Controller - Antilock Brake

The antilock brake controller (CAB) is a microprocessor-based device that monitors wheel speeds and controls the antilock and traction control 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 traction control function and turn on the warning lights.

There are two controllers. One is used for ABS only and one is used for ABS and traction control (LTCS) combined. Although they both look identical, they are functionally different. If the wrong controller is installed, the system will be disabled and a trouble code will be stored. The 1995 ABS controllers without LTCS have white part labels. The 1995 ABS controller with LTCS has an orange part label. The part numbers are also different.

The CAB on the LH is located in the engine compartment, on the left front inner frame rail. The CAB is connected directly to the modulator assembly by a 14-way electrical connector. The CCD bus [1][2]data link connector is located under the dash, to the left of the steering column.

The primary functions of the CAB are to:

^ detect wheel spin tendencies
^ detect wheel locking tendencies
^ 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 one of two methods. For the Mark IV (LTCS) system, during an antilock stop, the brake pedal position is maintained within a controlled region by the pedal travel sensor (PTS) and the pump/motor. The PTS has seven voltage steps. Once an antilock control cycle has been initiated, the CAB will activate the pump/motor when the PTS level increases one step and will turn off the pump/motor once original position is reached again. For the Mark IVG system, pedal position is controlled 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 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 lamp. If the antilock function is disabled, the system will revert to standard base brake system operation.

The CAB inputs include the following:

^ ABS warning lamp voltage
^ diagnostic communication (CCD)
^ fluid level switch # 2 (LTCS only)
^ four wheel speed sensors
^ ignition switch
^ main relay voltage
^ pedal travel sensor (LTCS only)
^ pump/motor sensor signal
^ stop lamp switch
^ traction control switch (LTCS only)

The CAB outputs include the following:

^ ABS warning light actuation brake warning light actuation
^ diagnostic communication
^ eight valve/solenoid drivers (ten with traction control)
^ main relay actuation
^ pump/motor relay actuation
^ traction control active lamp actuation
^ traction control warning lamp actuation