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
--- | --- | --- | --- | ---
Class 2 Serial Data | 1 | U1001-U1254 | 1 | --
Ground (DLC, terminal 5) | -- | 1* | -- | --
* An open in the DLC ground circuit terminal 5 will only effect the communication with the scan tool. The vehicle modules will still communicate. 1. No communication on any class 2 serial data module

### Circuit Description

Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. Connecting a scan tool to the data link connector (DLC) allows communication with the modules for diagnostic purposes. DTCs may be set due to this symptom and during this diagnostic procedure.

### Diagnostic Aids

• | This test is used for a total class 2 communication failure. If only 1 module is not communicating and sets no DTC, ensure that the vehicle is equipped with the module, then use DTC U1001-U1254 for diagnostics.

• | An open in the class 2 serial data circuit between the DLC and the BCM will only effect the scan tool communication and will not effect module to module communication

• | An open between the data link connector (DLC) and The BCM will only effect the communication with the scan tool. The vehicle modules will still communicate, and the vehicle will start.

• | The engine may not start when there is a total malfunction of the class 2 serial data circuit.

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

- | The class 2 serial data circuit shorted to ground or voltage.

- | A module internal malfunction that causes a short to voltage or ground on the class 2 serial data circuit.

• | When the class 2 serial data circuit is shorted to ground or to voltage, the following DTCs may set as history. If the short is removed and communication returns, these DTCs indicate an intermittent condition.

- | U1300

- | U1301

- | U1305

### 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

1. 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.

Important: Cycle the ignition Key OFF before each module disconnect and ON to attempt communication.

2. Ignition OFF, disconnect the module harness connectors listed one at a time, attempting to communicate with the BCM after each disconnect. Communication should not be available.

• | Digital radio receiver - X1

• | Vehicle communication interface module (VCIM) - X2

• | Transfer case shift control module - X2

• | Instrument panel cluster (IPC) - X1

• | Radio - X1, X2

• | Powertrain control module (PCM) -X1

• | Inflatable restraint sensing and diagnostic module (SDM) - X1

• | Electronic brake control module (EBCM) - X1

• | Generator battery control module - X1

⇒ | If communications are available after a module disconnect, test all the serial data circuits to the last module disconnected for a short to ground or short to voltage. If the circuits test normal, replace the last module disconnected.

3. Ignition OFF, disconnect the BCM harness connector X1 and X2.
4. Connect a test lamp between B+ and the following BCM connector terminals. The test lamp should not illuminate.

• | X2, terminal A38

• | X2, terminal A39

• | X2, terminal A44

• | X1, terminal A41

• | X1, terminal A42

• | X1, terminal A47

⇒ | If the test lamp illuminates, test the serial data circuit for a short to ground.

5. Connect a test lamp between ground and the following BCM connector terminals. The test lamp should not illuminate.

• | X2, terminal A38

• | X2, terminal A39

• | X2, terminal A44

• | X1, terminal A41

• | X1, terminal A42

• | X1, terminal A47

⇒ | If the test lamp illuminates, test the serial data circuit for a short to voltage.

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

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

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

• | GMLAN Wiring Repairs