Teves Mark 20 System Description
Teves Mark 20 System DescriptionThe 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
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. Antilock Brake System (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 enabled 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" or "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 pump
- ABS Integrated Control Unit with Traction Control same as above but has 10 Valve Solenoids
- 2 Proportioning Valves (LR, RR)
- 4 Wheel Speed Sensor (WSS)/tone wheel assemblies
- ABS Warning Lamp
- Fuses and Wiring Harness
- Fluid Reservoir
ABS and Brake Warning Lamps
The amber Antilock Brake System (ABS) warning lamp is located in the information center. It is used to inform the driver that the antilock function has been turned off.
The ABS warning lamp is controlled by the Control Antilock Brake (CAB). If the 25 way CAB connector is not connected, the Body Control Module (BCM) will not receive a message from the CAB via the CCD bus and the BCM will send a message to the instrument cluster to turn on the ABS warning lamp.
The red BRAKE warning lamp is also located in the information center. It can be activated in several ways. Application of the parking brake or a low fluid signal from the fluid level switch located in the master cylinder reservoir will cause the lamp to come on.
Controller -- Antilock Brake (CAB)
The Controller Antilock Brake (CAB) is a microprocessor-based device that monitors wheel speeds and controls the antilock functions. The CAB contains two microprocessors that receive identical sensor signals and then independently process the information. The results are then compared to make sure that they agree. Otherwise, the CAB will turn off the antilock and turn on the Antilock Brake System (ABS) amber warning lamp.
The primary functions of the CAB are to:
- detect wheel locking tendencies
- control fluid pressure modulation to the brakes during antilock stop
- monitor the system for proper operation
- provide communication to the DRB while in diagnostic mode
- store diagnostic information in non-volatile memory
The CAB continuously monitors the speed of each wheel. When a wheel locking tendency is detected, the CAB will command the appropriate valve to modulate brake fluid pressure in its hydraulic unit. Brake pedal position is maintained during an antilock stop by being a closed system with the use of 2 accumulators. The CAB continues to control pressure in individual hydraulic circuits until a wheel locking tendency is no longer present. The CAB turns on the pump motor during an antilock stop.
The antilock brake system is constantly monitored by the CAB for proper operation If the CAB detects a system malfunction, it can disable the antilock system and turn on the ABS warning lamp. If the antilock function is disabled, the system will revert to standard base brake system operation.
CAB Inputs Include The Following:
- ABS Warning Lamp
- Diagnostic Communication
- Four Wheel Speed Sensors
- Ignition Switch
- Fused B+
- Stop Lamp Switch
- Traction Control Switch
CAB Outputs Include The Following:
- ABS Warning Light Actuation
- Voltage
- Eight Valves
- Ten Valves With Traction Control
- Diagnostic Communication
- Body Controller Communication
- Traction Control Lamp Illumination
ICU Mounting Location:
Hydraulic Control Unit (HCU)
General Information
The Hydraulic Control Unit (HCU) contains the valve block assembly, two accumulators, and pump/motor assembly. The HCU is mounted on the crossmember over the steering gear on the right side.
Valve Block AssemblY
The valve block assembly contains valves with four inlet valves and four outlet valves. The inlet valves are spring-loaded in the open position and the outlet valves are spring loaded in the closed position. During and antilock stop, these valves are cycled to maintain the proper slip ratio for each wheel. If a wheel detects slip, the inlet valve is closed to prevent and further pressure increase. Then the outlet valve is opened to release the pressure to the accumulators until the wheel is no longer slipping. Once the wheel is no longer slipping, the outlet valve is closed and the inlet valve is opened to reapply pressure. If the wheel is decelerating within its predetermined limits (proper slip ratio), the inlet valve will close to hold the pressure constant. On JX and NS Bodies which are equipped with a traction control system, there are two additional valves that isolate the master cylinder and rear wheels. During a traction control situation the brakes are applied to reduce wheel slippage.
