System Components
SYSTEM COMPONENTS - TRW EBC 125^ Controller antilock brake (CAB)
^ Hydraulic control unit (HCU)
^ One wheel speed sensor/tone wheel assembly
^ ABS warning indicator (amber)
^ Brake warning indicator (red)
^ Brake fluid level switch
^ 4WD input (if equipped)
^ Brake lamp switch
^ Fuses and wiring
TRW EBC 125 CONTROLLER ANTILOCK BRAKE (CAB)
The CAB is mounted directly to the hydraulic control unit (HCU) that includes a microprocessor and two solenoids that control the valves that control brake pressure during RWAL braking and circuits that:
^ Monitor the brake switch input to tell whether or not to prepare for possible antilock braking
^ Monitor the brake fluid level switch input to tell whether or not the state of the hydraulics has a problem
^ Monitor the wheel speed sensor input to determine when a rear wheel is tending to lock up
^ Operate the integral hydraulic control unit (HCU) during antilock braking based on comparing the speed sensor to information programmed in memory.
^ Detect RWAL system related problems and take diagnostic action
^ Able to execute self-tests and output control commands
SYSTEM COMPONENTS - TRW EBC 325
^ Controller antilock brake (CAB)
^ Hydraulic control unit (HCU)
^ Three wheel speed sensor/tone wheel assemblies
^ ABS warning indicator (amber)
^ Brake warning indicator (red)
^ Brake fluid level switch
^ 4WD input (if equipped)
^ Brake lamp switch
^ Fuses and wiring
TRW EBC 325 CONTROLLER ANTILOCK BRAKE (CAB)
The CAB is mounted directly to the hydraulic control unit (HCU) that includes a microprocessor and six solenoids that control valves that control brake pressure during antilock braking. The CAB also has circuits that monitor the following:
^ Brake switch input is monitored to determine whether or not to prepare for possible ABS braking
^ Monitor the brake fluid level switch input to tell whether or not the state of the hydraulics has a problem
^ Wheel speed sensors are monitored to determine when a wheel is tending to lock up. The CAB will operate the valves in the HCU to control braking pressure during ABS braking
^ Detect ABS system related problems and take diagnostic action
^ Able to execute self-tests and output control commands
ABS WARNING INDICATOR AND RED BRAKE WARNING INDICATOR (EBC 125 AND EBC 325)
The system is equipped with amber ABS and a red Brake warning indicator to alert the driver of a malfunction it has detected. The CAB can signal the operation of both the amber ABS warning indicator and the red Brake warning indicator via PCI Bus. The CAB controls the ABS warning indicator by making it do one of three things:
^ Light steady for 3.4 seconds during an initial test at the beginning of an ignition cycle to function as a bulb check.
^ Light steady when a system malfunction exists.
Flash if a vehicle tooth or tire parameter is faulty or missing.
The Instrument Cluster controls the amber antilock Brake system-warning indicator. All Dodge Truck Instrument Clusters have direct control over the ABS warning indicator and Brake warning indicator. The Instrument Cluster transmits a message over the PCI BUS relating to diagnostics and current lamp status for the ABS and Brake indicator. The Instrument Cluster will expect a PCI BUS message back from the CAB indicating whether the indicator should be illuminated on or off. In the event that no signal is received from the CAB for a certain amount of time, the Instrument Cluster will illuminate the ABS warning indicator. The indicator provides notice of ABS system-related problems and the need to take diagnostic action.
The vehicle Instrument Cluster has a red Brake warning indicator to alert the driver to the following conditions:
^ Brake fluid level low
^ Parking Brake applied
^ ABS system malfunction
The Instrument Cluster controls the red Brake warning indicator. There is a parallel path to chassis ground that will illuminate the red Brake warning indicator:
^ Through the parking Brake switch contacts
^ Through the Brake fluid level switch
^ Through the CAB via PCI BUS
The CAB can control the operation of both the amber ABS warning indicator and the red Brake warning indicator:
If the CAB is disconnected, the amber ABS warning indicator and red Brake warning indicator will be illuminated
^ If the axle type or tire size are not programmed properly, the amber indicator will blink
^ If any Foundation Brake system problem exists
^ If any ABS system DTC is set, both the amber and red indicators will be illuminated.
HYDRAULIC CONTROL UNIT
The HCU on the EBC 125 has an integral valve body for controlling the rear Brakes. The HCU on the EBC 325 has an integral valve body for controlling the front and rear Brakes.
Within the HCU are solenoids, valves, check valves, and a reset switch necessary to apply and release brake pressure as required to avoid wheel lockup, keep the wheels rolling, and maintain optimum deceleration.
The Isolation Valve(s) is/are normally open, allowing unrestricted flow from the master cylinder to the wheels. When the CAB determines antilock intervention is required, the valve(s) close to isolate the master cylinder hydraulic circuit(s). Fluid is trapped in the circuit(s) and then prevented from reaching the wheels.
The Dump Valve(s) is/are pulsed on and off by the CAB. The valve(s) cycles only if the isolation valve is closed. When the dump valve is on, it allows fluid to the low-pressure accumulator for temporary storage. This causes the pressure to the wheel to decrease. When the dump valve is off, fluid is allowed to the wheel.
The Brake Return Check Valve allows the HCU to drain faster after the antilock activation, when the brake is released.
The Reset Switch is used in the EBC 125 (RWAL) system only. The switch is positioned in the HCU to monitor the master cylinder (input), rear brake (output) and accumulator pressure. During normal braking, pressure is equal for input and output and the switch remains open. During an antilock stop the switch will close as the isolation valve is cycled and pressure becomes unequal between input and output. At the end of an antilock stop the isolation valve will open as the master cylinder pressure and rear pressure equalize. The switch is used to monitor correct operation of the HCU and to set a DTC if incorrect pressure is detected on the EBC 125 (RWAL) antilock system when not in an ABS event. When the brakes are applied, fluid is forced from the master cylinder outlet port(s) to the HCU inlet port(s). This pressure is transmitted through normally open isolation valve(s) inside the HCU, then through the outlet port(s) of the HCU to the wheels. If the CAB senses that a wheel is about to lock based on wheel speed sensor data, it pulls the normally open isolation valve closed for that circuit. This prevents any more fluid from entering that circuit. The CAB continues to look at the wheel speed sensor(s) signal to determine if the wheel is still decelerating. If deceleration is still taking place, the normally closed dump valve for that circuit is opened. This action dumps any pressure that is trapped between the normally open isolation valve and the brake back into an accumulator. Once the affected wheel comes back up to speed, the CAB returns the valve(s) to their normal condition allowing the affected brake to be reapplied. On the EBC 325 system, there is a pump/motor unit, two accumulators, three isolation valves, and three dump valves which are used to provide rapid response during the reapply sequence and to minimize pedal feedback due to the increased hydraulic circuits.