Brakes to an electronic control unit (located behind the passen- ger seat). The control unit’s main circuit interprets the speed sensor signals and calculates wheel speed, wheel retardation, and a vehicle reference speed. If the calculations indicate wheel lockup, the main circuit sig- nals the appropriate solenoid control valve to reduce braking pressure. During emergency braking, the sole- noid control valve alternately reduces, increases, or maintains air pressure supply in the brake chamber to prevent front and rear wheel lockup. CAUTION Never apply the service and spring parking brakes simultaneously.
To do so transmits excessive input force to the brake components, which could dam- age or cause eventual failure of brake actuating components. Meritor WABCO ® Antilock Braking System (ABS), Air Brake Systems The electronic control unit also has a safety circuit that constantly monitors the wheel sensors, solenoid control valves, and the electrical circuitry. The ABS warning light comes on after turning on the ignition switch. The warning light goes out, if all ABS components are work- ing properly. The Meritor WABCO ® Antilock Braking System (ABS) is an electronic wheel speed monitoring and control sys- tem that works with the air 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 air brake system is in effect. If, during vehicle operation, the safety circuit senses a failure in any part of the ABS system (a sensor, sole- noid control valve, wiring connection, short circuit, etc.), the ABS warning light comes on and the control circuit where the failure occurred is switched to normal braking action. The remaining control circuit will retain the ABS effect. Even if the ABS system is partially or completely inoperative, normal braking ability is maintained.
An ex- ception would be if a solenoid control valve (or combi- nation solenoid control valve) is damaged and inopera- tive. As these components are an integral part of the air brake system, normal braking may be impaired or inop- erative. IMPORTANT: For proper ABS system operation, do not change tire sizes. The sizes of the tires in- stalled during production are programmed into the electronic control unit. Installing different sized tires could result in a reduced braking force, lead- ing to longer stopping distances. ABS includes signal-generating tone wheels and sen- sors located in the wheel hubs of each sensed axle. The sensors transmit vehicle wheel speed information 7.3