System Components

TRW EBC 2 CONTROLLER ANTILOCK BRAKE (CAB)
The CAB is mounted under the hood, separately from the valve body, and has a microprocessor and circuits that:
^ monitor the input from the brake lamp switch to determine whether or not to prepare for possible antilock braking
^ monitor the wheel speed sensor input to determine when a rear wheel is tending to lock up.
^ operate the integral control unit assembly during antilock braking based on comparing the speed sensor to information programmed in memory.
^ detect ABS system related problems and take diagnostic action

TRW EBC 325 CONTROLLER ANTILOCK BRAKE (CAB)
The CAB is mounted directly to the hydraulic control unit and includes a microprocessor and six solenoids 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 antilock braking.
^ 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 antilock braking.

ABS WARNING INDICATOR AND RED BRAKE WARNING INDICATOR (EBC 2 AND EBC 325)
The system is equipped with an amber ABS and a red brake warning indicator to alert the driver of a malfunction it has detected. The CAB can control the operation of both the yellow ABS warning indicator and the red brake warning indicator via CCD 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.
^ light flashing if a vehicle speed signal 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 cluster will periodically transmit a message over the COD bus relating to diagnostics and current indicator status for the ABS and red brake indicator. The cluster will expect a periodic message back from the CAB indicating whether the indicators should be turned on or off. In the event that no signal is received from the CAB for a certain number of times' the cluster will turn on the ABS warning indicator. The indicator provides notice of 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:
^ parking brake applied
^ hydraulic failure in either front or rear channel
^ ABS system malfunction

The red brake warning indicator is controlled by the vehicle's instrument cluster. There is a parallel path to chassis ground that will illuminate the red brake warning indicator:
^ through the parking brake switch contacts
^ through the combination valve brake warning indicator switch
^ through the CAB via CCD Bus

The CAB can control the operation of both the yellow ABS warning indicator and the red brake warning indicator
^ If the CAB is disconnected, the yellow indicator will be illuminated.
^ If the axle type or tire size are not programmed properly, the yellow indicator will blink.
^ If a system DTC is set, both the yellow and red indicators will be illuminated.
^ If any foundation brake system problem exists.

BRAKE LAMP SWITCH CIRCUIT
An input from this switch prepares the CAB for a possible antilock stop.

The antilock system uses an input signal from the brake pedal actuated switch. With the brake pedal not applied, a ground is fed through the brake lamp switch to the CAB on the Brake Lamp Switch Sense Circuit. When the driver applies the brake pedal the ground signal is interrupted, alerting the CAB of a possible ABS event.

HYDRAULIC CONTROL UNIT
The HCU on the EBC 2 has only a separate rear valve body.

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 closes to isolate the master cylinder hydraulic circuit. Fluid is trapped in the circuit and then prevented from reaching the wheels.

The dump valve(s) is/are pulsed on and off by the CAB. This valve cycles only if the isolation valve is closed. When dump is on, it allows fluid to the low pressure accumulator for temporary storage. This causes the pressure to the wheel to decrease. When the valve is off, fluid pressure is maintained to the wheel.

The brake return check valve allows the HCU to drain faster after the ABS stop, when the brake is released.

The Reset 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 ABS stop the isolation valve will open as master cylinder pressure and rear pressure equalize. The switch is used to monitor correct operation of the HCU and to set a diagnostic trouble code if incorrect pressure is detected.

When the brakes are applied, fluid is forced from the master cylinder outlet ports to the HCU inlet ports. This pressure is transmitted through three normally open isolation valves inside the HCU, then through the outlet ports 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 sensor 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 dumps any pressure that is trapped between the normally open valve and the brake back into an accumulator. Once the affected circuit comes back up to speed, the CAB returns the valves to their normal condition allowing the affected brake to be reapplied. On the EBC 325 system, the pump and accumulators are used to provide rapid response during the reapply sequence and to minimize pedal feedback.

WHEEL SPEED SENSORS
The EBC 325 ABS system uses one wheel speed sensor on each front wheel, and one speed sensor mounted in the rear axle for the rear wheels.

The EBC 2 uses only one speed sensor mounted in the rear axle for the rear wheels.

The sensor measures the wheel speed by monitoring a rotating tone wheel. The signal generated by the sensor and tone wheel is transmitted to the CAB.

Each sensor has:
^ a metal tone wheel with teeth on its outside diameter
^ a magnetic coil pick-up (speed sensor) that is mounted to a fixed component
^ an air gap between the tone wheel and the speed sensor assembly

As the teeth of the tone wheel move through the magnetic field of the sensor, an AC voltage is generated. This signal frequency increases or decreases proportionally to the speed of the wheel. The CAB monitors this signal to check for a sudden change in single or multiple wheel decelerations. If the deceleration of one or more wheels is not within a predetermined amount, the antilock module takes control.

Diagnostically, the coil of wheel speed sensors have different amounts of resistance based upon the sensor type. When measured across the connector's two terminals, the resistance should be:

Front sensors 2500-2955 ohms

Rear sensor 1600-2300 ohms

(NOTE: For all resistance ranges add 30% to the value for extreme heat, subtract 30% for extreme cold.)

The rear wheel speed sensor used on all vehicles have the same resistance value, but there are two different sensors. Therefore, it is important to replace the rear wheel speed sensor with the proper part.

On a EBC 325 system each front wheel speed is monitored through the speed sensor mounted at the wheel end of the hub. On a EBC 326 and EBC 2 system the rear wheel speed is monitored through the speed sensor mounted in the rear axle assembly.

The CAB will disable antilock control, illuminate the ABS warning light circuit via COD, and store trouble codes if it detects any of the following on the wheel speed sensor circuits:
^ incorrect circuit resistance when checked with no vehicle movement
^ incorrect sensor output during vehicle movement
^ erratic sensor output during vehicle movement