System Components

SYSTEM COMPONENTS
ABS
^ controller antilock brake (CAB)
^ vacuum booster master cylinder
^ ABS integrated electronic control module/hydraulic control unit (HCU), valve block assembly: 8 valve solenoids (4 inlet valves, 4 outlet valves, 2 accumulators) 1 pump.
^ 4 wheel speed sensor/tone wheel assemblies
^ ABS warning indicator
^ fuses and wiring harness
^ fluid reservoir

ABS With Traction Control
^ CAB with Traction Control programming
^ HCU with two additional control valves.
^ TRAC ON/OFF Switch
^ TRAC/TRAC OFF indicators

ABS AND BRAKE WARNING INDICATORS
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. The CAB controls the lamp with a command over the PCI bus.

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 and TRAC OFF Indicators.

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.

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 ABS amber warning indicator.

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
^ manage traction control functions
^ 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 to modulate brake fluid pressure in its hydraulic unit. Brake pedal position is maintained during an antilock stop by being a closed system. 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
^ three power feeds: valve, pump, and microprocessor
^ brake switch
^ traction control switch The CAB outputs include the following:
^ ABS warning indicator actuation
^ 12 volts power to wheel speed sensors eight valves
^ ten valves with traction control
^ diagnostic communication
^ PCI bus communication
^ traction control lamp illumination

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

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 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 vehicles, 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 event 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 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 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 CAB monitors the pump motor operation internally.

ABS SWITCHES/SENSORS
Master Cylinder: The master cylinder is a standard tandem compensating port design for ABS and non ABS systems. Traction control vehicles use a dual center port master cylinder. For proper traction control operation 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 indicator by grounding the indicator circuit. This switch does not disable the ABS system.

Wheel Speed Sensors and Tone Wheels: One active wheel speed sensor (WSS) is located at each wheel and sends a small DC signal to the control module (CAB). This signal is generated when a toothed sensor ring (tone wheel) passes by a stationary wheel speed sensor. The CAB converts the signals for each wheel.

Because of internal circuitry, correct wheel speed sensor function cannot be determined by a continuity or resistance check through the sensor.

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 speed sensor is mounted though the bearing cover and the rear tone wheel is an integral part of the rear bearing 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 constant velocity (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 pressure 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 correct factory mini-spare.

ABS 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, check wheel speed sensor circuitry, and run the pump motor 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, the driver 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 indicator and TRAC OFF lamp if equipped.

ABS 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 DRBIII. The following are characteristics of diagnostic mode:
- The amber ABS warning indicator will blink rapidly. If a hard trouble code, such as Valve Power Feed Failure code is present, the indicator will be illuminated without blinking until the trouble condition is cleared.
- Antilock operation is disabled.
- The HCU valves cannot be actuated 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 DRBIII.

TRACTION CONTROL OPERATION (IF EQUIPPED)
The Controller Antilock Brake (CAB) monitors wheel speeds. If, during acceleration, the module detects front (drive) wheel slip and the brakes are not applied, the CAB 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 is similar to the 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 excess fluid volume to return to the reservoir when not used by the build/decay cycles. These are required because the pump supplies more volume than the traction control system requires.

If at any time the brake pedal is applied during a traction control cycle, the brake lamp switch will trigger the CAB 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. Only the "TRAC" portion of the "TRAC OFF" indicator will illuminate during a traction control event.

If the CAB calculates that the brake temperatures are high, the traction control system will become inoperative until a time-out period has elapsed. When in this thermal protection mode, the traction control "TRAC OFF" lamp will illuminate; however, a fault will not be registered.