Transfer Case - Service Techniques: xDrive: Introduction

xDrive is already in use on the E53 and E83. Because of the small amount of space available on the E60, E61 transmission tunnel, it was not possible to use the xDrive from the E83 (ATC 400).

The transfer case ATC 300 is used on the E60, E61. (ATC = Active Torque Control).

Mechanical Construction Of The Transfer Case

Fig 1: Cut-Away View Of xDrive Transfer Box

Courtesy of BMW OF NORTH AMERICA, INC.

Input/Output

Fig 2: xDrive Component Flow Diagram

Courtesy of BMW OF NORTH AMERICA, INC.

System Circuit Diagram

Fig 3: xDrive System Circuit Diagram

Courtesy of BMW OF NORTH AMERICA, INC.

xDrive consists of a link between the Dynamic Stability Control (DSC) and an electronically controlled multi-plate clutch in the transfer case.

The electronically controlled multi-plate clutch smoothly distributes the drive torque as required to the front axle.

The rear axle is always driven.

When the multi-plate clutch is open, the entire drive torque flows to the rear axle.

xDrive constantly communicates with the DSC. The drive-dynamic input information is computed in the DSC control unit.

Important information is:

Accelerator Pedal Position
Engine Torque
Drive-Dynamic State

The DSC calculates the necessary locking torque. The locking torque is communicated to xDrive. This allows xDrive to register whether the vehicle is able to follow the driver's commands. xDrive intervenes to counter the threat of any tendency for wheel spin, oversteer or understeer. xDrive regulates input torque distribution between the two axles to meet driving demands.

Only if xDrive cannot maintain the vehicle's course on its own will DSC also intervene. Engine output is reduced and individual wheels are specifically braked.

The input torque is delivered to the axle that has better traction when road conditions change, such as on snow, ice or a loose road surface.

Differences between ATC 300 in the E60, E61 and ATC 400/ATC 500 in the E83/E53:

ATC 300:
Drive torque to the front axle is transmitted via gears in the transfer case.

The control lever is actuated by a control cam.
ATC 400/ATC 500:
Drive torque to the front axle propeller shaft is transmitted by a chain in the transfer case.

The control lever is actuated by a control disc.

The Advantages:

xDrive delivers the following advantages through demand-oriented input torque distribution:

Outstanding driving stability fully up to the limit range
Optimal forward momentum
Excellent traction in all road situations