Hydraulic Control Assembly - Antilock Brakes: Description and Operation
INTEGRATED CONTROL UNIT (ICU)Integrated Control Unit (ICU):
The hydraulic control unit (HCU) and the controller antilock brake (CAB) used with this antilock brake system are combined (integrated) into one unit, which is called the integrated control unit (ICU). The ICU is located on the driver's side of the vehicle, and is mounted to the left front frame rail below the master cylinder.
The ABS with traction control ICU consists of the following components: the CAB, eight (build/decay) solenoid valves (four inlet valves and four outlet valves), two hydraulic shuttle valves, two traction control valves, valve block, fluid accumulators, a pump, and an electric pump/motor.
The replaceable components of the ICU are the HCU and the CAB. No attempt should be made to service any components found inside of the HCU or CAB.
CONTROLLER ANTILOCK BRAKE (CAB)
The controller antilock brake (CAB) is a microprocessor-based device which monitors the ABS system during normal braking and controls it when the vehicle is in an ABS stop. The CAB is mounted to the bottom of the HCU. The CAB uses a 25-way electrical connector on the vehicle wiring harness. The power source for the CABs through the ignition switch in the RUN or ON position. The CAB is on the PCI bus.
The primary functions of the (CAB) are to:
1. monitor the antilock brake system for proper operation.
2. detect wheel locking or wheel slipping tendencies by monitoring the speed of all four wheels of the vehicle.
3. control fluid modulation to the wheel brakes while the system is in an ABS mode or the traction control system is activated.
4. store diagnostic information.
5. provide communication to the DRB scan tool while in diagnostic mode.
The CAB constantly monitors the antilock brake system for proper operation. If the CAB detects a fault, it will send a message to the mechanical instrument cluster (MIC) instructing it to turn on the amber ABS warning lamp and disable the antilock braking system. The normal base braking system will remain operational.
The CAB continuously monitors the speed of each wheel through the signals generated by the wheel speed sensors to determine if any wheel is beginning to lock. When a wheel locking tendency is detected, the CAB commands the CAB command coils to actuate. The CAB command coils then open and close the valves in the HCU that modulate brake fluid pressure in some or all of the hydraulic circuits. The CAB continues to control pressure in individual hydraulic circuits until a locking tendency is no longer present.
The CAB contains a self-diagnostic program that monitors the antilock brake system for system faults. When a fault is detected, the amber ABS warning lamp is turned on and the fault diagnostic trouble code (DTC) is then stored in a diagnostic program memory. These DTC's will remain in the CAB memory even after the ignition has been turned off. The DTC's can be read and cleared from the CAB memory by a technician using the DRB scan tool. If not cleared with a DRB scan tool, the fault occurrence and DTC will be automatically cleared from the CAB memory after the identical fault has not been seen during the next 3,500 miles of vehicle operation.
CONTROLLER ANTILOCK BRAKE INPUTS
^ wheel speed sensors (four)
^ stop lamp switch
^ ignition switch
^ system relay voltage
^ ground
^ traction control lamp
^ diagnostic communication (PCI)
CONTROLLER ANTILOCK BRAKE OUTPUTS
^ amber ABS warning lamp actuation (through MIC)
^ red BRAKE warning lamp actuation (through MIC)
^ traction control lamp actuation (through MIC)
^ diagnostic communication. (PCI)
HYDRAULIC CONTROL UNIT (HCU)
The hydraulic control unit (HCU) is mounted to the CAB as part of the ICU. The HCU controls the flow of brake fluid to the brakes using a series of valves and accumulators. A pump/motor is mounted on the HCU to supply build pressure to the brakes during an ABS stop.
VALVES AND SOLENOIDS
The valve block contains four inlet valves and four outlet solenoid valves. The inlet valves are spring loaded in the open position and the outlet valves are spring-loaded in the closed position during normal braking. The fluid is allowed to flow from the master cylinder to the wheel brakes.
During an ABS stop, these valves cycle to maintain the proper slip ratio for each wheel. The inlet valve closes preventing further pressure increase and the outlet valve opens to provide a path from the wheel brake to the HCU accumulators and pump/motor. This releases (decays) pressure from the wheel brake, thus releasing the wheel from excessive slippage. Once the wheel is no longer slipping, the outlet valve is closed and the inlet valve is opened to reapply (build) pressure.
There are four other valves in the HCU. These four remaining valves are used for traction control. Two traction control (TC) valves, mounted in the HCU valve block, are normally in the open position and close only when the traction control is applied. There are also two shuttle valves which control pressure return to the master cylinder under ABS and traction control conditions.
These TC valves are used to isolate the rear (nondriving) wheels of the vehicle from the hydraulic pressure that the HCU pump/motor is sending to the front (driving) wheels when traction control is being applied. The rear brakes need to be isolated from the master cylinder when traction control is being applied so the rear wheels do not drag. For more information on the TC and shuttle valves, see Traction Control System; Description and Operation.
BRAKE FLUID ACCUMULATORS
There are two fluid accumulators in the HCU: one for the primary hydraulic circuit, and one for the secondary hydraulic circuit. Each hydraulic circuit uses a 3 cc accumulator.
The fluid accumulators temporarily store brake fluid that is removed from the wheel brakes during an ABS cycle. This stored fluid is used by the pump/motor to provide build pressure for the brake hydraulic system. When the antilock stop is complete, the accumulators are drained by the pump/motor.
There are two noise dampening chambers in the HCU on this vehicle equipped with traction control.
PUMP/MOTOR
There are two pump assemblies in the HCU: one for the primary hydraulic circuit, and one for the secondary hydraulic circuit. Both pumps are driven by a common electric motor. This DC-type motor is integral to the HCU and is controlled by the CAB.
The pump/motor provides the extra amount of brake fluid needed during antilock braking. Brake fluid is released to the accumulators when the outlet valve is opened during an antilock stop. The pump mechanism consists of two opposing pistons operated by an eccentric camshaft. In operation, these pistons are used to purge fluid from the accumulators back into the master cylinder circuits. When the antilock stop is complete, the pump/motor drains the accumulators.
The pump motor is also used to build pressure when the system goes into traction control mode. For more information, refer to Traction Control System.
The CAB may turn on the pump/motor when an antilock stop is detected. The pump/motor continues to run during the antilock stop and is turned off after the stop is complete. Under some conditions, the pump/motor runs to drain the accumulators during the next drive-off.
The pump/motor is not a serviceable item; if it requires replacement, the HCU must be replaced.