Antilock Brakes / Traction Control Systems: Description and Operation

Fig 1 Rear Wheel Antilock Brake System:

NOTE: The Rear-Wheel Anti-Lock Brake System is the standard system available on this model.

DESCRIPTION
The Anti-lock brake controller (CAB) contains a microprocessor which continuously monitors the inputs and controls the modulator valve. It also contains a self-check program which is activated when the ignition system is turned "ON". The brake warning and anti-lock indicator lamps are illuminated for approximately two seconds during the system self test cycle. If the system determines a circuit to be at fault, the CAB will illuminate both lamps as an indication that a fault code has been stored in memory.

PURPOSE
This system prevents rear wheel lock-up by monitoring rear wheel speed and altering brake pressure to the rear wheels. Rear brake fluid application is controlled according to wheel speed, degree of wheel slip and rate of deceleration. The main components of the system are:

- an electronic brake control module
- a hydraulic pressure control valve
- a speed sensor and exciter ring.
- A standard master cylinder and power booster are used.

OPERATION
During light brake application, rear wheel deceleration and/or slip is not sufficient to activate the ABS system. Brake fluid apply pressure at the rear wheels is normal. When braking effort, degree of wheel slip and/or rate of deceleration approach predetermined limits, sensor inputs will cause the module to activate the ABS system.

Normal Braking Mode
In normal braking mode, the antilock solenoid valves are inactive. The valves are open allowing normal fluid flow to the rear wheel cylinders.

Start Of Antilock Mode Braking
When high pedal effort braking occurs, the decrease in exciter wheel rotating speed is noted and converted into an electronic signal by the sensor. The signal is transmitted to and processed by the control module. The module determines that acceptable limits of wheel slip/deceleration are about to be exceeded. The module activates the antilock solenoid valves at this point.

The module transmits a signal that activates the solenoid valves. The valves then decrease, or isolate fluid apply pressure to the rear wheel cylinders as needed.

Closing the valves prevents further fluid flow to the rear wheel cylinders. This action isolates the rear brakes from the master cylinder. The net effect is to decrease rear brake apply pressure to the point where the wheels will continue to rotate but not lock.

When rear brake pressure requirements return to normal levels, the solenoid valves are inactivated. This action allows the resumption of normal fluid flow to the wheel cylinders.

Solenoid Valve Cycle Times
Activation (opening/closing) of the solenoid valves is not static during antilock mode braking. Valve operation is continuous as they are rapidly cycled in response to sensor inputs and control module commands. Cycle times are measured in milliseconds.

As the demand for antilock mode brake operation is decreased, the module deactivates the hydraulic valve components to restore normal brake operation.

CODES
System faults are identified by a series of flash codes that operate through the anti-lock warning lamp. The CAB will store and display one fault code at a time. Flash codes may be retrieved by temporarily grounding the anti-lock diagnostic connector.