The purpose of the Antilock Brake System (ABS) is to minimize wheel slip during heavy braking. The ABS performs this function by monitoring the speed of each wheel and controlling the brake fluid pressure to each wheel independently during an braking event. This allows the driver to maintain directional stability while minimizing stopping distance.
During normal braking, the ABS system is transparent to the operator. Internally, each control piston is in the uppermost or home position allowing brake fluid pressure to pass to the wheels unrestricted. A small internal Expansion Spring Brake (ESB) device is applied to each piston, preventing it from being forced downward by the pressurized fluid passing above it.
When impending wheel lock is noted at one or more wheels, the Electronic Brake Control Module (EBCM) commands the system into ABS mode. Solenoids in each front wheel circuit close. The brakes on the pistons are released and the pistons are driven downward by the electric motors through a system of driven gears and against the spring pressure. The amount of current applied to the motors controls the speed and distance traveled. As the motors move backwards, the piston moves downward, allowing a check valve to seat. The brake pressure to the wheel is now a function of the controlled volume within the piston chamber.
To reduce pressure, the motor continues to drive the piston downward. If an increase in pressure is necessary, the piston is driven upward. Total pressure available is limited to the amount present when ABS was entered.
The rear brakes are controlled in similar fashion. Wheel speed signals are received from each rear wheel. The EBCM uses a Select Low strategy, controlling a single motor to control output to the rear brakes based on the wheel with the greatest tendency to lock.
Many of the service procedures, including troubleshooting and control assembly overhaul require the use the Saturn Portable Diagnostic Tool (PDT) or an equivalent scan tool. If a tool is not available for these procedures, the vehicle should be taken to a qualified repair shop.