Electronic Brake Control Module: Description and Operation

TRW EBC 125 SYSTEM DESCRIPTION
The EBC 125 (RWAL) system can be identified by he controller-antilock brake (CAB) and hydraulic control unit (HCU) being an integral electronic/ hydraulic unit. One 14 way connector and mounted near the master cylinder next to the Power Distribution Center (PDC).

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 325 SYSTEM DESCRIPTION
The EBC 325 (ABS) system can be identified by he controller-antilock brake (CAB) and hydraulic control unit (HCU) being an integral electronic/ hydraulic unit with a pump/motor unit. One 14 way connector, one 4 way connector and mounted under he hood in close proximity to the master cylinder next to the Power Distribution Center (PDC).

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 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

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 he 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

BRAKE LAMP SWITCH CIRCUIT
This switch prepares the CAB for a possible antilock stop. The antilock system uses an input signal from the brake pedal switch when activated. A released brake pedal will close the switch circuit and read 0 volts. When the driver applies the brake pedal, the circuit voltage is 12 volts. This signal ells the CAB that the pedal is depressed. The brake lamp switch is located on the brake pedal under the driver dash.

BRAKE FLUID LEVEL SWITCH CIRCUIT
This switch signals the CAB for a hydraulic fluid problem. The antilock system uses input from the brake fluid level switch when activated. A low brake fluid condition in the master cylinder will close the switch circuit and read 0 volts. When the brake fluid level is at specifications, the switch will open and circuit sense voltage will read 5 volts. This signal tells the CAB if there is enough brake fluid in he master cylinder to have an antilock operation.

The brake fluid level switch is located in the master cylinder reservoir with a 10k resistor. The switch is a Hall effect design and uses a float to determine level.

HYDRAULIC CONTROL UNIT
The HCU on the EBC 125 has an integral valve body for controlling the rear Brakes. The HCU on he EBC 325 has an integral valve body for control ling 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 of the wheels. When the CAB determines antilock intervention is required, the valve(s) close to isolate he master cylinder hydraulic circuit(s). Fluid is rapped 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 of 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. he Brake Return Check Valve allows the HCU of drain faster after the antilock activation, when he 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 he switch remains open. During an antilock stop he 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 he 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. his 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.

WHEEL SPEED SENSORS
The EBC 125 (RWAL) system uses only one speed sensor mounted in the rear axle for the rear wheels.

The EBC 325 (ABS) system uses one wheel speed sensor on each front wheel, and one 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 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, and AC voltage is generated. This signal frequency increases or de creases proportionally to the speed of the wheel. he CAB monitors this signal to check for a sudden change in single or multiple wheel decelerations. If he deceleration of one or more wheels is not within a predetermined amount, the CAB takes control for antilock action through the HCU.

Diagnostically, the coils of the wheel speed sensors have different amounts of resistance based upon the location. When measured across the connector two terminals, the resistance should be:

Front sensor:
90°C (194°F) 2259-2761 Ohms
25°C (77°F) 1800-2200 Ohms
-40°C (40°F) 1332-1628 Ohms
Rear sensor:
90°C (194°F) 2900-3500 Ohms
25°C (77°F) 1600-2300 Ohms
-40°C (1-40°F) 1000-1400 Ohms

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

On the EBC 325 (ABS) system each front wheel speed is monitored through the speed sensor mounted at the wheel end of the hub. On the EBC 125 (RWAIL) and EBC 325 (ABS) systems the rear wheel speed is monitored through the speed sensor mounted in the rear axle assembly.

The CAB will disable antilock control, illuminate he amber ABS warning indicator circuit via the PCI BUS, and store diagnostic trouble codes if it detects a problem with any or all of the wheel speed sensors:
^ Incorrect circuit resistance when checked with no vehicle movement
^ Incorrect sensor output during vehicle movement
^ Erratic sensor output during vehicle movement

4WD INPUT
Vehicles equipped with four wheel drive (4WD) have an input to the CAB, which tells whether or not the vehicle is 4WD. This input comes from the 4WD switch status which is sent to the CAB via PCI Bus. While in four-wheel drive, the front and rear axles will operate together. With this input, the CAB is able to modify its operation to allow for 4WD operation. Should a need for antilock operation occur while in four-wheel drive, the CAB will extend he amount of allowable dump valve cycles.

DIAGNOSTIC COMMAND MODES
The system software includes several self-tests hat are performed every time the ignition is turned on and the vehicle is driven. Some of the self-tests occur immediately, while the pump motor active est occurs under normal driving while not in antilock operation with speeds above 15 MPH. Also, when over 8 MPH the CAB checks continuously for a missing or erratic wheel speed sensor signal.

SELF-TEST AT IGNITION TURN ON

RAM AND ROM CHECKS
RAM and ROM are major parts of the CAB in the EBC 125 and EBC 325 systems. Read-Only Memory (ROM) is a permanent memory that contains the instructions that perform all calculations and decisions that make up antilock braking. Random-Access-Memory (RAM) is an erasable, re written memory that is used to keep track of all numeric computations and decisions. RAM and ROM are both verified at ignition turn on.

WATCHDOG CHECK
The external watchdog circuit is coupled to the valve power relay. If the external signal is not active, the valve power relay cannot be operated. he watchdog circuit is checked by disabling the external watchdog signal. The relay command is issued with the watchdog signal turned off, and if the relay does turn on, then the watchdog circuit is not functioning properly, and a ignition-on DTC (fault) is issued.

BULB CHECKS
The CAB is not responsible for the diagnosis of any lamp failures, no explicit checks are made on he lamps at ignition turn-on other than requesting he Instrument Cluster to illuminate the amber ABS and the red Brake indicators for a few seconds. he bulb check test confirms correct CAB and bulb operation.

VALVE, POWER RELAY, AND PUMP MOTOR TEST
After completion of the bulb check, an active test of the power relay and the solenoid valve coils occur. Tests are made for shorted relay contacts, open relay, and shorted isolation and dump valve solenoids (open valves take somewhat longer to diagnose; open valve checks are made during normal operation while not in antilock mode). The pump motor works on Pulse Width Modulation (PWM). During self-test mode and during antilock state, you will not be able to hear the pump motor operation. The pump motor self-test will happen when vehicle speed is over 15 MPH.

PEDAL STUCK IN THE APPLIED POSITION CHECK
In order to minimize the possibility for false activation of the antilock system, the antilock mode cannot be entered until a brake-applied signal is received from the brake pedal switch. If the brake pedal is stuck in the applied position, the safeguard is defeated. The CAB will attempt to diagnose this condition by only enabling antilock if the brake pedal switch indicates the brake is off.

SELF-TEST AT INITIAL IGNITION ON
The system undergoes a power-up self-test when he vehicle has its initial ignition turn-on. The power-up self-tests are abbreviated if the vehicle ignition switch is turned from some other position for RUN, as might occur in restarting a stalled engine with a vehicle rolling.

DRBIII DIAGNOSTIC COMMANDS

RESET MODULE
This command resets the EBC 125 and EBC 325 systems as follows:
^ Turns off both warning indicators
^ Clears all DTC's and counters in the NVRAM
^ Reactivates the system

The CAB accepts the reset module command only if the vehicle's wheels are not rotating.

OUTPUT CONTROL COMMAND
Provisions have been made to allow actuation of all valves, power relay, and pump motor (325).