System Components

SYSTEM COMPONENTS
ABS
^ controller antilock brake (CAB)
^ vacuum booster
^ master cylinder (w/center valves)
^ hydraulic control unit (HCU)
^ valve block assembly:
- 8 valve solenoids (4 inlet valves, 4 outlet valves)
^ pump/motor assembly:
- 1 motor
- 2 pumps
^ 4 wheel speed sensor/tone wheel assemblies
^ ABS warning indicator
^ fuses and wiring harness
^ fluid reservoir (integral part of master cylinder assembly)

PEDAL FEEL/VEHICLES CHARACTERISTICS
There are several pedal feel/vehicle characteristics that are considered normal for antilock braking that may require further explanation.

When stopping conditions activate the antilock brakes, the driver may feel some vibrations/ pulsations in the brake pedal and may hear the solenoid valves clicking and the pump motor running. The vibrations/pulsations are caused by the isolating, building and decaying of brake fluid pressure within the brake lines. The ABS system prevents complete wheel lock-up, but some wheel slip is required for the best braking performance. This slip may result in some tire chirping, depending on the road surface. This chirping should not be interpreted as total wheel lock-up. Total wheel lock-up leaves black tire marks on dry pavement. Antilock braking may leave some light marks.

At the end of an ABS stop, the ABS may function all the way down to near 0 km/h (0 mph). There may be a slight brake pedal drop anytime the ABS is deactivated.

In case of braking on a bumpy surface, the ABS system may detect wheel locking tendencies due to wheel hop and cycle the ABS. In that event the brake pedal may pulsate with a perceived loss of deceleration. ABS braking may also be activated at times while on dry pavement with sand, gravel, or other loose debris on the road.

It should be noted that the pulsating pedal feel characteristic will not illuminate the brake warning lamps or set a trouble code that is stored in the Controller Antilock Brake (CAB). When investigating a hard pedal feel, inspect the sensor and tone wheel teeth for chips/broken teeth, damaged sensor pole tips, excessive runout of the tone wheel, or excessive air gap.

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. The CAB controls the lamp indirectly The CAB monitors its own functions. If the CAB determines that the ABS warning 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 indicator will remain lit during every key cycle until the 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 Indicators.

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

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 CAB indicates an Electronic Brake Distribution (EBD) failure.

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
^ detect wheel slip
^ control fluid pressure modulation to the brakes during antilock stop and traction control operation
^ 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 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 proper operation. If the CAB detects a system malfunction, it can disable the antilock system and turn on the antilock warning lamp. 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 (PCI BUS)

The CAB outputs include the following:
^ eight valve/solenoid drivers
^ pump/motor actuation
^ ABS warning indicator actuation (PCI BUS)
^ red brake warning indicator actuation (PCI BUS)
^ 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 inlet valves, outlet valves, and shuttle 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 may turn 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 wheel. The sensor has internal circuitry powered by 12 volts from the controller antilock brake (CAB). The sensor generates and sends a DC voltage signal back to the CAB. The signal is toggled in proportion to the speed of the toothed tone wheel as it passes the sensor pole. The CAB uses the signal to activate ABS functions as required.

Because of the internal circuitry, correct sensor function cannot be determined by a resistance check across the pins of the sensor.

The front wheel sensors are attached to a boss in the steering knuckle. The tone wheels are an integral part of the front axle shaft. The rear speed sensors are mounted in the caliper adapter plate and the rear tone wheels are an integral part of the rear rotor hubs. The wheel speed sensor air gap is NOT adjustable.

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 correct factory mini-spare.