Antilock Brakes / Traction Control Systems: Description and Operation

Anti-Lock Brake System

With the Ignition Switch in RUN, current flows through Fuse 18 to the Anti-lock Brake Module and the Anti-lock Brake coil. Anti-lock Brake Module grounds the relay coil, the coil energizes and the relay contacts change position. Current now flows through the Fuse Link Cartridge S and the Anti-lock Brake Relay to the Anti-lock Brake Module, the Anti-lock Brake Motor Relay and the ABS Hydraulic Control Unit.

The Anti-lock Brake Module grounds pin 52 to illuminate the ANTI-LOCK BRAKE Indicator until a self-check of the entire Anti-lock System has been completed.

If brake pedal travel reaches approximately 3 inches during and anti-lock stop, the Anti-lock Brake Pedal Travel Switch contacts open. The Anti-lock Brake Module then grounds pin 15 to energize the Anti-lock Brake Motor Relay coil and close the Relay contacts. Current flows through the Fusible Link Cartridge B and the Anti-lock Brake Motor Relay to the pump motor on the ABS Hydraulic Control Unit. The pump motor runs to build up pressure in the brake master cylinder, forcing the brake pedal back up. When the pedal returns enough to close the Anti-lock Brake Pedal Travel Switch contacts, the Anti-lock Brake Module "ungrounds" pin 15 and the pump motor stops.

Master Cylinder Fluid Reservoir

The brake master cylinder fluid reservoir contains a fluid level switch, which is open with fluid level above a predetermined level. If brake fluid level drops too low the switch contacts close, turning on the red BRAKE warning indicator in the instrument cluster. The Anti-lock Brake System is not affected by this condition.

Hydraulic Control Unit Fluid Reservoir

The ABS Hydraulic Control Unit fluid reservoir also contains a fluid level switch, which is closed with fluid level above a predetermined level. If brake fluid level in the reservoir drops too low, the switch contact open. The Anti-Lock Brake Module responds to this input by turning on the amber ANTI-LOCK BRAKE Indicator and disabling the anti-lock functions of the brake system.

Wheel And Toothed Ring Sensor

Four individual wheel sensors and toothed sensor rings indicate to the Anti-lock Brake Module the speed of each wheel. Each wheel has a sensor and toothed sensor ring. As the wheel rotates, the teeth on the sensor ring make and break the magnetic field in the wheel sensor, generating a pulsating voltage. The Anti-lock Brake Module monitors these signals and senses impending wheel lockups.

Traction Assist

Vehicle equipped with the Anti-lock Brake System are also equipped with the Traction Assist System. With this system, the Anti-lock Brake Module detects rear wheel spin and pulses the spinning wheel's brake until traction is re-established.