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 | 2* | 1 | --
Ground (DLC, terminal 5) | -- | 1* | -- | --
* An open between the data link connector (DLC) and JX 205, or DLC terminal 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, or group of 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 when the ignition switch is in any position other than OFF. The low speed GMLAN serial data circuit uses JX 205 and JX 405 as common connection points between some modules and the data link connector (DLC).

### Diagnostic Aids

• | Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:

- | A control module on the data communication circuit was disconnected while the communication circuit is awake.

- | Power to one or more modules was interrupted during diagnosis

- | A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.

- | Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time.

- | If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.

• | Do not replace a control module reporting a U code. The U code identifies which control module needs to be diagnosed for a communication issue.

• | Communication may be available between the BCM and the scan tool with the high speed GMLAN serial data system inoperative. This condition is due to the BCM using both the high and low speed GMLAN systems.

• | An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle. When the scan tool attempts to communicate with the vehicle a message "no CANdi module detected" will be displayed.

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

• | Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed GMLAN communications with the scan tool.

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

| These conditions may be caused by the installation of an aftermarket navigation radio module (see bulletins). Some customers may comment of one or more of the following concerns:

- | Vehicle will not crank.

- | Vehicle cranks but will not start.

- | Vehicle stability enhancement system warning lights and messages.

- | PRNDL gear indicator position errors.

- | Tire Pressure Monitor (TPM) system warning lights.

### Reference Information

#### Schematic Reference

Data Communication Schematics

#### 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

### Circuit/System Testing

Note: Use the schematic to identify the following:

• | Control modules the vehicle is equipped with

• | Control module locations on the low speed GMLAN serial data circuit

• | Each control module's low speed GMLAN serial data circuit terminals

1. Attempt to communicate with all control modules on the low speed GMLAN serial data circuit, refer to Data Link References. Communications should not be available with two or more control modules on the low speed GMLAN serial data circuit.

⇒ | If only one control module is not communicating, diagnose that control module only, refer to DTC U0100-U0299.

⇒ | If one or more control modules are communicating but not all, refer to Testing the Serial Data Circuit for an Open/High Resistance.

2. Disconnect the scan tool from the DLC. The following tests will be done at the DLC connector.
3. All access doors closed, ignition OFF for 60 seconds, test for less than 10 Ω between the ground circuit terminal 5 and ground.

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

4. Ignition ON, test for less than 4.5 V between the serial data circuit terminal 1 and ground.

⇒ | If greater than the specified range, test the serial data circuit for a short to voltage, refer to Testing the Serial Data Circuit for a Short to Voltage.

5. Ignition OFF for 60 seconds, test for greater than 100 Ω between the serial data circuit terminal 1 and ground.

⇒ | If not the specified value, test the serial data circuit for a short to ground, refer to Testing the Serial Data Circuits for a Short to ground.

6. Disconnect the harness connector at the JX 205 splice pack.
7. Test for less than 2 Ω between the serial data circuit terminal 1 and the JX 205 splice pack terminal 1.

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

#### Testing the Serial Data Circuits for a Short to Voltage

1. Ignition OFF, disconnect the harness connectors at the JX 205 and JX 405 splice packs, as equipped.
2. Ignition ON, test for less than 4.5 V between the DLC serial data circuit terminal 1 and ground.

⇒ | If greater than the specified range, repair the short to voltage.

3. Test for less than 4.5 V between each serial data circuit and ground.

⇒ | If greater than the specified range, perform the appropriate test listed below:

• | Serial data circuits with one control module, test the serial data circuit for a short to voltage. If the circuit tests normal replace the control module.

• | Serial data circuits with 2 or more control modules, test each section of the serial data circuit for a short to voltage. If the circuits test normal, replace the control module that causes the short to voltage when connected.

#### Testing the Serial Data Circuits for a Short to Ground

1. Ignition OFF, disconnect the harness connectors at the JX 205 and JX 405 splice packs, as equipped.
2. Test for infinite resistance between the DLC serial data circuit terminal 1 and ground.

⇒ | If not the specified value, repair the short to ground.

3. Ignition OFF for 60 seconds, test for greater than 100 Ω between each serial data circuit and ground.

⇒ | If less than the specified range, perform the appropriate test listed below:

• | Serial data circuits with one control module, test the serial data circuit for a short to ground. If the circuit tests normal replace the control module.

• | Serial data circuits with 2 or more control modules, test each section of the serial data circuit for a short to ground. If the circuits test normal, replace the control module that causes the short to ground when connected.

#### Testing the Serial Data Circuit for an Open/High Resistance

1. If equipped with a JX 405 splice pack, ignition ON, verify the scan tool communicates with one or more control modules connected to the JX 405 splice pack.

⇒ | If all control modules on the JX 405 spice pack do not communicate, test the serial data circuit between the JX 205 and JX 405 splice packs for an open/high resistance. If the circuit tests normal, test each splice pack for an open/high resistance.

2. Ignition OFF, disconnect one of the harness connectors listed below:

• | JX 205 if the control modules not communicating are connected to the JX 205 splice pack

• | JX 405 if the control modules not communicating are connected to the JX 405 splice pack

3. Install a 3A fused jumper wire between the disconnected splice pack terminal 1 and a low speed GMLAN serial data circuit that is not communicating.
4. Ignition ON, verify the scan tool communicates with the control modules connected to the low speed GMLAN serial data circuit.

⇒ | If any control module does not communicate, test each section of the serial data circuit for an open/high resistance. If the circuits test normal, replace the control module that does not communicate when connected.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

• | GMLAN Wiring Repairs

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