Antilock Brakes / Traction Control Systems: Description and Operation

The anti-lock braking system prevents the wheels from locking in an emergency stop, until shortly before the vehicle comes to halt, thus assuring full steerability and directional stability. In addition, the system optimizes the braking distance corresponding to the varying degree of grip between the wheel and road surface.
The ABS control unit is matched to the approved tire dimensions. The use of unapproved tires can lead to different wheel speeds which the control unit interprets as different road speeds at the vehicles axles. If the rolling radius exceeds a certain amount, the control unit deactivates the ABS and the ABS pilot lamp illuminates.
The three-channel ABS system manufactured by Bosch has a separate speed sensor at each wheel. A front/rear braking circuit split is used, one braking circuit acts on the front axle (push-rod circuit) with the second acting on the rear axle (floating circuit).