Component Operations

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 the amount of allowable dump valve cycles.

DIAGNOSTIC COMMAND MODES
The system software includes several self-tests that 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 test 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, rewritten 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. The 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 the lamps at ignition turn-on other than requesting the Instrument Cluster to illuminate the amber ABS and the red Brake indicators for a few seconds. The 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 the 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 to RUN, as might occur in restarting a stalled engine with a vehicle rolling.

DRB III 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).

THE ON-BOARD DIAGNOSTIC SYSTEM
The CAB monitors the antilock system. The CAB monitors critical input and output circuits for correct operation.

Some circuits are tested continuously; others are checked only under certain circumstances.

Each circuit malfunction detected by the CAB has a corresponding trouble code:
^ If the CAB diagnostic system senses that one of the circuits is malfunctioning, it stores the corresponding trouble code in memory
^ If the malfunction goes away after the trouble code is stored, the trouble code will be erased after 50 key cycles, so long as no other DTC's were set within these 50 cycles