Modules connected to the high speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules when the ignition switch is in any position other than OFF. The high speed GMLAN serial data circuits must be operational for the vehicle to start due to body control module (BCM) and engine control module (ECM)/powertrain control module (PCM) communications. The vehicle theft deterrent (VTD) module and ECM/PCM exchange information using the BCM as the gateway module allowing communication between the high and low speed serial data busses. The low speed GMLAN serial data circuit must also be operational for vehicle starting. The high speed GMLAN serial data buss uses two 120 ohm terminating resistors that are in parallel with the high speed GMLAN (+) and (-) circuits. One of the resistors is in the rear fuse block and the other is internal to the ECM/PCM.

Schematic Reference

Connector End View Reference

Master Electrical Component List

Electrical Information Reference

1. With a scan tool verify that communication is not available for all high speed serial data modules.

⇒	If any high speed module communicates and any DTC beginning with a U exists, refer to Diagnostic Trouble Code (DTC) List - Vehicle for diagnostics.

⇒	If any high speed module communicates and no DTC beginning with a U exists, Refer to DTC U0100-U0299 for diagnostics.

2. Test for less than 1.0 ohm of resistance between the DLC ground circuit terminal 5 and ground.

⇒	If greater than 1.0 ohm, test the ground circuit for open/high resistance.

3. Ignition OFF, disconnect the harness connectors of the BCM, as necessary.
4. Test for less than 5 ohms of resistance between each ground circuit of the BCM and ground.

⇒	If greater than 5 ohms, repair the ground circuit for an open/high resistance.

5. Ignition ON, test for battery voltage between each voltage input circuit of the BCM and ground.

⇒	If less than battery voltage, test each voltage output circuit of the BCM for a short to ground, and each voltage input circuit of the BCM for a short to ground or an open/high resistance. If the circuits test normal, replace the BCM.

6. Remove the terminator resistor located in the rear fuse block.
7. Test for 110-130 ohms of resistance between the terminals of the resistor.

⇒	If not within the specified range, replace the resistor.

8. Ignition OFF, disconnect the harness connector of the suspension control module.
9. Attempt to communicate with the EBCM. Communication should not be available.

⇒	If communication is established, test the serial data circuits for a short between them, short to ground or a short to voltage between the suspension control module and the terminator resistor. If the circuits test normal, replace the suspension control module.

10. Ignition OFF, disconnect the harness connector of the distance sensing cruise control module.
11. Attempt to communicate with the EBCM. Communication should not be available.

⇒	If communication is established, replace the distance sensing cruise control module.

12. Ignition OFF, disconnect the harness connector of the vehicle communication interface module.
13. Attempt to communicate with the EBCM. Communication should not be available.

⇒	If communication is established, test the serial data circuits for a short between them, short to ground or a short to voltage between the vehicle communication interface module and the suspension control module. If the circuits test normal, replace the vehicle communication interface module.

14. Ignition OFF, disconnect BCM harness connector C3.
15. Test for infinite resistance between the serial data circuits of BCM harness connector C3 terminal 8 and ground and between BCM harness connector C3 terminal 9 and ground.

⇒	If less than infinite, test the circuit for a short to ground.

16. Ignition OFF, disconnect the harness connector of the engine control module. Connect the harness connector C3 of the BCM.
17. Attempt to communicate with the EBCM. Communication should not be available.

⇒	If communication is established, replace the engine control module.

18. Ignition OFF, disconnect the harness connector of the transmission control module.
19. Attempt to communicate with the EBCM. Communication should not be available.

⇒	If communication is established, test the serial data circuits for a short between them, short to ground or a short to voltage between the transmission control module and the engine control module. If the circuits test normal, replace the transmission control module.

20. Ignition OFF, connect the harness connector of the suspension control module. Disconnect the harness connector of the EBCM.
21. Attempt to communicate with the suspension control module. Communication should not be available.

⇒	If communication is established, test the serial data circuits for a short between them, short to ground or a short to voltage between the EBCM and the transmission control module. If the circuits test normal, replace the EBCM.

22. Ignition OFF, disconnect the harness connector C3 of the BCM.
23. Test for less than 1 volt between the high speed GMLAN serial data circuit of the DLC terminal 6 and ground, and terminal 14 and ground.

⇒	If greater than 1 volt, test the serial data circuit for a short to voltage.

24. Test for infinite resistance between the high speed GMLAN serial data circuit of the DLC terminal 6 and ground, and terminal 14 and ground.

⇒	If less than infinite resistance, test the serial data circuit for a short to ground.

25. Test for infinite resistance between the high speed GMLAN serial data circuits of the DLC terminal 6 and 14.

⇒	If less than infinite resistance, test the serial data circuits for a short between them.

26. If the circuits test normal, replace the BCM.

Perform the Diagnostic Repair Verification after completing the repair.