The Rear Wheel Antilock Brake System (RWAL) is designed to prevent rear wheel lockup under heavy braking conditions. The main system components are: the RWAL valve, controller (CAB), rear wheel speed sensor mounted on the axle, and exciter ring on the differential.
When the brakes are applied, hydraulic fluid is routed from the master cylinder's secondary circuit through the combination valve, to the RWAL valve. From there the fluid is routed to the rear brake wheel cylinders. The CAB monitors rear wheel speed through the sensor on the axle. If a wheel is about to lock up, the CAB signals the RWAL valve, which then modulates hydraulic pressure to the rear wheels to prevent lock up.
During light brake application, rear wheel deceleration is not sufficient to activate the antilock system. During a normal stop the brake fluid flows without restriction to the rear wheel cylinders to stop the vehicle. The antilock solenoids are inactive. The isolation valve is open and the dump valve is closed allowing normal fluid flow to the rear wheel cylinders.
If the CAB senses impending rear wheel lockup, it will energize the isolation solenoid. This prevents a further increase of driver induced brake pressure to the rear wheels. If this initial action is not enough to prevent rear wheel lock-up, the CAB will momentarily energize a dump solenoid. This opens the dump valve to vent a small amount of isolated rear brake pressure to an accumulator. The action of fluid moving to the accumulator reduces the isolated brake pressure at the wheel cylinders. The dump (pressure venting) cycle is limited to very short time periods (milliseconds). The CAB will pulse the dump valve until the rear wheel deceleration reaches the desired slip rate programmed into the CAB. The system will switch to normal braking once wheel locking tendencies are no longer present.
Note that the RWAL system controls both rear wheel brakes simultaneously.
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 HCU provides three channel ABS or four channel With Traction Control pressure control to the front and rear brakes. 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 valves are operated by the antilock electronic module.
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 that 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 convert 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 indicate 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.