Operation of the Pneumatic ABS
Pneumatic ABS is an electronic wheel speed monitoring and control system that works with the air brake system. It 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 air brake system is in effect.
The Meritor WABCO ABS is a four-sensor system. It combines one front-axle control channel with one rear-axle control channel to form one control circuit.
Example: The sensor and solenoid control valve at the left front wheel form a control circuit with the sensor and solenoid valve at the right rear axle.
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.
Important: For proper ABS system operation, do not change tire sizes. The sizes of the tires installed during production are programmed into the electronic control unit. Installing different sized tires could result in a reduced braking force, leading to longer stopping distances.
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.
Although the ABS system improves vehicle control during emergency braking situations, the driver still has the responsibility to change driving styles depending on the existing traffic and road conditions. For example, the ABS system cannot prevent an accident if the driver is speeding or following too closely on slippery road surfaces.
Even if the ABS system is partially or completely inoperative, normal braking ability is usually maintained.
Important: If a solenoid control valve (or combination solenoid control valve) is damaged and inoperative, normal braking may be impaired.