BCM to EBCM Communication Codes: Description and Operation

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

COMMUNICATION CIRCUITS

ABS/TCS Fault

The ABS/TCS Fault line (circuit 825, between EBCM pin 24 and BCM pin 3D3) is at 12 volts when the BCM is requesting the EBCM to disable the TCS function. This line will pulse a 5 Hz signal if TCS is enabled (either actively controlling traction or ready to perform traction). In the event of an ABS/TCS malfunction, the EBCM may ground the circuit to notify the BCM when ABS/TCS function has been disabled.

Any time that the ABS/TCS Fault line is grounded, the BCM will cause the ANTILOCK DISABLED and TRACTION DISABLED messages to be displayed on the Climate Control Driver Information Center (CCDIC).

TCS Enable

The TCS Enable line (circuit 856, between EBCM pin 27 and BCM pin 2A10) is at 12 volts when the BCM is commanding the EBCM to disable TCS. It will pulse at 5 Hz when it is requesting the EBCM that TCS can be enabled (ready to perform traction).

Traction Active

The Traction Active line (circuit 933, between EBCM pin 40 and BCM pin 3C12) is at 0 volts when the EBCM is actively controlling wheel slip. This line will pulse at 5 Hz when TCS is enabled but not controlling wheel slip. This line can also be at 12 volts when the BCM is requesting the EBCM to disable TCS.

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 TCS Enable line goes to 12 volts. When the BCM is done performing its power-up sequence, the TCS Active line will be pulled up to 12 volts. The ABS/TCS Fault line is held at ground until the accumulator has reached its proper operating level. The ANTILOCK DISABLED and TRACTION DISABLED messages will remain on until the correct pressure is reached. The BRAKE lamp will also remain on until the accumulator is charged.

NORMAL OPERATION

When the correct accumulator pressure has been reached, the ABS/TCS Fault line will pull up to 12 volts.

If the TCS Enable and Traction Active lines initialize correctly, then the BCM will begin to send a 5 hertz signal to the EBCM on the TCS Enable line, indicating the TCS should be enabled by the EBCM. The EBCM will echo this response back to the BCM on the ABS/TCS Fault line, indicating that the TCS is functional and no faults have been detected. The Traction Active line will also echo this signal back to the BCM, indicating that traction control is possible if it is required.

If EBCM detects that traction control is required the EBCM will indicate to the BCM that it is controlling wheel slip by pulling the Traction Active line low. The ABS/TCS Failure line will continue to pulse if no errors have been detected. The TCS Enable line also continues to pulse the 5 hertz signal. The Traction Active line will remain high until the EBCM is no longer controlling wheel slip.

If at any time the EBCM detects an error, the ABS/TCS line will go low. The BCM will discontinue pulsing the TCS Enable line (disabling TCS) when the vehicle reaches 15 mph, preventing a sudden loss 0f traction at higher speeds.

The BCM can disable traction control if certain conditions are present, such as a system overheated or mini-spare disable condition. When the BCM disables the TCS, it will stop pulsing the TCS Enable line and will send a 12 volt signal to the EBCM. The EBCM disable TCS and echoes back the high signal on the Traction Active line. The ABS/TCS Fault line also echoes back the high signal.

The following BCM codes will set if failures occur between the BCM/EBCM communications at key-on initialization or during normal driving:

BCM Code B421 - OUTPUT DRIVER #2

The BCM monitors the status of the TCS Enable line at all times. If the voltage state on this circuit does no match the commanded state, Code B421 will set. The TCS Enable line should never be low when the BCM is not commanding it low. Code B421 will normally set during key-on if the TCS Enable line never reaches to 12 volts when the key is turned to RUN.

BCM Code B480 - ABS/TCS FAULT

If the TCS Enable line is pulsing at 5 Hz, indicating that TCS should be enabled, and the ABS/TCS Fault line stays at 12 volts (not pulsing), then Code B480 will set.

BCM Code B483 - TCS ACTIVE FAULT

The BCM also monitors the Traction Active line continuously. BCM Code B483 will set at key-on if this line never pulls up. If B483 is set, the TRACTION DISABLED message will be displayed.