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 | U0100-U0299* | 1 | --
Ground (DLC, terminal 5) | -- | 1* | -- | --
* An open between the data link connector serial data circuit terminal 1 or ground circuit terminal 5 and the BCM circuit will only effect the communication with the scan tool. 1. No communication on any low speed GMLAN module

### Circuit/System Description

The serial data is transmitted over a single wire to the appropriate control modules. The transmission speed for GMLAN low speed is up to 83.33 Kb/s. Under normal vehicle operating conditions, the speed of the buss is 33.33 Kb/s. The control modules toggle the serial data circuit between 0-5 V during normal communications. To wake the control modules connected to the GMLAN low speed serial data circuit, a voltage wake up pulse of 10 V is sent out. If serial data is lost, control modules will set a no communication code against the non-communicating control module. Note: a loss of serial data communications DTC does not represent a failure of the module that set it.

### 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’re 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 low 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 low speed GMLAN serial data circuit between the splice pack and a module will only affect that 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 but not communicate with the vehicle. When the scan tool attempts to communicate with the vehicle a No CANdi Module Detected message will be displayed.

• | 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.

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

| 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 light.

### 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

• | The low speed GMLAN serial data circuit terminals for each control module

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 less than the specified range, 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 appropriate harness connector at the JX 200 splice pack.
7. Test for less than 2 Ω between the DLC serial data circuit terminal 1 and the JX200 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 appropriate harness connectors at the JX 200 splice pack.
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 at the splice pack connector 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 appropriate harness connectors at the JX 200 splice pack.
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 at the splice pack 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 voltage. 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. Ignition OFF, disconnect the appropriate harness connectors at the JX 200 splice pack.
2. Install a 3 A fused jumper wire between the disconnected splice pack connector terminal 1 and a low speed GMLAN serial data circuit terminal that is not communicating.
3. 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.

4. If all circuits test normal, replace the splice pack.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

• | GMLAN Wiring Repairs

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