Antilock Brakes / Traction Control Systems: Description and Operation

ABS SYSTEM OPERATION
The Anti-Lock Brake System (ABS) prevents wheel lockup by automatically modulating the brake pressure during an emergency stop. By not locking the wheels, the driver can improve steering control during hard braking and stop the vehicle in the shortest possible distance under most conditions. ABS controls each brake separately. The brake pedal force required to engage ABS function may vary with the road surface conditions. A dry surface requires a higher force, while a slippery surface requires much less force. During ABS operation, the driver will sense a slight pulsation in the brake pedal. In addition, a mechanical noise from the engine compartment may be heard. The pedal effort and pedal feel during normal braking are similar to that of a conventional power brake system.

HYDRAULIC ANTI-LOCK ACTUATOR
Components
The hydraulic anti-lock actuator consists of a valve body assembly, pump and a motor assembly. During normal braking, fluid from the brake master cylinder enters the hydraulic anti-lock actuator through two inlet ports located on the bottom of the hydraulic anti-lock actuator. The fluid then passes through four normally open Isolation (ISO) valves, one to each of the four wheels.

System Operation
If the anti-lock brake control module senses that a wheel is about to lock, it pulses the appropriate isolation valve, which closes that valve. This prevents any more fluid from entering the affected brake. The ABS control module then looks at that wheel again. If it is still decelerating, the ABS control module pulses the normally closed dump valve, which decreases the pressure trapped in the line. The hydraulic anti-lock actuator, the valve body, the pump and the motor are not serviceable separately. The ABS control module is a non-repairable on-board diagnostic unit consisting of one microprocessor and the necessary circuitry for its operation.

ANTI-LOCK BRAKE SENSOR
The anti-lock brake system uses four variable-reluctance sensors and toothed anti-lock sensor indicator rings to determine the rotational speed of each wheel. The sensors operate on the magnetic induction principle. As the teeth on the speed indicator ring rotate past the stationary sensor, a signal proportional to the speed of the rotation is generated and sent to the ABS control module through a twisted pair wire cable and wiring harness.

ANTI-LOCK BRAKE CONTROL MODULE
Primary Functions
The two primary functions of the anti-lock brake control module are control of the hydraulic anti-lock actuator during ABS operation and system monitoring during normal driving, as well as during anti-lock braking. Under normal braking conditions, the microprocessor produces short test pulses to check the electrical system without any mechanical reaction. The ABS control module continuously monitors the speed of each wheel to determine if any wheel is beginning to lock. When a wheel locking tendency is detected, the ABS control module signals the appropriate isolation valve in the hydraulic anti-lock actuator to open or close as well as the pump motor to turn "ON" and return brake fluid back to the base braking system.

Operation
The ABS control module monitors the electromechanical components of the system. Malfunction of the ABS system will cause the ABS control module to shut off or inhibit the system. However, normal power assisted braking remains. Malfunctions are indicated by the ABS warning indicator inside the vehicle. The ABS system is self-monitoring. When the ignition switch is placed in the "RUN" position, the ABS control module will perform a preliminary self check on the anti-lock electrical system indicated by a two to three second illumination of the amber anti-lock brake warning indicator in the instrument cluster. At 6.4 km/in (4 mph), the valves and motor will be powered briefly to verify proper function. During vehicle operation, including normal and anti-lock braking, the ABS control module monitors all electrical anti-lock functions and some hydraulic operations.

Diagnostic Trouble Codes (DTC)
Most faults that occur in the anti-lock brake system will be stored as a DTC in the Non-volatile Random Access Memory (NVRAM) of the ABS control module. Once a DTC is stored, it will be retained by the ABS control module even with the key in the "OFF" position. DTCs can be retrieved by following the on-board diagnostics procedures.