Antilock Brakes / Traction Control Systems: Description and Operation
DESCRIPTIONThe 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 pedal position (BPP) switch.
- Anti-lock brake warning indicator.
MECHANICAL OPERATION
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.
The anti-lock brake system (ABS) is self-monitoring. When the ignition switch is in the ON position, the anti-lock brake control module will perform a preliminary self-check on the anti-lock electrical system indicated by a three to five second illumination of the amber anti-lock brake warning indicator in the instrument cluster. During vehicle operation, including normal and anti-lock braking, the anti-lock brake control module monitors all electrical anti-lock functions and some hydraulic operations.
For detected malfunctions of the ABS, the amber anti-lock brake warning indicator will illuminate. The sequence of illumination for this warning indicator (when used to perform ABS trouble code retrieval) combined with the symptoms can determine the appropriate diagnostic tests to perform. Most malfunctions are recorded as a coded number in the brake anti-lock control module memory to assist in pinpointing the component needing service.
The red brake system warning indicator on the instrument cluster will light when the brake fluid in the brake master cylinder reservoir is too low or when the parking brake is applied.
ELECTRICAL OPERATION
The anti-lock brake system (ABS) electrical system consists of:
- Hydraulic pump motor relay.
- Hydraulic solenoid relay.
- Hydraulic anti-lock actuator assembly.
- Anti-lock brake control module.
- Front brake anti-lock sensors.
- Rear brake anti-lock sensors.
The hydraulic anti-lock actuator assembly consists of three solenoids which control hydraulic fluid flow to each wheel. This is a three-solenoid, four-channel system.
When the ignition switch is in START or ON, the 10A ELECTRON IGN fuse in the I/P Fuse/Relay Panel applies voltage to the anti-lock brake control module.
The anti-lock brake control module processes inputs from the brake pedal position (BPP) switch, front brake anti-lock sensors and rear brake anti-lock sensors.
The BPP switch monitors brake pedal position. When the brake pedal is depressed, the BPP switch contacts close and send input to the anti-lock brake control module.
The front brake anti-lock sensors and rear brake anti-lock sensors generate a varying voltage signal that is proportional to wheel speed. The anti-lock brake control module monitors the brake anti-lock sensor signals and compares them to determine if anti-lock brake system (ABS) operation is required.
The anti-lock brake control module controls the operation of the following:
- Hydraulic solenoid relay.
- Hydraulic pump motor relay.
- Anti-lock brake warning indicator.
- Hydraulic anti-lock actuator assembly.
The hydraulic solenoid relay is grounded by the anti-lock brake control module to power up the system. The anti-lock brake control module also grounds the hydraulic pump motor relay to run the pump motor during anti-lock cycling.
The anti-lock brake control module prevents wheel lockup by pulsing the solenoid valves in the hydraulic anti-lock actuator assembly.
The anti-lock brake warning indicator bulb is grounded by the anti-lock brake control module to illuminate the anti-lock brake warning indicator. The anti-lock brake warning indicator is illuminated for several seconds during system power-up and when the anti-lock brake control module detects a problem with the ABS.