Pump/Motor Assembly
The pump/motor assembly provides the extra amount of fluid needed during antilock braking. The pump is supplied fluid that is released to the accumulators when the outlet valve is opened during and antilock stop. The pump is also used to drain the accumulator circuits after the antilock stop is complete. The pump is operated by an integral electric motor. This DC-type motor is controlled by the Control Antilock Brake (CAB). The CAB may turn on the pump motor when an antilock stop is detected. The pump continues to run during the antilock stop and is turned off after the stop is complete. Under some conditions, the pump/motor will run to drain the accumulators during the next drive off. The pump mechanism consists of two opposing pistons operated by an eccentric camshaft secondary hydraulic circuit. In operation, one piston craw fluid from the accumulators. The opposing piston pumps fluid to the valve body solenoids. The CAB monitors the pump/motor operation internally.
Accumulators
The accumulators provide temporary fluid storage during an antilock stop and are drained by the pump/motor. Each of the diagonal circuits uses a 3cc. accumulator.
MASTER CYLINDER
The master cylinder is a standard tandem compensation port design for ABS and non ABS systems. Traction control vehicles use a dual center port master cylinder. For proper traction control the standard master cylinder must not be used.
A fluid level switch is located in the master cylinder fluid reservoir. The switch closes when a low fluid level is detected. The fluid level switch turns on the brake warning lamp by grounding the lamp circuit. This switch does not disable the ABS system.
WHEEL SPEED SENSORS AND TONE WHEELS
One Wheel Speed Sensor (WSS) is located at each wheel and sends a small AC signal to the control module Controller Antilock Brake (CAB). This voltage is generated by magnetic induction when a toothed sensor ring (tone wheel) passes by a stationary magnetic sensor (wheel speed sensor). The CAB converts the AC signals into digital signals for each wheel.
The front wheel speed sensor is attached to a boss in the steering knuckle. The tone wheel is an integral part of the front axle shaft.
The rear sensors are mounted to the rear brake backing plates or rear disc adapters. The rear tone wheels is pressed on the rear hub. The wheel speed sensor air gap is not adjustable. Refer Wheel Speed Sensor air gap and resistance specifications.
The four wheel speed sensors are serviced individually. The front tone wheels are serviced as an assembly with the outer C.V. joint housing. The rear tone wheels are serviced as an assembly.
Correct antilock system operation is dependent on tone wheel speed signals from the wheel speed sensors. The vehicle's wheels and tires should all be the same size and type to generate accurate signals. In addition, the tires should be inflated to the recommended pressures for optimum system operation. Variation in wheel and tire size or significant variations in inflation pressure can produce inaccurate wheel speed signals; however, the system will continue to function when using the mini-spare.
DIAGNOSTIC MODE
To enter diagnostic mode, a vehicle speed must be below 10 km/h (6 mph) and Antilock Brake System (ABS) condition present. If vehicle speed is not below 10 km/h (6 mph), a "No Response" message could be displayed by the DRB. The following are characteristics of diagnostic mode:
- The amber ABS warning lamp will blink rapidly (about 1/2 second ON and 1/2 second OFF). If a hard trouble code, such as CAB Power Feed Circuit Failure code is present, the lamp will be illuminated without blinking until the trouble condition is cleared.
- Antilock operation is disabled.
- You will be unable to actuate the valves in the valve body when the vehicle speed is above 8 km/h (5 mph). If valve actuation is attempted above 8 km/h (5 mph), a "No Response" message will be displayed through the DRB.
SYSTEM INITIALIZATION SELF TESTS
System initialization starts when the key is turned to "run". At this point, the CAB performs a complete self-check of all electrical components in the antilock systems.
Between 8 - 17 km/h (5 - 10 mph), a dynamic test is performed. This will momentarily cycle the inlet and outlet valves and run the pump/motor at 25 km/h (15 mph), the Controller Antilock Brake (CAB) will try to test the pump/motor. If the driver has their foot on the pedal the test will be run at 40 km/h (24 mph) regardless of brake switch state. If, during the dynamic test, the driver has his/her foot on the brake pedal, he/she may feel the test through brake pedal pulsations. This is a normal condition.
If any component exhibits a trouble condition during system initialization or dynamic check, the CAB will illuminate the ABS warning lamp and TRAC OFF lamp if equipped.