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 | 2¹ | 1 | --
Ground (DLC, Terminal 5) | -- | 1¹ | -- | --
¹ An open between the data link connector (DLC) and JX 200, or DLC pin 5 ground will only effect the communication with the scan tool. See Diagnostic Aids. 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 JX 200 as the common connection between the modules and the data link connector (DLC).

### Diagnostic Aids

• | Use the Data Link References to identify the low speed GMLAN serial data modules.

• | This test is used for a total low speed GMLAN communication failure. 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.

• | An open in the low speed GMLAN serial data circuit between the splice pack and a module will only effect the specific module. 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 DLC and JX 200 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.

### Reference Information

#### Schematic Reference

• | Data Communication Schematics

• | Control Module References

#### Connector End View Reference

• | Component Connector End Views

• | Splice Pack 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 ohm between the DLC ground circuit terminal 5 and ground.

⇒ | If greater than 1 ohm, test the ground circuit for open/high resistance.

2. Ignition OFF, disconnect the harness connector (splice pack comb) at JX 200. Install a 3A fused jumper wire between pin A and pin M of JX 200, harness side.
3. Ignition ON, with a scan tool, attempt to communicate with the BCM. Communication should be available.

⇒ | If no communication, test the following circuits: serial data circuit for a short to ground, a short to voltage or an open/high resistance between the BCM and DLC, BCM voltage inputs for open/high resistance or short to ground, BCM grounds for open/high resistance. If the circuits test normal, replace the BCM.

4. Install another 3A fused jumper wire to pin A of JX 200, harness side.
5. Use the other end of the jumper wire to connect all other pins, harness side, one at a time and verify that low speed communication remains available to the BCM.

⇒ | If low speed communication is interrupted after connecting an individual module at JX 200, test the serial data circuit between JX 200 and the last module connected for a short to ground and short to voltage. If the circuit tests normal, replace the module that caused no communication.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

• | GMLAN Wiring Repairs

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