Antilock Brakes / Traction Control Systems: Description and Operation

COMPONENTS OF THE ABS
The anti-lock braking system (ABS) components are combined with a vacuum booster and tandem master cylinder (TMC) to provide a two-circuit braking system. The anti-lock braking system comprises the following components:
^ Four inductive wheel speed sensors, hub end mounted
^ ABS warning light
^ Hydraulic module.

NOTE:
^ The hydraulic module consists of an electric motor driven pump, two low pressure accumulators, valve block and an ABS electronic control module.
^ Electronic control modules for vehicles without traction control are designated ABS CM. Control modules for vehicles with traction control are designated ABS/TC CM.

VALVE BLOCK
The valve block houses solenoid operated valves which are activated by voltage signals from the control module. The signals are generated using wheel speed information received from the wheel speed sensors.

CIRCUIT OPERATION
With Traction Control
For vehicles with traction control the valves operate on four circuits. During ABS control the rear wheel are controlled on a 'select low' principle (as above), but during traction control operation the rear wheels are controlled individually.

Less traction control
For vehicles without traction control the valves operate on three circuits, two front and one rear, as necessary to prevent wheel locking during braking. Brake pressure is modulated individually at the front wheels and collectively at the rear. Rear wheel control operates on a "select low" principle i.e. locking in either wheel is sensed, and controlled brake pressure is applied to both wheels.