Antilock Brakes / Traction Control Systems: Description and Operation

Current is supplied at all times to the Anti-Lock Brake System (ABS) Relay through the ABS PUMP fuse. With the Ignition Switch in the START or RUN position, current is supplied to the ABS Control Module and the coil of the ABS Relay through the TURN fuse. The ABS Control Module supplies a ground to the coil of the Solenoid Relay and the contacts of the relay close. With the relay closed, current flows from the ABS PUMP fuse to the coil of the Hydraulic Pump Relay and the coils of the ABS Actuator Assy.

Wheel Speed Sensor

When brake pedal is depressed, the Stop Lamp Switch closes and current is supplied to the ABS Control Module. The ABS Control Module monitors the Wheel Speed Sensor signals. When one wheel begins to lock up, the wheel speed is slower than the other three wheels and a locking condition is detected by the Wheel Speed Sensor.

If a locking condition is detected, the ABS Control Module grounds the Solenoid Valves for the locking wheel. The Solenoid Valves, once activated, act as a modulating relief valve. When open, these valve relieve brake fluid pressure on the wheel that is beginning to lock up. This pressure release prevents the wheel from locking, greatly improving braking performance.

Hydraulic Pump

Opening any of the Solenoid Valves, however, lowers pressure throughout the entire braking system. In order to maintain system pressure, a Hydraulic Pump is incorporated into the ABS Actuator Assy. For vehicles built before March 1991, a system pressure is monitored so that if a drop in pressure below 14000 kPa (2050 psi) occurs, it will cause a Pressure Control Switch to open, and a low pressure signal is sent to the ABS Control Module. Upon receiving this signal, the ABS Control Module grounds the coil of the Hydraulic Relay. With the relay coil grounded, the relay contacts are closed and current flows to the Hydraulic Pump Motor. The Pump operates until pressure builds up and the Pressure Control Switch closes.

ABS Control Module

The ABS Control Module continuously runs self-test procedures to detect a malfunction in the ABS system. If a malfunction occurs, the ABS Control Module will deactivate the ABS system and normal braking operation will be maintained.