System Components

SYSTEM COMPONENTS

ANTILOCK BRAKE SYSTEM
^ controller antilock brake (CAB)
^ vacuum booster
^ master cylinder (w/center valves)
^ hydraulic control unit (HCU)
^ valve block assembly: 6 valve solenoids (3 inlet valves, 3 outlet valves) 3 accumulators
^ pump/motor assembly:
1 motor
2 pumps
^ internal G (acceleration) sensor
^ 1 internal proportioning valve
^ 3 wheel speed sensor/tone wheel assemblies
^ ABS warning indicator
^ 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. On the KJ, the 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 an "are you there" 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 activated 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 brake fluid level switch circuit becomes open or shorted to ground.

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
^ control fluid pressure modulations to the brakes during antilock stop operation
^ control the ABS warning indicator
^ 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 wheel speed sensors. When a wheel locking tendency is detected, the CAB will command the appropriate HCU valve to modulate brake fluid pressure to that wheel. Brake pedal position is maintained during an antilock stop by being a closed system with the use of 3 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 activate 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:
^ three wheel speed sensors
^ brake lamp switch
^ ignition switch
^ battery voltage
^ diagnostic communication (PCI BUS)
^ G-sensor (internal acceleration sensor)

The CAB outputs include the following:
^ six valve/solenoid drivers
^ pump/motor actuation
^ ABS warning indicator actuation
^ 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 6 valves with three inlet valves and three outlet valves. The inlet valves are springloaded 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. 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 disporportionately, 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.
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 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 CAB monitors the pump/motor operation internally.

SENSORS
Wheel Speed Sensors and Tone Wheels: One wheel speed sensor (WSS) is located at each front wheel and another mounted to the rear axle. 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 sensor is attached to the hub housing. The tone wheel is an integral part of the front hub. The rear speed sensor is mounted to the axle housing. The rear tone wheel is bolted to the ring gear inside the rear differential housing. The wheel speed sensor air gap is NOT adjustable. Because of internal circuitry, a resistance check of wheel speed sensors will not determine correct or incorrect function.

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. When driven over rough road surfaces, the rear wheel speed sensor signals may be erratic and cause a false trouble code.

G (Acceleration) Sensor: The CAB monitors the acceleration sensor at all times. The sensor assembly contains three mercury sensors that monitor vehicle deceleration rates (G-force). Sudden rapid changes in vehicle and wheel deceleration rate trigger the sensor, sending a signal to the CAB. The sensor assembly provides three deceleration rates; two for forward braking and one for rearward braking. The G-Sensor is mounted inside the CAB and is not separately serviceable.