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
--- | --- | --- | --- | ---
High Speed GMLAN Serial Data Bus (+) | 1 | 2 | 1 | --
High Speed GMLAN Serial Data Bus (-) | 1 | 2 | 1 | --
Ground (DLC pin 5) | -- | 1 | -- | --
1. No communication on any high speed GMLAN module 2. An open in only one high speed GMLAN serial data circuit will allow degraded communication between the modules

### Circuit/System Description

Modules connected to the GMLAN serial data circuit monitor for serial data communications on the GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. The high speed GMLAN serial data buss uses two 120 ohm terminating resistors that are in parallel with the high speed GMLAN (+) and (-) circuits. One of the resistors is connected near the data link connector (DLC) end of the link and the other is internal to the ECM/PCM, end of the link. When a module detects a bus-off condition a DTC U2100 will be set. This DTC can be retrieved as history only.

### Diagnostic Aids

• | When testing for a short between the high speed GMLAN (+) and (-) circuits (ECM/PCM disconnected), a resistance reading of 120 ohms is normal. To test for shorts to voltage or ground in the high speed GMLAN (+) and (-) circuits ensure that all modules, resistors and the scan tool are disconnected from the buss. A normal resistance reading across the high speed GMLAN buss with the resistors, all module and scan tool disconnected is infinite ohms.

• | The engine may not start when there is a total malfunction of the high speed GMLAN serial data bus.

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

- | A short between high speed GMLAN (+) and high speed GMLAN (-) circuits

- | Any of the high speed GMLAN serial data circuits shorted to ground or voltage

- | A module internal malfunction that causes a short to voltage or ground on the high speed GMLAN circuits

### Reference Information

#### Schematic Reference

Data Communication Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation Reference

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 the specified range, test the ground circuit for open/high resistance.

2. Ignition OFF, disconnect the harness connector X 3 at the engine control module (ECM).
3. Test for less than 1 ohm of resistance between each ground circuit of the ECM and ground.

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

4. Ignition ON, test for battery voltage between each voltage input circuit of the ECM and ground.

⇒ | If less than specified, test each voltage input circuit of the ECM for a short to ground or open/high resistance, and each voltage output circuit of the ECM for a short to ground. If the circuits test normal, replace the ECM.

5. Test for infinite resistance between DLC serial data circuit terminal 14 and ground.

⇒ | If less than the specified range, test both serial data circuits for a short to ground. If the circuits test normal, replace the transmission control module (TCM).

6. Test for 0 volts between DLC serial data circuit terminal 14 and ground.

⇒ | If greater than the specified value, test both serial data circuits for a short to voltage. If the circuits test normal, replace the TCM.

7. Ignition OFF. Test for less than 1 ohm of resistance between the DLC serial data circuit terminal 14 and the ECM serial data circuit terminal 33.

⇒ | If greater than the specified range, test the serial data circuit for open/high resistance. If the circuits test normal, replace the TCM.

8. Ignition OFF. Test for less than 1 ohm of resistance between the DLC serial data circuit terminal 6 and the ECM serial data circuit terminal 53.

⇒ | If greater than the specified range, test the serial data circuit for open/high resistance. If the circuits test normal, replace the TCM.

9. Ignition OFF, disconnect the harness connector at the TCM.
10. Test for 110-130 ohms between the DLC serial data circuit terminal 6 and terminal 14.

⇒ | If greater than the specified range, replace the terminator resistor.

⇒ | If less than the specified range, test for a short between the + and - serial data circuits. If the circuits test normal, replace the terminator resistor.

11. Connect the harness connector at the TCM.
12. Test for 115-125 ohms between the DLC serial data circuit terminal 6 and terminal 14.

⇒ | If less than the specified range, test for a short between the + and - serial data circuits between the TCM and ECM. If the circuits test normal, replace the TCM.

13. If the circuits test normal, replace the ECM.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

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

• | GMLAN Wiring Repairs