ABS System Components
Electronic Brake Control Module (EBCM)The Electronic Brake Control Module (EBCM) is a small control microprocessor, which monitors the speed of each wheel and the electrical status of the brake pressure modulator valve assembly It is mounted under the left side of the instrument panel above the brake pedal on the DERM bracket. The primary functions of the EBCM are to detect wheel locking tendencies, control the brake system while in antilock mode and monitor the system for proper electrical operation. The EBCM also controls display of ABS diagnostic trouble codes while in diagnostic mode.
The EBCM continuously checks the speed of each wheel to determine if any wheel is beginning to lock. If a wheel locking tendency is detected, the EBCM commands appropriate valve positions to modulate brake fluid pressure in some or all of the hydraulic circuits to prevent wheel lockup and provide optimum braking. The EBCM continues to control pressure in individual hydraulic circuits until a locking tendency is no longer present.
The Antilock Brake System is electrically monitored by the EBCM for proper operation. If the EBCM detects an error, it can disable the antilock function and turn on the amber "ANTILOCK" warning lamp in the instrument panel cluster.
Wheel Speed Sensors
A wheel speed sensor is present at each front wheel on the steering knuckle and on the rear axle differential. The sensors transmit wheel speed information to the EBCM by means of a small AC voltage. This voltage is generated by magnetic induction caused by passing a toothed sensor ring past a stationary sensor. The signal is transmitted to the EBCM through a pair of wires.
The front wheel speed sensor is mounted in the front suspension knuckle and is non-adjustable. Sensor gap is set at time of assembly. The connectors for each speed sensor are located near each underhood strut tower.
The rear wheel speed sensor is mounted in the rear axle differential case assembly and is non-adjustable. Sensor gap is set at the time of assembly.
In this system, the speed of each toothed sensor ring is measured separately. The front wheels are controlled individually, the rear wheels together. The control of the rear wheels works on the "select low" principle where the wheel with the greater tendency to lock determines the level of control.
Brake Pressure Modulator Valve Assembly
The brake pressure modulator valve assembly, mounted on the left front side of the engine compartment, provides brake fluid modulation for each of the individual wheel circuits as required during antilock braking. During the antilock mode, the modulator can maintain or reduce brake fluid pressure independent of the pressure generated in the master cylinder. The modulator does not provide more pressure than what is applied by the master cylinder. The modulator alone cannot apply the brakes.
The brake pressure modulator valve assembly is an integral, non-serviceable component. The brake pressure modulator valve assembly should never be disassembled.
The brake pressure modulator valve assembly uses a three circuit configuration during normal braking. Individual circuits are provided for the left front and right front wheels, while a shared circuit is used for the rear wheels. The front and rear circuits are hydraulically isolated so that a leak or malfunction in one circuit will allow continued braking ability on the other.
The brake pressure modulator valve assembly consists of several other components which are described as follows:
Solenoid Valves
The solenoid valves of the modulator are an integral portion of the brake pressure modulator valve assembly and are not serviceable. The solenoid valves increase, decrease or maintain brake fluid pressure to the individual wheel circuits. The solenoid valves are controlled by the EBCM which commands the valves to their correct position during antilock braking. During normal braking, the valves are positioned to pressure increase or open. Braking is accomplished as a normal power assist system. During antilock mode, the pressure in each hydraulic circuit can be increased, decreased or held constant by changing the valve position to various combinations. Valve position can be changed very rapidly during antilock mode.
Pump Motor
The brake pressure modulator valve assembly also consists of a small pump which circulates brake fluid back to the master cylinder during the antilock mode. The pump is located on the bottom of the brake pressure modulator valve assembly and cannot be serviced separately.