four corners of your vehicle. I suggest using four jack stands and slowly raising vehicle to required height. Vehicle needs to be level when checking and replacing fluid. Remove lower splash shields. See our tech article on engine splash shield removing. Remove the exhaust system. See our tech article on exhaust system replacing. With the exhaust removed, you now have access to the heat shield. Remove the five 8mm fasteners and three 10mm nuts. Then remove the heat shield from the vehicle.
The red arrows point to the location of the out of sight E10 fasteners from the previous step. Photo shows fasteners removed. With fasteners removed, pull servo-motor out of transfer case and remove from vehicle. Reverse steps to install n e servo-motor. Be sure to replace sealing gasket and program / adapt VTG module using an advanced BMW scan tool.