Principle of Operation: TCC

Regulation of the transfer case clutch (TCC) locking pressure allows stepless coupling of the front axle to the drivetrain. The driving torque on the front axle can be increased or decreased depending on the driving situation and road conditions. Obviously, when more driving torque is transmitted to the front axle, driving torque on the rear axle is proportionally reduced due to lack of traction. The advantages of variable distribution of driving torque to the front and rear axles are:

• Optimum utilization of the cornering and longitudinal wheel forces on the front and rear axles.
• DSC brake interventions only become necessary at a significantly later stage, an increase in comfort refinement.
• Compared with an "open" differential transfer case and DSC, xDrive significantly improves driving torque distribution when traction on the front and rear axles is notably different. The DSC control unit influences control of the transfer case clutch. Even when DSC is deactivated, TCC remains active for the purpose of maximum traction and driving dynamics.

Permanent four-wheel drive is only completely deactivated in three control situations:

• During very tight cornering with low engine torque to allow speed compensation between the front and rear axles (e.g. parking).
• At speeds greater than 112 mph.
• When the vehicle dramatically understeers.

The transfer case clutch control logic is described in three main modules:

• Pre-control.
• Traction control/driving dynamics control.
• Tire tolerance logic.