The ABS module measures the speed on each individual wheel using four speed sensors. If the speed on one wheel is too slow, the ABS automatically opens the corresponding brake to prevent the wheel from locking. Since the wheels on tractors have a high inertia, due to their size and weight, the ABS module also communicates with the tractor control unit. The control unit can then drive the affected wheel via the Vario transmission, if opening the brake is not enough to maintain the required speed. Both front wheels are controlled together to maintain the stability of the 900 Vario, even if there are different surfaces under each wheel. The rear wheels are controlled individually by the ABS module.