System Components
SYSTEM COMPONENTS^ Controller Antilock Brake (CAB)
^ vacuum booster
^ master cylinder
^ ABS integrated control unit Hydraulic Control Unit (HCU), 1 motor pump.
^ ABS integrated control unit with traction control same as above but has 10 valve solenoids. 2 valves are isolation valves.
^ 4 wheel speed sensor/tone wheel assemblies
^ ABS warning indicator
^ fuses and wiring harness
^ fluid reservoir
^ brake lamp switch
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 indirectly controlled by the CAB. The CAB will send a message across the PCI Bus that informs the instrument cluster to turn the ABS warning indicator ON or OFF. If the 24-way CAB connector is not connected, the instrument cluster will not receive a message from the CAB via the PCI BUS circuit. The instrument cluster will then 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 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 an antilock stop
^ 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 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 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 lamp switch
^ traction control switch (if equipped)
The CAB outputs include the following:
^ ABS warning indicator actuation
^ valve actuation
^ diagnostic communication
^ traction control indicator illumination (if equipped)
HYDRAULIC CONTROL UNIT
The Hydraulic Control Unit (HCU) contains the valve block assembly, and pump/motor assembly.
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. The CAB monitors wheel speeds. If the CAB detects a wheel deceleration that is disproportionate to the other wheels, it will close the inlet valve to that wheel. This prevents any increase in fluid pressure. If the wheel continues to decelerate disproportionately, the CAB opens the outlet valve for that wheel to release fluid pressure from that channel. The released fluid is routed to the accumulators. When the wheel speed is no longer disproportionate to the other wheels, the inlet valve will return to its normally open position and the outlet valve will return to the normally closed position. 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 at a slipping drive wheel 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 required. 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: All vehicles including Traction control vehicles use a dual center port master cylinder.
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. The sensors use an electronic principle known as magneto-resistive to help increase performance, durability and low speed accuracy. The sensors convert wheel speed into a small digital sound. A toothed gear tone wheel serves as the trigger mechanism for each sensor.
The CAB sends 12 volts to power an Integrated Circuit (ICC) in the sensor. The IC supplies a constant 7 mA signal to the CAB. The relationship of the tooth on the tone wheel to the permanent magnet in the sensor, signals the IC of the sensor to toggle a second 7 mA power supply on or off. The output of the sensor, sent to the CAB, is a DC voltage signal with changing voltage and current levels. The CAB monitors the changing amperage (digital signal) from each wheel speed sensor. The resulting signal is interpreted by the CAB as the wheel speed.
Because of internal circuitry, correct wheel speed sensor function cannot be determined by a continuity or resistance check through the sensor.
Correct antilock system operation is dependent on accurate 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 tires 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.
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, 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 depressed 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 depressed 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 TCS indicator (if equipped).
DIAGNOSTIC MODE
To enter diagnostic mode, a vehicle speed must be below 10 km/h (6 mph) and no ABS/TCS 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 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.
- Valve actuation with the DRBIII(D is disabled 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.