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 steering column control module (SCCM) and engine control module (ECM) communications. The vehicle theft deterrent (VTD) module and ECM exchange information using the SCCM 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 that are in parallel with the high speed GMLAN (+) and (-) circuits. One of the resistors is in the rear cargo area and the other is internal to the ECM.

### Diagnostic Aids

• | Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:

- | A control module on the data communication circuit was disconnected while the communication circuit is awake.

- | Power to one or more modules was interrupted during diagnosis

- | A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.

- | Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time.

- | If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.

• | Do not replace a control module reporting a U code. The U code identifies which control module needs to be diagnosed for a communication issue.

• | 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.

• | An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle. When the scan tool attempts to communicate with the vehicle a message "no CANdi module detected" will be displayed.

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

• | Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed GMLAN communications with the scan tool.

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

| These conditions may be caused by the installation of an aftermarket navigation radio module (see bulletins). Some customers may comment of one or more of the following concerns:

- | Vehicle will not crank.

- | Vehicle cranks but will not start.

- | Vehicle stability enhancement system warning lights and messages.

- | PRNDL gear indicator position errors.

- | Tire Pressure Monitor (TPM) system warning lights.

### 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.

⇒ | If any high speed module communicates and no DTC beginning with a U exists, refer to DTC U2105-U2199.

### Circuit/System Testing

1. Ignition OFF, test for less than 1.0 ohm between the data link connector (DLC) ground circuit terminal 5 and ground.

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

2. Ignition OFF, disconnect the harness connector at the steering column control module (SCCM). Test for less than 2 ohms between the DLC serial data circuit terminal 6 and SCCM serial data circuit terminal 21.

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

3. Test for infinite resistance between the serial data circuits of the DLC listed below and ground.

• | Terminal 6

• | Terminal 14

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

4. Ignition ON, test for less than 1 volt between the serial data circuits of the DLC listed below and ground.

• | Terminal 6

• | Terminal 14

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

5. 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.

6. Ignition OFF, disconnect the X1 harness connector of the power steering control module (PSCM). Connect the harness connector of the SCCM.
7. Ignition ON, attempt to communicate with the EBCM. Communication should not be available.

⇒ | If communication is established, replace the PSCM.

8. Ignition OFF, disconnect the X1 harness connector of the electronic brake control module (EBCM).
9. Ignition ON, attempt to communicate with the vehicle communication interface module (VCIM). Communication should not be available.

⇒ | If communication is established, replace the EBCM.

10. Ignition OFF, disconnect the X2 harness connector of the underhood electrical center (UEC).
11. Ignition ON, attempt to communicate with the VCIM. Communication should not be available.

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

• | PSCM terminal 2

• | PSCM terminal 3

• | PSCM terminal 2 and terminal 3

• | EBCM terminal 21

• | EBCM terminal 23

• | EBCM terminal 21 and terminal 23

12. Ignition OFF, disconnect the X4 harness connector of the VCIM. Install 3A fused jumper wires between the terminals listed below:

• | Terminal 2 and terminal 8.

• | Terminal 3 and terminal 9

13. Reconnect the UEC. Ignition ON, attempt to communicate with the UEC. Communication should not be available.

⇒ | If communication is established, replace the VCIM.

14. Test for less than 1 volt between the serial data circuits listed below and ground.

• | VCIM terminal 2

• | VCIM terminal 3

• | VCIM terminal 8

• | VCIM terminal 9

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

15. Test for infinite resistance between the serial data circuits listed below and ground.

• | VCIM terminal 2

• | VCIM terminal 3

• | VCIM terminal 8

• | VCIM terminal 9

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

16. Test infinite resistance between the serial data circuits listed below:

• | VCIM terminal 2 and VCIM terminal 3

• | VCIM terminal 8 and VCIM terminal 9

⇒ | If less than the specified range, test the serial data circuits for being shorted together.

17. If all circuits test normal, replace the SCCM.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

• | GMLAN Wiring Repairs

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