Antilock Brakes / Traction Control Systems: Description and Operation

Anti-Lock Brake System:

Fig. 2 ABS Hydraulics:

Fig. 3 Hydraulic Actuation Assembly:

The Anti-Lock Brake System (ABS) functions by releasing and applying fluid pressure to the brake calipers during certain braking conditions. During normal driving conditions the system does not function and has no effect on front-to-rear brake proportioning. When one or more wheels approaches a slip condition. The ABS automatically senses the slip and activates the pressure control function.
Through pre-programming, the control unit decides which wheel or wheels' brakes pressures need modulation. The control unit sends appropriate signals
to solenoid valves located in the actuation assembly. The control valves then modulate fluid pressure which results in a pressure reduction at wheel cylinder to prevent further lock up.
The ABS electrical system consists of a control module (mounted under the lefthand kick panel), hydraulic actuation assembly and relay. These components work together along with the wheel speed sensors, sensor rotors and other brake system components to control braking.
This three-channel system has independent front wheel control and select low resistance rear wheel control. The hydraulic actuation assembly, Fig. 3, contains four solenoid valves, flow control valves and the pump motor.
Two solenoids for the front wheels receive independent braking signals dependent on wheel speed. The control module selects the rear wheel with the lowest resistance and sends both rear solenoids the same braking signal.
The control module continuously calculates wheel speed and activates the ABS system only when wheel lock-up is sensed under severe braking conditions. When the module senses a lock-up, it sends a pressure reduction signal through the relay to the actuation assembly. Hydraulic fluid flow into a buffer chamber in the actuation assembly is controlled by the solenoid.
When the wheel is no longer in danger of locking up, the module sends pressure increase signals through the relay to the actuation assembly. One signal directs the solenoid to allow fluid pressure to increase while the other activates the pump motor to further increase pressure by returning fluid from the buffer chamber. This operation continues to all wheels until no wheel is in danger of locking up.
A fail-safe function in the control module reverts the system to normal braking operation if a malfunction is detected. The control module stores malfunction codes in its internal memory to aid in diagnosis. If a failure is present, the ABS warning lamp will illuminate.