Controller Antilock Brake (CAB)

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 1993 and 1994 ABS controllers without LTCS have white part labels. The 1993 ABS controller with LTCS has a blue part label. The 1994 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 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 DRB 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. In order to increase pressure at the wheel brake again, the master cylinder is opened to the wheel brake, which causes the pedal to drop. 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. The CAB continues to control pressure in individual hydraulic circuits until a wheel locking tendency is no longer present.

Because the CAB is continuously monitoring wheel speed, it can also detect wheel spin (traction loss) conditions. If the CAB detects drive wheel spin and the brakes are not applied, it will enter the "traction control" mode. When a wheel spin condition is detected, the CAB will close the normally open traction control valves. When the traction control valves are closed, the master cylinder primary and secondary chambers are isolated from the drive wheel brake channels. The pump motor is started to supply fluid volume/pressure to the drive wheel brake channels. The pump runs continuously during traction control operation. The inlet and outlet valves are opened and closed to modulate the hydraulic pressure in the appropriate brake channel. By cycling the inlet and outlet valves, the CAB can control wheel spin by applying the brake on the wheel that has lost traction.

The antilock brake and traction control systems are constantly monitored by the CAB for proper operation. If the CAB detects a malfunction, it can disable the antilock and traction control functions and turn on the warning lamps. 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
^ stop lamp switch
^ pedal travel sensor
^ ignition switch
^ main relay voltage
^ pump/motor sensor signal
^ diagnostic communication (CCD)
^ fluid level switch # 2
^ traction control switch
^ pressure switch (1993 only)
^ ABS warning lamp voltage

The CAB outputs include the following:
^ eight valve/solenoid drivers (ten with traction control)
^ main relay actuation
^ pump/motor relay actuation
^ antilock light actuation
^ brake warning light actuation
^ traction control warning lamp actuation
^ traction control active lamp actuation
^ diagnostic communication