System Components

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
^ 4 wheel speed sensor/tone wheel assemblies
^ ABS warning indicator
^ fuses and wiring harness
^ fluid reservoir

ABS AND BRAKE WARNING LAMPS
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. The ABS warning indicator is controlled by the CAB. If the 24 way CAB connector is not connected, the BCM will not receive a message from the CAB via the PCI bus and the BCM will send a message to the instrument cluster to turn on the ABS warning indicator.

The ABS Warning Indicator will remain lit during every key cycle until a circuit or component fault is repaired and the CAB no longer detects the fault. After repair of a sensor signal fault or a pump motor fault, the CAB must sense all four wheels at 25 km/h (15 mph) before it will extinguish the ABS Indicator.

The Instrument Cluster will illuminate the ABS Warning Indicator if it loses communication with the CAB.

The red BRAKE warning indicator is also located in the instrument cluster. 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 indicator to come on. The status of the red BRAKE warning indicator is not monitored by the CAB.

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 system and turn on the ABS warning indicator.

The primary functions of the CAB are to:
^ detect wheel locking tendencies
^ control fluid pressure modulation to the brakes during an antilock stop
^ monitor the system for proper operation
^ provide communication to the DRB III 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 via the hydraulic control unit (HCU). 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 indicator. If the antilock function is disabled, the system will revert to standard base brake system operation.

The CAB inputs include the following:
^ diagnostic communication
^ four wheel speed sensors
^ ignition switch
^ fused B+
^ brake switch

The CAB outputs include the following:
^ ABS warning indicator actuation
^ ABS pump control
^ eight valve control signals
^ diagnostic communication
^ body controller communication

HYDRAULIC CONTROL UNIT

The hydraulic control unit (HCU) contains the valve block assembly and pump motor assembly. The HCU and the CAB are mounted as an assembly and are not separately serviceable.

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 an antilock stop, these valves are cycled to maintain the proper slip ratio for each wheel. If the CAB detects slip, the inlet valve is closed to prevent any further pressure increase. If necessary, 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.

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 motor is controlled by the CAB. The CAB will 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 CAB monitors the pump motor operation internally.

SWITCHES/SENSORS

Master Cylinder: A fluid level switch is located in the master cylinder fluid reservoir. The switch closes when a fluid level is detected. The fluid level switch turns on the brake warning indicator by grounding the indicator circuit in the Instrument Cluster. This switch does not disable the ABS system.

Wheel Speed Sensors and Tone Wheels: One Wheel Speed Sensor (WSS) is located at each wheel. Each sensor sends a small digital signal to the control module (CAB). The CAB sends 12 volts to the sensor. The sensor has an internal magneto resistance 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 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 disc adapters. The rear tone wheels are pressed on the rear hub. The wheel speed sensor air gap is not adjustable.

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.

SYSTEM INITIALIZATION
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 check wheel speed sensor circuitry. At 25 km/h (15 mph), the CAB will try to test the pump motor. If the brake pedal is applied, the test will be run at 40 km/h (24 mph) regardless of brake switch state. If, during the dynamic test, the brake pedal is applied, brake pedal pulsations may be felt. 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 indicator.

DIAGNOSTIC MODE
To enter diagnostic mode, a 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 DRB III. The following are characteristics of diagnostic mode:
- The amber ABS warning indicator will blink rapidly. If a hard trouble code, such as CAB Power Feed Circuit Failure code is present, the indicator 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 on the DRB III.