Electronic Brake Control Module: Description and Operation
INTEGRATED CONTROL UNIT (ICU)DESCRIPTION
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 in front of the driver's side front tire, behind the inner fender splash shield.
Two different ICU's (HCU and CAB) are used on this vehicle depending on whether or not the vehicle is equipped with traction control. The HCU on a vehicle equipped with traction control has a valve block that is approximately one inch longer than a HCU on a vehicle that is equipped with ABS only.
The ABS-only ICU consists of the following components: the CAB, eight (build/decay) solenoid valves (four inlet valves and four outlet valves), valve block, fluid accumulators, a pump, and an electric motor.
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 traction control (ASR) valves, two hydraulic shuttle valves, valve block, fluid accumulators, a pump, and an electric motor.
The replaceable components of the ICU are the HCU and the CAB. No attempt should be made to service any individual components of the HCU or CAB.
OPERATION
^ CONTROLLER ANTILOCK BRAKE (CAB)
^ HYDRAULIC CONTROL UNIT (HCU)
For information on the ICU's hydraulic circuits, refer to Antilock Brake System, Description and Operation.
CONTROLLER ANTILOCK BRAKE (CAB)
DESCRIPTION
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 CAB is through the ignition switch in the RUN or ON position. The CAB is on the PCI bus.
OPERATION
The primary functions of the CAB are to:
^ Monitor the antilock brake system for proper operation.
^ Detect wheel locking or wheel slipping tendencies by monitoring the speed of all four wheels of the vehicle.
^ Control fluid modulation to the wheel brakes while the system is in an ABS mode.
^ Store diagnostic information.
^ Provide communication to the DRB scan tool while in diagnostic mode.
^ Illuminate the amber ABS warning lamp.
^ (with traction control only) Illuminate the TRAC ON lamp in the message center on the instrument panel when a traction control event occurs.
^ (with traction control only) Illuminate the TRAC OFF lamp when the amber ABS warning lamp illuminates.
The CAB constantly monitors the antilock brake system for proper operation. If the CAB detects a fault, it will turn on the amber ABS warning lamp and disable the antilock braking system. The normal base braking system will remain operational.
NOTE: If the vehicle is equipped with traction control, the TRAC OFF lamp will illuminate anytime the amber ABS warning lamp illuminates.
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 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 3,600 miles.
CONTROLLER ANTILOCK BRAKE INPUTS
^ wheel speed sensors (four)
^ brake lamp switch
^ ignition switch
^ system and pump voltage
^ ground
^ traction control switch (if equipped)
^ diagnostic communication (PCI)
CONTROLLER ANTILOCK BRAKE OUTPUTS
^ amber ABS warning lamp actuation (via BUS)
^ instrument cluster (MIC) communication (PCI)
^ traction control lamps (if equipped)
^ diagnostic communication (PCI, via BUS)
HYDRAULIC CONTROL UNIT (HCU)
DESCRIPTION
Teves Mark 20 ICU:
The Hydraulic Control Unit (HCU) is mounted to the CAB as part of the ICU. The ICU is located in front of the driver's side front tire, behind the inner fender splash shield. 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.
The HCU on a vehicle equipped with traction control has a valve block that is approximately one inch longer than a HCU on a vehicle that is equipped with ABS only in order to incorporate the additional valves.
OPERATION
The operation of the HCU's hydraulic circuits can be found in Brakes, Antilock Brake System, Description And Operation, Hydraulic Circuits And Valve Operation.
The following topics explain how the different components within the HCU operate.
VALVES AND SOLENOIDS
The valve block contains four inlet valves and four outlet 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.
For information on the valves used with the traction control system, refer to Traction Control System, Description and Operation.
BRAKE FLUID ACCUMULATORS
Teves Mark 20 ICU:
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 5 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.
PUMP/MOTOR
Teves Mark 20 ICU:
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, one piston draws fluid from the accumulators, and the opposing piston pumps fluid to the master cylinder circuits. When the antilock stop is complete, the pump/motor drains the accumulators.
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.