The four-wheel Antilock Brake System (ABS) is designed to prevent wheel lock-up during braking under virtually any road surface conditions. This allows the driver to retain greater control of the vehicle during braking.
The major components of the system are:
The four-wheel ABS has a three-channel design. The front brake antilock valve provides two-channel pressure control of the front brakes. Each front wheel brake unit is controlled separately. Two solenoid valves are used in each control channel.
The rear brake antilock valve controls the rear wheel brakes in tandem. The rear brake valve contains two solenoid valves.
The front and rear antilock valves contain electrically operated solenoid valves. The solenoid valves modulate brake fluid apply pressure during antilock braking. The antilock electronic module operates the valves.
The antilock electrical system is separate from other electrical circuits in the vehicle. A specially programmed electronic control module is used to operate the system components.
Some models have Electronic Brake Distribution (EBD) designed into the system which eliminates the combination/proportioning valve.
The ABS electronic control module monitors wheel speed sensor inputs continuously while the vehicle is in motion. The module will not activate the ABS system as long as sensor inputs indicate normal braking.
During normal braking, the master cylinder, power booster and wheel brakes units all function as they would in a vehicle without ABS. The solenoid valves are not activated.
The wheel speed sensors converts wheel speed into electrical signals. These signals are transmitted to the module for processing and determine wheel lock-up and deceleration rate. When a wheel speed sensor signal indicates the onset of wheel lock-up the ABS braking is activated.
The antilock system retards the lockup conditions by modulating fluid apply pressure to the wheel brake units. The pressure is modulated according to wheel speed, degree of lock-up and rate of deceleration. The solenoid valves are cycled continuously to modulate pressure. Solenoid cycle time in antilock mode can be measured in milliseconds.