Anti-lock braking systems are designed to prevent locked-wheel skidding during hard braking or during braking on slippery surfaces. The front wheels of a vehicle cannot properly steer the vehicle if they are locked and sliding; the vehicle will continue in its previous direction of travel. The four wheel anti-lock brake system holds each wheel just below the point of locking, thereby allowing steering response and preventing the rear of the vehicle from sliding sideways.
The system monitors and compares wheel speed based on the inputs from the wheel speed sensors. The brake pressure is controlled according to the impending lock-up computations of the ABS control unit.
As a wheel approaches lock-up, the controller actuates the appropriate build/decay or isolation solenoid. Depending on the inputs from each respective wheel speed sensor, the ECM may cycle one of the 2 isolation valves; thus stopping the flow of fluid from the master cylinder to either the LF/RR or RF/LR brake circuit or the ECM may cycle any combination of the 4 build/decay solenoids; thus allowing the system to build brake pressure at the caliper or release (decay) pressure to the modulator sump. This reduces the tendency of the vehicle to skid sideways under braking.
The hydraulic unit or modulator contains the 6 solenoid valves, 4 shuttles, 2 sumps, release check valves, proportioning valves, accumulator and the pump/motor assembly which provides pressurized fluid for the anti-lock system when necessary.
Hydraulic units are not interchangeable between vehicles. The units are not serviceable; if any fault occurs within the hydraulic unit, the entire unit must be replaced.
The electronic control unit (ECU) receives inputs from the 4 speed sensors, brake light switch and the diagnostic connector. The ECU outputs signals to the 6 solenoid valves, ABS warning light relay and pump motor relay if detecting an impending wheel lock-up.
Each wheel is equipped with a magnetic sensor which produces a small AC voltage linear to wheel speed (11 Hz/mph). The sensor is mounted a fixed distance from a toothed ring which rotates with the wheel. The sensors are replaceable but not interchangeable; each must be fitted to its correct location. The toothed rings are replaceable although disassembly of the hub or axle shaft is required.
Both the amber ANTI-LOCK light and red BRAKE light are located on the instrument cluster. Each lamp warns the operator of a possible fault in the respective system. A fault in one system may cause the other lamp to illuminate depending on the nature and severity of the problem. The operation or behavior of the amber warning lamp is one of the prime diagnostic tools for the system.
At no time should the warning lamp be lit while the engine is running. Once the vehicle speed exceeds 3 mph, the system performs a self-check of the pump and solenoid function. Any failure in these circuits will cause the warning lamp to re-light, warning the operator of a problem.
See Figures 1 through 8