Meritor WABCO® Antilock Braking System (ABS)
The Meritor WABCO® Antilock Braking System (ABS) is an electronic wheel speed monitoring and control system that works with the hydraulic brake system. ABS passively monitors vehicle wheel speed at all times, but controls wheel speed during an emergency or reduced-traction stop. In normal braking applications, the standard hydraulic brake system is in effect.
Important: For proper ABS system operation, the front and rear tires must rotate at speeds within 16% of one another. Installing different sized tires on just the front or rear axle will result in inaccurate wheel speed signals to the ECU resulting in a reduced braking force, which could lead to longer stopping distances.
ABS includes signal-generating tone wheels and sensors located in the wheel hubs of each sensed axle. The sensors transmit vehicle wheel speed information to an electronic control unit (located on the frame rail) that interprets these signals to calculate wheel speed and a vehicle reference speed. If the calculations indicate wheel lockup, the appropriate control circuit signals the brake pressure modulator to increase or decrease braking pressure until wheel slip has been corrected.
The electronic control unit also has a safety circuit that constantly monitors the wheel sensors, brake pressure modulator, and the electrical circuitry. The dash warning light (ABS) comes on after the ignition is switched to the ON position. Once the vehicle moves faster than about 4 mph (6 km/h), the warning light goes out only if all of the vehicle's ABS components are working properly.
If during vehicle operation, the safety circuit senses a failure in any part of the ABS system (a sensor, brake pressure modulator, wiring connection, short circuit, etc.), the warning light (ABS) comes on and the control circuit where the failure occurred is switched to normal braking action. Even if the ABS system is partially or completely inoperative, normal braking ability is maintained. An exception would be if the brake pressure modulator or hydraulic fluid line is damaged. Since these components are an integral part of the hydraulic brake system, normal braking may be impaired or inoperative.
Important: If the ABS warning light does not work as described above or comes on while driving, repair the ABS system immediately to ensure full antilock braking capability.
See the diagnostic menu screens in Chapter 2 or Chapter 3 as applicable, and follow the steps to read the fault codes in the instrument panel diagnostic menu.
During emergency or reduced-traction stops, fully depress the brake pedal until the vehicle comes to a safe stop; do not pump the brake pedal. With the brake pedal fully depressed, the ABS system will control all wheels to provide steering control and a reduced braking distance.