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
--- | --- | --- | --- | ---
Low speed GMLAN serial data | 1 | 2 | 1 | --
1. No communication on any low speed GMLAN module 2. No communication on one or a specific group of low speed GMLAN modules

### Circuit/System Description

Modules connected to the low speed GMLAN serial data circuit 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 low speed GMLAN serial data circuit must be operational for the vehicle to start so the vehicle theft deterrent (VTD) module and steering column control module (SCCM) can communicate.

### 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 Testing

#### LS GMLAN initial test

1. Ignition OFF, disconnect the X1 harness connector at the steering column control module (SCCM).
2. Ignition ON, test for less than 1.0 volt between the low speed GMLAN serial data circuit of the DLC and ground.

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

3. Test for infinite resistance between the serial data circuit of the DLC and ground.

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

4. Test for less than 1.0 ohm in the serial data circuit between the DLC and the SCCM.

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

5. Disconnect the X1 harness connector at the SDM, install a 3A fused jumper wire from the SDM terminal 35 to the harness side of X1.
6. With a scan tool, attempt to communicate with the IPC.

⇒ | If communication is established, refer to Diagnostic 2.

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

#### LS GMLAN test 1

1. Remove the jumper wire from the SDM.
2. Disconnect the following harness connectors for the following modules.

• | REC X5

• | UEC X2

• | IPC X1

• | SCCM X1

3. Test each serial data circuit of the disconnected modules for a short to voltage or a short to ground, with the modules disconnected. If the circuit tests normal, install each module one at a time in the following order, while attempting to communicate after each module is connected until communications stop.

• | SCCM X1

• | IPC X1

• | UEC X2

• | REC X5

4. Replace the last module connected that caused communication to stop.

#### LS GMLAN test 2

1. Remove the jumper wire from the SDM.
2. Install a 3A fused jumper wire between terminal 37 and 38 on the harness side of the SDM X1 harness connector.
3. Attempt to communicate with the AHS, communication should not be available.

⇒ | If communication is established, replace the SDM.

4. Disconnect the jumper wire for the harness side of the SDM X1 harness connector.
5. Disconnect the following harness connectors of the following modules.

• | Auxiliary HVAC Module X1

• | TPM X1

• | Infotainment display unit X1

• | SCCM X1

6. Test each serial data circuit of the disconnected modules for a short to voltage or a short to ground, with the modules disconnected. If the circuit tests normal, install each module one at a time in the following order, while attempting to communicate after each module is connected until communications stop.

• | SCCM X1

• | Infotainment display unit X1

• | SDM X1

• | TPM X1

• | Auxiliary HVAC Module X1

7. Replace the last module connected that caused communication to stop.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

• | GMLAN Wiring Repairs

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