BCM to EBCM Communication Codes: Description and Operation

The Anti-Lock Brake System communicates with the Body Computer Module (BCM) for the purpose of displaying failure messages and ABS diagnostic trouble codes. Communication occurs on three circuits; failures in the ABS may result in up to three BCM trouble codes and/or two Driver Information Center Messages.

COMMUNICATION CIRCUITS

ABS Fault

The ABS Fault line (circuit 825, between EBCM pin 29 and BCM pin 3D3) is held at 12 volts by the BCM. In the event of an ABS malfunction, the EBCM may ground the circuit to notify the BCM when anti-lock function has been disabled. Additionally, the ABS Fault line is grounded by the valve relay (through circuit 851 and the ABS diode) any time that the valve relay is not engaged.

Any time that the ABS Fault line is grounded, the BCM will cause the "ANTI-LOCK DISABLED" message to be displayed on the Climate Control Driver Information Center (CCDIC).

Diagnostic Enable

The diagnostic Enable line (circuit 856, between EBCM pin 15 and BCM pin 2A10) is held at 12 volts by the EBCM. ABS diagnostic trouble code display is enabled any time the Diagnostic Enable line is held to ground. When ABS diagnostics are requested through diagnostic service mode, the BCM grounds circuit 856.

Low Brake Fluid

The Low Brake Fluid line (circuit 853, between the fluid level sensor, EBCM pin 17 and BCM pin 3D1) is held at 12 volts by both the BCM and the EBCM. Any time that this circuit is grounded by the fluid level sensor, the following action takes place:

^ The EBCM disables anti-lock function, grounds the ABS Fault Line and grounds the BRAKE lamp circuit.

^ The BCM enables the "LOW BRAKE FLUID" message and turns on the BRAKE warning lamp in the Instrument Panel Cluster (IPC). In response to the EBCM grounding the ABS Fault line, the BCM will also enable the "ANTI-LOCK DISABLED" message

The "ANTI-LOCK DISABLED" message has a higher priority than the "LOW BRAKE FLUID" message; as a result, the "LOW BRAKE FLUID" message will be seen only at key-on.

KEY-ON SYSTEM INITIALIZATION

At key-on, the BCM performs a check to see if the EBCM is responding the BCM inputs. After the ignition is moved to the "RUN" position, the BCM grounds the Diagnostic Enable line (circuit 856). When the Diagnostic Enable line is grounded, the EBCM should respond by grounding the ABS Fault line (circuit 825) until the Diagnostic Enable line is allowed to return to 12 volts. If the initialization test fails, the following BCM codes may set:

BCM Code B421

The BCM monitors the status of the Diagnostic Enable line at all times. If the voltage state on this circuit does not match the commanded state, Code B421 will set.

BCM Code B480

If the EBCM does not respond properly to the key-on initialization test (by grounding the ABS Fault line), Code B480 will set.

BCM CODE B481 - ANTI-LOCK FAULT

BCM Code B481 will set if the ABS Fault line remains grounded for approximately 2-1/2 minutes.