Antilock Brakes / Traction Control Systems: Description and Operation

System Power-up

With the Ignition Switch in the START or RUN position, voltage is present at the Anti-Lock Brake Control Module through the CLUSTER Fuse. The Module applies voltage at pin 9 to energize the Anti-lock Power Relay and to activate the Anti-lock Brake System. The Module also grounds pin 26 to turn on the amber ANTI-LOCK Indicator in the Instrument Cluster. If no system faults are detected, the Module turns off the Indicator about 4 seconds after the engine starts.

Pressure Control

With the Ignition Switch in the RUN position, voltage is present at the Hydraulic Pump Motor Relay through the ANTI-LOCK Fuse. The Relay is controlled by the Pressure Control Contacts in the Anti-lock Pressure Switch. When system pressure is low, the Pressure Control Contacts close, grounding the Hydraulic Pump Motor Relay. The Relay energizes and turns on the Hydraulic Pump Motor to build up system pressure. When system pressure is sufficient, the Pressure Control Contacts opens, deenergizing the Relay and shutting off the Hydraulic Pump Motor.

Pressure Warnings

If system pressure drops below the point where the Pressure Control Contacts close, the Pressure Warning Contacts will change position. One pair of contacts will open, breaking the circuit between pins 25 and 27 of the Anti-lock Brake Control Module. This prompts the Module to turn on the amber ANTI-LOCK Indictor. The other pair of Pressure Warning Contacts will close, turning the red BRAKE Indicator.

Fluid Level Warnings

The brake fluid reservoir cap includes a Brake Fluid Level Sensor with two sets of contacts. If the fluid level drops too low, the upper contacts will close, turning on the red BRAKE Indicator. This does not affect Anti-lock Brake System operation. If fluid level continues to drop, the lower contacts will also open, breaking the circuit between pins 25 and 27 of the Anti-lock Brake Control Module. This prompts the Module to turn on the amber ANTI-LOCK Indicator and to inhibit operation of the Anti-lock Brake System.

Wheel Speed Sensor Inputs

The Anti-lock Brake Control Module senses impending wheel lock-up by monitoring the speed of each wheel. This information is provided by four Brake Sensors and toothed sensor ring assemblies: one for each of the four wheels. As the wheel rotates, the toothed ring attached to the brake rotor (front wheel) or axle shaft (rear wheel) also rotates and the teeth "make and break" the magnetic field in the Sensor, generating a pulsing voltage that is supplied to the Anti-lock Brake Control Module.

Solenoid Control Valve Outputs

If data provided by the Brake Sensors indicates that a wheel is about to lock-up, the Anti-lock Brake Control Module will energize the necessary solenoids in the Solenoid Control Valve Body to prevent lock-up. The Solenoid Control Valve Body contains 6 solenoids: an inlet and outlet solenoid for each front wheel, and an inlet and outlet solenoid for both rear wheels.

To prevent a wheel from locking up, the Anti-lock Brake Control Module closes that wheel's inlet solenoid and opens the outlet solenoid to reduce brake pressure at the wheel. As soon as the near lock-up condition is gone, the Module returns the solenoids to their normal positions. The opensing and closing of the solenoid valves takes only milliseconds and can take place several times each second for each brake circuit (left front, right front or rear).