For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
High Speed GMLAN Serial Data Bus (+) | 1 | 2¹ | 1 | --
High Speed GMLAN Serial Data Bus (-) | 1 | 2¹ | 1 | --
Ground (DLC, Terminal 5) | -- | 1 | -- | --
¹ An open between the data link connector (DLC) and the first splice will only effect the communication with the scan tool. The vehicle modules will still communicate.

1. No communication on any high speed GMLAN module
2. An open in only one high speed GMLAN serial data circuit may allow degraded communication between the modules.

### Circuit/System Description

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 theft deterrent 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.

### Diagnostic Aids

• | Use the Data Link References to identify the high speed GMLAN serial data modules.

• | This test is used for a total high speed GMLAN communication failure. If only one module is not communicating and sets no DTC, ensure that the vehicle is equipped with the module, then use DTC U0100-U0299 for diagnostics.

• | An open in the DLC ground circuit terminal 5 will allow the scan tool to operate to set up the vehicle on the tool and then not communicate with the vehicle. When the scan tool is to the point of communicating with the vehicle, a message on the scan tool will indicate "no CANdi module detected" and will not communicate.

• | The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.

| The following conditions may cause a total loss of high speed GMLAN data communication:

- | A short between high speed GMLAN (+) and high speed GMLAN (-) circuits

- | Any of the high speed GMLAN serial data circuits shorted to ground or voltage

- | A module internal malfunction that causes a short to voltage or ground on the high speed GMLAN circuits

• | Use the DMM MIN/MAX function to capture/locate intermittent conditions.

### Reference Information

#### Schematic Reference

• | Data Communication Schematics

• | Control Module References

#### Connector End View Reference

Master Electrical Component List

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

### Circuit/System Testing

1. Test for less than 1 ohm of resistance between the DLC ground circuit terminal 5 and ground.

⇒ | If greater than 1 ohm, test the ground circuit for open/high resistance.

2. Ignition OFF, disconnect the harness connector of the BCM.
3. 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 and open/high resistance. If the circuits test normal, replace the faulty module.

4. Test for less than 1 ohm of resistance between each ground circuit of the BCM and ground.

⇒ | If greater than 1 ohm, repair the ground circuit for an open/high resistance.

5. Attempt to communicate with the electronic brake control module (EBCM). Communication should not be available.

⇒ | If communication is established, test or replace the resistor.

6. Ignition OFF, disconnect the harness connector of the suspension control module.
7. 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.

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

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

10. Ignition OFF, disconnect the harness connector of the vehicle communication interface module (VCIM).
11. 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 between the VCIM and the suspension control module. If the circuits test normal, replace the VCIM.

12. Ignition OFF, disconnect BCM harness connector C3.
13. 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.

14. Ignition OFF, disconnect the harness connector of the ECM. Connect the harness connector C3 of the BCM.
15. Attempt to communicate with the EBCM. Communication should not be available.

⇒ | If communication is established, replace the ECM.

16. Ignition OFF, disconnect the harness connector of the transmission control module (TCM).
17. 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 TCM and the ECM. If the circuits test normal, replace the TCM.

18. Ignition OFF, connect the harness connector of the suspension control module. Disconnect the harness connector of the EBCM.
19. 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 TCM. If the circuits test normal, replace the EBCM.

20. Ignition OFF, disconnect the harness connector C3 of the BCM.
21. Test for less than 1 volt between the high speed GMLAN serial data circuits of the DLC and ground.

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

22. Test for infinite resistance between the high speed GMLAN serial data circuit of the DLC and ground.

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

23. Test for infinite resistance between the high speed GMLAN serial data (+) and (-) circuits of the DLC.

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

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

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

• | GMLAN Wiring Repairs

• | Control Module References for module replacement, setup, and programming