System Components

SYSTEM COMPONENTS ANTILOCK BRAKE SYSTEM
^ Integrated Controller Unit (ICU)
^ mounting bracket
^ controller antilock brake (CAB)
^ hydraulic control unit (HCU) pump/motor assembly
^ vacuum booster
^ master cylinder (w/center valves and fluid reservoir)
^ 4 wheel speed sensor/tone wheel assemblies ABS warning indicator (cluster)
^ Trac off indicator (cluster) (if equipped)
^ fuses and wiring harness

ABS AND RED BRAKE WARNING INDICATOR
The amber ABS warning indicator is located in the instrument cluster. It is used to inform the driver that the antilock function has been turned off due to a system malfunction. The Controller Antilock Brake (CAB) controls the lamp indirectly. The CAB monitors its own functions. If the CAB determines that the ABS indicator should be on, the CAB sends a message via the PCI BUS to the instrument cluster and the cluster turns on the indicator. The instrument cluster sends a message over the PCI BUS, if the CAB does not respond the instrument cluster will illuminate the ABS indicator.

The red brake warning indicator is located in the instrument cluster. It can be actuated by application of the parking brake, a leak in the front or rear wheel brake hydraulic circuit which causes the master cylinder reservoir to be low on fluid, or by turning the ignition switch to the start position. The red brake warning indicator can also be turned on if the CAB indicates an Electronic Variable Brake Proportioning (EVBP) failure.

CONTROLLER ANTILOCK BRAKE (CAB)
The antilock brake controller (CAB) is a microprocessor-based device that monitors wheel speeds and controls the antilock functions.

The primary functions of the CAB are:
^ monitor wheel speeds
^ detect wheel locking tendencies
^ detect wheel slip
^ control fluid pressure modulation to the brakes during antilock stop and traction control (if equipped)
^ monitor the system for proper operation
^ provide communication to the DRBIII 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 in its hydraulic unit to modulate brake fluid pressure. 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 the proper operation. If the CAB detects a system malfunction, it can disable the antilock system and turn on the antilock warning indicator. If the antilock function is disabled, the system will revert to standard base brake system operation.

The CAB inputs include the following:
^ four wheel speed sensors
^ brake lamp switch
^ ignition switch
^ battery voltage
^ diagnostic communication (PCI BUS)
^ traction control switch (if equipped)

The CAB outputs include the following:
^ ten valve/solenoid drivers (TCS)
^ eight valve/solenoid drivers (non-TCS) pump/motor actuation
^ ABS warning indicator actuation (PCI BUS)
^ red brake warning indicator actuation (PCI BUS)
^ TRAC OFF lamp (PCI BUS) (if equipped)
^ TRAC lamp (PCI BUS) (if equipped)
^ diagnostic communication (PCI BUS)

HYDRAULIC CONTROL UNIT
The hydraulic control unit (HCU) contains the valve block assembly, and the pump/motor assembly.

Valve Block Assembly: The valve block assembly contains inlet valves, outlet valves, hydraulic shuttle valves, and traction control valves (if equipped). The inlet valves are spring-loaded in the open position and the outlet valves are spring loaded in the closed position. During an antilock stop, these valves are cycled to maintain the proper slip ratio for each channel. If a wheel locks, the inlet valve is closed to prevent any 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), both valves will close to hold the pressure constant. The valves are also cycled during a traction control event (if equipped).

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 an 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 motor is controlled by the Controller Antilock Brake (CAB). The CAB turns on the motor when an antilock stop is detected. The pump continues to run during the antilock stop and is turned off approximately 3-5 seconds after the stop is complete. The pump mechanism consists of two opposing pistons operated by an eccentric cam. One piston supplies the primary hydraulic circuit. The opposing piston supplies the secondary hydraulic circuit. The CAB monitors the pump/motor operation internally.

SENSORS
Wheel Speed Sensors and Tone Wheels: One Wheel Speed Sensor (WSS) is located at each wheel and sends a small digital signal to the Controller Antilock Brake (CAB). The CAB sends 12 volts down to the sensor. The sensor has an internal magnetoresistance bridge that alters the voltage and amperage of the signal circuit. This voltage and amperage is changed by magnetic induction when a toothed sensor ring (tone wheel) passes by a stationary magnetic sensor (wheel speed sensor). The CAB measures the voltage and amperage of the digital signals for each wheel.

The front wheel sensor is attached to a boss in the steering knuckle. The tone wheel is an integral part of the front axle shaft. The rear speed sensor is mounted in the caliper adapter plate and the rear tone wheel is an integral part of the rear rotor hub.
^ The wheel speed sensor air gap is NOT adjustable.
^ Wheel speed sensors cannot be checked for resistance to determine if they are ok.

Correct antilock system operation is dependent on 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. Variations 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.

SYSTEM INITIALIZATION
System initialization starts when the key is turned to "RUN". At this point, the Controller Antilock Brake (CAB) performs a complete selfcheck of all electrical components in the antilock brake systems. The ABS, brake warning lamps, TRAC OFF and TRAC (if equipped) will illuminate for 4 seconds with ignition on.

AT 20 km/h (12 mph) a dynamic test is performed. This will momentarily run the pump/motor. 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 diagnostic trouble code during system initialization or dynamic check, the CAB will illuminate the ABS and TRAC off (if equipped) warning indicators.

DIAGNOSTIC MODE
On a Mark 20e system to enter a diagnostic mode, vehicle speed must be below 10 km/h (6 mph) and no ABS condition present. If vehicle speed is not below 10 km/h (6 mph), a "no response" message could be displayed by the DRBIII 30. The following are characteristics of diagnostic mode:
^ The amber ABS and red brake warning indicator will blink rapidly. If a hard trouble code is present, such as CAB Power Feed Circuit, the indicator will be illuminated without blinking until the diagnostic trouble condition is corrected.
^ Antilock operation is disabled.