Antilock Brakes / Traction Control Systems: Description and Operation
PURPOSE OF ABS- Prevents wheel lockup during an emergency stop by modulating brake pressure.
- Permits the driver to maintain steering control and stop the vehicle in the shortest possible distance under most conditions.
ABS OPERATING CHARACTERISTICS
- The Anti-lock Brake System (ABS) is a four wheel system which prevents wheel lock-up by automatically modulating the brake pressure during an emergency stop.
- By not locking the wheels, it enables the driver to maintain steering control and stop in the shortest possible distance under most conditions.
- During normal braking the ABS pedal feel will be the same as a non-ABS pedal.
- During ABS operation, a pulsation can be felt in the brake pedal, accompanied with a fall and rise in brake pedal height and a "clicking" sound.
ABS COMPONENTS-The ABS system consists of the following:
- Power brake booster.
- Brake master cylinder.
- Brake pressure control valves.
- Hydraulic brake tubes.
- Hydraulic Control Unit (HCU).
- ABS Control Module.
- Front brake anti-lock sensor.
- Rear brake anti-lock sensor.
ABS OPERATION
- If the ABS module senses that a wheel is about to lock, based on brake anti-lock sensor data, it pulses the normally open solenoid valve closed for that circuit. This prevents any more fluid from entering that circuit.
- The anti-lock brake module then looks at the brake anti-lock sensor signal from the affected wheel again.
- If that wheel is still decelerating faster than the other three wheels, it opens the normally closed solenoid valve for that circuit. This dumps any pressure that is trapped between the normally open valve and the brake back to the brake master cylinder reservoir.
- Once the affected wheel returns to speed, the ABS control module returns the valves to their normal condition allowing fluid flow to the affected brake.
ABS SELF-DIAGNOSTICS (DTC)
- The anti-lock brake module monitors the electro-mechanical components of the system.
- Malfunction of the anti-lock brake system will cause the anti-lock brake module to shut off or inhibit the system. However, normal power-assisted braking remains.
- Malfunctions are indicated by a warning indicator in the instrument cluster.
- The 4-wheel anti-lock brake system is self monitoring. When the ignition switch is placed in the "RUN" position, the anti-lock brake module will perform a preliminary self check on the anti-lock electrical system indicated by a two second illumination of the amber ABS indicator in the instrument cluster.
- During vehicle operation, including normal and anti-lock braking, the anti-lock brake module monitors all electrical anti-lock functions and some hydraulic operations.
- In most malfunctions of the anti-lock brake system, the amber ABS indicator will be illuminated.
- Most malfunctions are recorded as a diagnostic trouble code (DTC) in the anti-lock brake module keep-alive random access memory and assist in pinpointing the component needing service.
ELECTRICAL SYSTEM
Power is provided to the anti-lock brake control module through three separate fused lines for electronic logic control, hydraulic pump motor and solenoid valves.