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 | --
¹ An open between the data link connector (DLC) and the splice pack will only effect the communication with the scan tool. The vehicle modules will still communicate. 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. The low speed GMLAN serial data circuit uses SP 200 as a centralized common contact point for the buss.

### 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 1 module is not communicating and sets no DTC, ensure that the vehicle is equipped with the module, then use DTC U0100-U0299 or U2105-U2199 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. Depending on which module is effected, the vehicle may not start.

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

• | Use the DMM MIN/MAX function to capture/locate intermittent conditions.

• | 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

#### 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 References

Control Module References for Scan Tool Information

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the low speed GMLAN splice pack.
2. Ignition ON, test for less than 1 volt between the low speed GMLAN serial data circuit of the DLC and ground.

⇒ | If greater than 1 volt, repair 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, repair the serial data circuit for a short to ground.

4. Test for less than 5 ohm of resistance in the serial data circuit between the DLC and the splice pack.

⇒ | If greater than 5 ohm, repair the serial data circuit for an open/high resistance.

5. Install a 3-amp fused jumper wire between pin A and pin C of the splice pack. Install another 3-amp fused jumper wire to pin C and leave the other end of it free to use as a probe.
6. Remove the harness connectors containing the GMLAN low speed buss from the DRR and the VCIM.
7. Using the extra jumper wire from pin C attempt to establish communications with any other module connected to the splice pack. At least one module should be able to communicate.

⇒ | If communications can not be established with any module, test the serial data circuit of the BCM for a short to voltage, short to ground or open/high resistance. If the serial data circuit test normal test the BCM B+ and ground circuits for there respective potentials. If all of the aforementioned circuits are analyzed functional then replace the BCM.

8. Using the extra jumper wire at pin C, continue testing each serial data circuit at the splice pack for communications. All modules should be able to communicate.

⇒ | If there is only one module connected, test the serial data circuit for a short to voltage or a short to ground. If the circuit tests normal, replace the faulty module.

⇒ | If there are multiple modules, test each serial data circuit 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 starting with the closest module to the splice pack until communications stop. Replace the faulty module.

9. If all circuits test normal, test or replace the splice pack.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

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

• | GMLAN Wiring Repairs