Antilock Brakes / Traction Control Systems: Description and Operation

Fig. 1 Anti-lock Brake System:

These models are equipped with a four wheel anti-lock brake system (ABS), Fig. 1. The system prevents lockup by automatically modulating brake system pressure during an emergency stop situation.
By controlling wheel lockup, the vehicle's operator can maintain steering control and stop the vehicle in the shortest possible distance, under most driving conditions.
The ABS system controls the front brakes separately and the rear brakes as a pair, whenever wheel lockup begins. The brake pedal application force required to engage the system function may vary with road surface conditions. A dry surface will require a higher force, while a slippery surface will require a much less force.
During system operation, the vehicle's operator will sense a slight pulsation in the brake pedal, accompanied by a rise in brake pedal height and a clicking sound.
The hydraulic pump maintains system pressure between approximately 2030-2610 psi, within the accumulator and is connected by a high pressure hose to the booster chamber portion of the hydraulic actuation assembly and a control valve. When the brakes are applied, a scissor lever mechanism activates the control valve and a pressure, proportional to brake pedal travel, enters the booster chamber portion of the hydraulic actuation assembly. This pressure is transmitted through the normally open solenoid valve through the proportioning valve to the rear brakes. The same pressure moves the booster piston against the master cylinder piston, shutting off the central valves in the master cylinder. This applies pressure to the front wheels through the two front normally open solenoid valves.
The electronic controller monitors the electro-mechanical components of the system. Malfunction of the ABS system will cause the electronic controller to shut off or inhibit the ABS function, while retaining normal power assisted braking. Malfunctions are indicated by one or two warning lamps inside the vehicle.
Loss of hydraulic fluid or power boost pressure will disable the ABS system.
The four wheel ABS system is self monitoring. When the ignition switch is turned to the RUN position, the electronic controller will perform a preliminary self check on the ABS electrical system indicated by a three or four second illumination of the amber ``Check Anti-Lock Brakes'' lamp in the instrument cluster. During vehicle operation, including normal and anti-lock braking, the electronic controller monitors all electrical anti-lock functions and some hydraulic system operation.
For most malfunctions of the ABS system, the amber ``Check Anti-Lock Brakes'' and/or the red ``Brake'' lamp will be illuminated. The sequence of illumination of these warning lamps, combined with the problem symptoms, can determine the appropriate diagnostic test to perform. Most malfunctions are recorded as a coded number in the controller memory, pinpointing the exact component requiring service.