Antilock Brakes / Traction Control Systems: Description and Operation

Hydraulic Schematic:

ABS Component Location:

System Operation Flow Chart:

The anti-lock brake system (ABS) improves braking performance under all severe braking conditions, and in poor weather conditions. The ABS uses electronic and hydraulic components to prevent the four wheels from locking during brake application. The ABS ensures proper wheel tracking and steering performance even during extremely hard braking. The ABS does not function during normal braking applications.

Components of the ABS include:
- Anti-lock brake control module.
- Hydraulic anti-lock actuator assembly.
- Hydraulic anti-lock actuator motor relay, located in the hydraulic anti-lock actuator assembly.
- Hydraulic anti-lock actuator solenoid relay, located in the hydraulic anti-lock actuator assembly.
- Brake master cylinder.
- Power brake booster.
- Front brake anti-lock sensors and rear brake anti-lock sensors.
- Front brake anti-lock sensor indicators and rear brake anti-lock sensor indicators. Front disc brakes.
- Rear drum brakes.
- Brake on/off (BOO) switch.
- Anti-lock brake warning indicator.

When the brakes are applied, fluid is forced from the brake master cylinder outlet ports to the hydraulic anti-lock actuator assembly inlet ports. This pressure is transmitted through three normally open solenoid valves inside the hydraulic anti-lock actuator assembly, then through the outlet ports of the hydraulic anti-lock actuator assembly to each wheel. The primary (front) circuit of the brake master cylinder feeds the right front and left rear brakes. The secondary (rear) circuit of the brake master cylinder feeds the left front and right rear brakes. If the anti-lock brake control module senses that a front wheel is about to lock, based on front brake anti-lock sensor data, it closes the normally open solenoid valve for that circuit. This prevents any more fluid from entering that circuit. The anti-lock brake control module then looks at the front anti-lock brake sensor signal from the affected front wheel again. If that front wheel is still decelerating, it opens the normally closed solenoid valve for that circuit. This dumps any pressure that is trapped between the normally open solenoid valve and the disc brake caliper back to the brake master cylinder reservoir. After the affected front wheel comes back to speed, the anti-lock brake control module returns the solenoid valves to their normal condition, allowing fluid flow to the affected disc brake caliper.

The anti-lock brake control module monitors the electromechanical components of the anti-lock system. A detected malfunction of the anti-lock system will cause the anti-lock brake control module to shut off or inhibit the anti-lock system. However, normal power assisted braking remains.