Antilock Brake System Description

TEVES MARK 20 SYSTEM DESCRIPTION
The controller antilock brake module is used to monitor wheel speeds and to modulate (control) hydraulic pressure in each brake channel. The modulated hydraulic pressure is used to prevent wheel lock-up during braking.

The Teves Mark 20 system uses a diagonal split hydraulic brake system. In the standard brake mode the master cylinder primary circuit supplies pressure to the right front and left rear wheel brakes, and the secondary master cylinder circuit supplies pressure to the left front and right rear wheel brakes.

During an antilock stop the Mark 20 system uses four-channel operation. This means that during antilock operation each wheel brake is independently controlled. By using separate hydraulic channel for each wheel, the system is able to retain directional stability and steering control while applying maximum braking. The system provides maximum braking even when road conditions vary.

TRACTION CONTROL SYSTEM (TCS) DESCRIPTION
The Traction (control System is available on this vehicle. The main purpose of traction control is to reduce wheel slip and maintain traction at the driven wheels when road surfaces are wet or snow covered. The traction control system reduces wheel slip by braking the wheel that is losing traction. The system is designed to operate at speeds below 56 km/h (35 mph).

The control module monitors wheel speed. If during acceleration the module detects front (drive) wheel slip and the brakes are not applied, the control module will enter traction control mode. Traction control works in the following order when drive wheel slip is detected.
1. Close the (normally open) isolation valves.
2. Start pump/motor and supply volume/pressure to front hydraulic circuits (pump runs continuously during traction control).
3. Open and close build and decay valves to maintain minimum wheel slip and maximum traction.

The cycling of the build and decay valves works similar to ABS except that they work to control wheel spin by applying brakes. ABS function is to control wheel skid by releasing brakes.

Two pressure relief valves allow pressure/volume to return to the reservoir when not consumed by the build/decay cycles. These are required because the pump supplies more volume than the system requires.

If at any time the brakes are applied during a traction control cycle, the brake switch will trigger the control module to switch off the traction control.

The traction control system will be enable at each ignition cycle. It may be turned off by depressing the traction control switch. The traction control system function lamp will illuminate "TRAC OFF" immediately upon depressing the traction control switch button. The lamp will display "TRAC ON" during a traction control cycle. If the controller calculates that the brake temperatures are high, the traction control system will become inoperative until a time-out period has elapsed. When in this thermo protection mode, the traction control "TRAC OFF" lamp will illuminate; however, a fault will not be registered.

SYSTEM COMPONENTS
^ controller antilock brake (CAB)
^ vacuum booster
^ master cylinder
^ ABS integrated control unit hydraulic control unit (HCU), valve block assembly: 8 valve solenoids (4 inlet valves, 4 outlet valves, 2 accumulators) 1 motor, 2 pumps.
^ ABS integrated control unit with traction control same as above but has 10 valve solenoids. 2 valves are isolation valves.
^ 2 proportioning valves (LR, RR)
^ 4 wheel speed sensor/tone wheel assemblies
^ ABS warning indicator fuses and wiring harness
^ fluid reservoir