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
- G (acceleration) switch
- 1 proportioning valve
- 4 wheel speed sensor/tone wheel assemblies
- ABS warning indicator
- fuses and wiring harness
- fluid reservoir (integral part of master cylinder assembly)

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 due to a system malfunction. On a TJ model the warning indicator is controlled by the CAB and the ABS relay. The CAB controls the indicator by directly grounding the ABS relay control circuit. The ABS relay grounds the indicator circuit when it is de-energized.

The red brake warning indicator is located in the instrument cluster. It can be activated by application of the parking brake, low brake fluid level or by turning the ignition switch to the start position.

CONTROLLER ANTILOCK BRAKE (CAB)
The controller antilock brake (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 modulation to the brakes
- during antilock stop operation
- monitor the system for proper operation
- provide communication to the DRBIII while in diagnostic mode
- control the ABS relay
- 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 the HCU 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 three 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:
- four wheel speed sensors
- brake lamp switch
- ignition switch
- battery voltage
- diagnostic communication
- G switch (acceleration switch)

The CAB outputs include the following:
- six valve/solenoid drivers
- pump/motor actuation
- ABS warning indicator control
- diagnostic communication

HYDRAULIC CONTROL UNIT
The hydraulic control unit (HCU) contains the valve block assembly, three accumulators, 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 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.
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 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. In operation, one piston draws fluid from the accumulators. The opposing piston pumps fluid to the valve body solenoids. The CAB monitors the pump/motor operation.
Accumulators: The accumulators provide temporary fluid storage during an antilock stop and are drained by the pump/motor.

RELAYS/SWITCHES
Relays: The ABS Relay is located in the power distribution center (PDC). When the relay is deenergized, it supplies a ground to turn on the ABS warning indicator. The ABS relay is controlled by the CAB. The CAB energizes the relay when it receives an ignition input signal.

SENSORS
Wheel Speed Sensors and Tone Wheels: One wheel speed sensor (WSS) is located at each wheel and sends a small AC signal to the Controller Antilock Brake (CAB). This voltage is generated by magnetic induction when a toothed sensor ring (tone wheel) passes by a stationary magnetic sensor (wheel speed sensor). The CAB converts the AC signals into 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 (rear disc only) and the rear tone wheel is an integral part of the rear rotor hub. The front wheel speed sensor air gap is NOT adjustable. The correct front wheel speed sensor air gap is from 0.40 mm to 1.3 mm (0.016" to 0.051"). Preferred rear sensor air gap for drum brake systems is 1.1 mm (0.043"). Acceptable air gap range is 0.92 mm to 1.275 mm (0.036" to 0.050"). All wheel speed sensors have a resistance between 900 and 1300 ohms.

Correct antilock system operation is dependent on correct wheel speed signals from the wheel speed sensors. The vehicle's wheels and tires should all be the same size and type. 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) Switch: The CAB monitors the acceleration switch at all times. The switch assembly contains three mercury switches that monitor vehicle deceleration rates (G-force). Sudden, rapid changes in vehicle and wheel deceleration rate trigger the switch, sending a signal to the CAB. The switch assembly responds to three deceleration rates; two for forward braking and one for rearward braking.