For 1990-2009 cars

### Diagnostic Fault Information

Perform the Diagnostic System Check - Vehicle.

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Low speed GMLAN serial data | 1 | 2 | 1 | --
Ground (DLC, terminal 5) | -- | 1 | -- | --

1. No communication on any low speed GMLAN module
2. No communication on a specific low speed GMLAN module

### 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 body control module (BCM) can communicate. The low speed GMLAN serial data circuit uses X95 as the common connection between the modules and the DLC.

### Diagnostic Aids

• | Use the Data Link References.

• | This test is used for a total low speed GMLAN 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 U0100-U0299 for diagnostics.

• | An open in the low speed GMLAN serial data circuit between the splice pack and a module(s) will only effect the specific module(s). This type of failure will set a loss of communication DTC for each module effected, and the other modules will still communicate.

• | 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.

• | An open between the data link connector (DLC) and X95 will only effect the communication with the scan tool. The vehicle modules will still communicate, and the vehicle will start.

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

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

- | The low speed GMLAN serial data circuit shorted to ground or voltage.

- | A module internal malfunction that causes a short to voltage or ground on the low speed GMLAN circuit.

• | 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 1.0 ohm between the ground circuit terminal 5 of the DLC and ground.

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

2. Ignition OFF, disconnect the harness connector at X95.
3. Ignition ON, test for less than 1 volt between the serial data circuit terminal 1 of the DLC and ground.

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

4. Test for infinite resistance between the serial data circuit terminal 1 of the DLC and ground.

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

5. Test for less than 2 ohms on the serial data circuit between terminal 1 of the DLC and terminal A of X95.

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

6. Install a 3-amp fused jumper wire between terminal A and terminal M of X95. Attempt to communicate with the body control module (BCM). Communication should be available.

⇒ | If communication is not available test the serial data line between X95, terminal M and harness connector X3, terminal 10 at the BCM for an open/high resistance.

7. Install another 3-amp fused jumper wire to terminal A of X95.

Important: Allow communications for 5 seconds after connecting a module with the jumper to determine if communication is interrupted or not.

8. With BCM communications established, use the other end of the jumper wire to connect all other terminals of X95, harness side, one at a time and verify that low speed communication remains available to the BCM.

⇒ | If communication is interrupted after connecting an individual module at X95, test the serial data circuit between X95 and the last module connected for a short to voltage or short to ground. If the circuit tests normal, replace the module that caused no communication.

### Repair Instructions

Perform the Diagnostic Repair Verification.

• | Control Module References

• | GMLAN Wiring Repairs