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
--- | --- | --- | --- | ---
Battery Positive Voltage (B+) | 1 | 1 | -- | --
Ground (DLC, Terminal 5) | -- | 1 | -- | --
High Speed GMLAN Serial Data (+) | 1 | 2* | 1 | --
High Speed GMLAN Serial Data (-) | 1 | 2* | 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) communications. The vehicle theft deterrent (VTD) module and ECM exchange information using the BCM as the gateway module allowing communication between the high and low speed serial data buses. The low speed GMLAN serial data circuit must also be operational for vehicle starting. The high speed GMLAN serial data bus uses two 120-ohm terminating resistors, one between the DLC and the BCM and one internal to the ECM. These resistors are placed in parallel with the high speed GMLAN (+) and (-) circuits.

### Diagnostic Aids

• | Communication may be available between the BCM and the scan tool with the high speed GMLAN serial data system inoperative. This condition is due to the BCM using both the high and low speed GMLAN systems.

• | 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 malfunction. 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.

• | Some modules may not have internal protection for specific voltage outputs and may open a battery positive voltage or ignition voltage source fuse. If a voltage input fuse is open and no short is found in that circuit, ensure that no module output voltage circuit is shorted to ground before replacing the module.

• | An open in the data link connector (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

### Reference Information

#### Schematic Reference

Data Communication Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Data Link Communications Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

Attempt to communicate with all modules on the high speed GMLAN bus.

⇒ | 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 or U2105-U2199 for diagnostics.

### Circuit/System Testing

#### High Speed GMLAN Initial Test

1. Test for less than 1.0 ohm between the DLC ground circuit terminal 5 and ground.

⇒ | If greater than the specified range, test the ground circuit for an open/high resistance.

2. Disconnect the harness connector at the EBCM.
3. Ignition ON, attempt to communicate with the BCM.

⇒ | If communication is established refer to DIAGNOSTIC 2.

⇒ | If communication is not established, refer to DIAGNOSTIC 1.

#### Diagnostic 1

1. Ignition OFF, disconnect the X2 harness connector at the BCM.
2. Ignition OFF, test for less than 2 ohms between the DLC serial data circuit terminal 6 and BCM serial data circuit terminal 1.

⇒ | If greater than the specified range, test the serial data circuit for open/high resistance.

3. Ignition OFF, disconnect the X1 and X4 harness connectors at the BCM.
4. Test for less than 5 ohms between the following ground circuit terminals and ground.

• | X1 terminal 61

• | X4 terminal E12

⇒ | If greater than the specified range, test the ground circuit for an open/high resistance.

5. Ignition OFF, disconnect the X3 harness connectors at the BCM.
6. Ignition ON, verify a test lamp illuminates between the following B+ circuit terminals and ground.

• | X3 terminal D12

• | X3 terminal D1

⇒ | If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit fuse is open, also test the B+ output circuits for a short to ground.

7. Ignition OFF, test for infinite resistance between the following serial data circuits of the DLC and ground.

• | Terminal 6

• | Terminal 14

⇒ | If less than the specified value, test the serial data circuits for a short to ground.

8. Ignition ON, test for less than 1 volt between the following serial data circuits of the DLC and ground.

• | Terminal 6

• | Terminal 14

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

9. Ignition OFF, test for infinite resistance between the DLC serial data circuit terminal 6 and terminal 14.

⇒ | If less than the specified value, test for a short between the serial data circuits.

10. Connect the X1, X2, X3 and X4 harness connector at the BCM.
11. Ignition OFF, disconnect the X4 harness connector at the VCIM.
12. Ignition ON, attempt to communicate with the BCM. Communication should not be available.

⇒ | If communication is established, test the following serial data circuits between the VCIM and the EBCM for a short between the circuits, short to ground or a short to voltage. If the circuits test normal, replace the VCIM.

• | Terminal 8

• | Terminal 9

13. Ignition OFF, disconnect the X2 harness connector at the BCM.
14. Ignition OFF, test for infinite resistance between the following serial data circuits and ground.

• | Terminal 2

• | Terminal 20

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

15. Ignition ON, test for less than 1 volt between the following serial data circuits and ground.

• | Terminal 2

• | Terminal 20

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

16. Ignition OFF, test for infinite resistance between the serial data circuit terminal 2 and terminal 20.

⇒ | If less than the specified value, test for a short between the serial data circuits.

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

#### Diagnostic 2

1. Connect the harness connector at the EBCM.
2. Ignition OFF, disconnect the harness connector at the appropriate harness connector at the ECM.

• | RPO LE5 - X3

• | RPO LNF - X2

3. Ignition ON, attempt to communicate with the BCM. Communication should not be available.

⇒ | If communication is established, replace the ECM.

4. Ignition OFF, disconnect the harness connector at the TCM.
5. Ignition ON, attempt to communicate with the BCM. Communication should not be available.

⇒ | If communication is established, test the following serial data circuits between the ECM and the TCM for a short between the serial data circuits, short to ground or a short to voltage. If the circuits test normal replace the TCM.

• | Terminal F33

• | Terminal F32

6. Ignition OFF, disconnect the harness connector at the EBCM.
7. Ignition ON, attempt to communicate with the BCM. Communication should not be available.
8. If communication is established, test the following serial data circuits between the EBCM and the TCM for a short between the serial data circuits, short to ground or a short to voltage.

• | RPO LNF - Terminal 12

• | RPO LNF - Terminal 14

• | RPO LNF - Terminal 12 and 14

• | RPO LE5 - Terminal 22

• | RPO LE5 - Terminal 24

• | RPO LE5 - Terminal 22 and 24

9. If all circuits test normal replace the EBCM.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for control module replacement, setup, and programming