• Perform the Diagnostic System Check prior to using this diagnostic procedure: Diagnostic System Check - Vehicle
• Review the description of Strategy Based Diagnosis: Strategy Based Diagnosis
• An overview of each diagnostic category can be found here: Diagnostic Procedure Instructions

The devices connected to the chassis high speed GMLAN serial data circuits (circuit numbers 6105 & 6106) monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the devices when the S39 Ignition Switch (or S38 Ignition Mode Switch) is in any position other than OFF. The chassis high speed GMLAN serial data bus uses terminating resistors that are in parallel with the chassis high speed GMLAN (+) and (−) circuits.

• Data Communication Schematics
• Control Module References

Master Electrical Component List

Data Link Communications Description and Operation

• Circuit Testing
• Connector Repairs
• Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs

Control Module References for scan tool information

1. Ignition ON/Vehicle In Service Mode.
2. Verify that the scan tool is communicating with vehicle.
• If the scan tool is not communicating with vehicle

  Refer to Scan Tool Does Not Communicate with Vehicle.

• If the scan tool is communicating with vehicle
3. Attempt to communicate with all devices on the chassis high speed GMLAN bus. Refer to Data Link References to determine how many devices should be communicating on the chassis high speed GMLAN bus.
4. Verify two or more devices are not communicating on the chassis high speed GMLAN bus.
• If only one device is not communicating

  Refer to Circuit/System Testing – Testing the Device Circuits.

• If two or more devices are not communicating
5. Ignition/Vehicle OFF, disconnect the scan tool from the X84 Data Link Connector. The following tests will be done at the X84 Data Link Connector.
6. Ignition ON/Vehicle In Service Mode.
7. Test for less than 4.5 V between each of the serial data circuit terminals listed below and ground.
• Terminal 12
• Terminal 13
• If 4.5 V or greater

  Repair the short to voltage in the serial data circuit between the X84 Data Link Connector and the K56 Serial Data Gateway Module.

• If less than 4.5 V
8. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
9. Test for greater than 100 Ω between each of the serial data circuit terminals listed below and ground.
• Terminal 12
• Terminal 13
• If 100 Ω or less

  Repair the short to ground in the serial data circuit between the X84 Data Link Connector and the K56 Serial Data Gateway Module.

• If greater than 100 Ω
10. Test for 110–130 Ω between the serial data circuit terminals 12 and 13.
• If less than 110 Ω

  Repair the short together in the serial data circuits between the X84 Data Link Connector and the K56 Serial Data Gateway Module.

• If greater than 130 Ω

  Repair the open/high resistance in the serial data circuit between the X84 Data Link Connector and the K56 Serial Data Gateway Module.

• If between 110–130 Ω
11. Refer to Circuit/System Testing – Serial Data Starting Point.

1. Ignition/Vehicle OFF, disconnect the harness connectors with the chassis high speed GMLAN serial data circuits (circuit numbers 6105 & 6106) at an easily accessible device that is not communicating.
2. Ignition ON/Vehicle In Service Mode.
3. Test for less than 4.5 V between each of the chassis high speed GMLAN serial data circuit terminals at the device connectors that was just disconnected and ground.
• If 4.5 V or greater

  Refer to Testing the Serial Data Circuits for a Short to Voltage.

• If less than 4.5 V
4. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
5. Test for greater than 100 Ω between each of the chassis high speed GMLAN serial data circuit terminals at the device connectors that was just disconnected and ground.
• If 100 Ω or less

  Refer to Testing the Serial Data Circuits for a Short to Ground.

• If greater than 100 Ω
6. Test for 110–130 Ω between each pair of the chassis high speed GMLAN serial data circuit terminals at the device connectors that was just disconnected.
• If less than 110 Ω

  Refer to Testing the Serial Data Circuits for a Short between the Circuits.

• If greater than 130 Ω

  Refer to Testing the Serial Data Circuits for an Open/High Resistance.

• If between 110–130 Ω
7. Replace the device that was just disconnected.

1. Ignition/Vehicle OFF, disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at another device, in the direction of the circuit shorted to voltage.
2. Ignition ON/Vehicle In Service Mode.
3. Test for greater than 4.5 V between each of the chassis high speed GMLAN serial data circuit terminals at the device connectors that was just disconnected and ground.
• If each serial data circuit is 4.5 V or less

  Replace the device that was disconnected.

• If any serial data circuit is greater than 4.5 V
4. Repeat step 1 until one of the following conditions are isolated:
• A short to voltage on the serial data circuit between two devices or splice packs, if equipped.
• A short to voltage on the serial data circuit between a device and a terminating resistor.

1. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
2. Disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at another device, in the direction of the circuit shorted to ground.
3. Test for greater than 100 Ω between each of the chassis high speed GMLAN serial data circuit terminals at the device connectors that was just disconnected and ground.
• If each serial data is 100 Ω or greater

  Replace the device that was disconnected.

• If any serial data circuit is less than 100 Ω
4. Repeat step 1 until one of the following conditions are isolated:
• A short to ground on the serial data circuit between two devices or splice packs, if equipped.
• A short to ground on the serial data circuit between a device and a terminating resistor.

1. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
2. Connect the harness connectors at the device that was disconnected.
3. Disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at another device, in the direction of the circuit shorted together.
4. Test for greater than 110 Ω between each pair of the chassis high speed GMLAN serial data circuit terminals at the device connectors that was just disconnected.
• If each pair of serial data circuits is 110 Ω or greater

  Replace the device that was disconnected.

• If any pair of serial data circuits is less than 110 Ω
5. Repeat step 1 until one of the following conditions are isolated:
• Serial data circuits shorted together between two devices or splice packs, if equipped.
• Serial data circuits shorted together between a device and a terminating resistor.
• A shorted terminating resistor.

1. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
2. Connect the harness connectors at the device that was disconnected.
3. Disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at another device, in the direction of the circuit with the open/high resistance.
4. Test for less than 130 Ω between each pair of the chassis high speed GMLAN serial data circuit terminals at the device connectors that was just disconnected.
• If each pair of serial data circuits is 130 Ω or less

  Replace the device that was disconnected.

• If any pair of serial data circuits is greater than 130 Ω
5. Repeat step 1 until one of the following conditions are isolated:
• An open/high resistance on the serial data circuit between two devices or splice packs, if equipped.
• An open/high resistance on the serial data circuit between a device and a terminating resistor.
• An open/high resistance terminating resistor.

1. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. Disconnect the harness connectors at a chassis high speed GMLAN device that is not communicating. It may take up to 10 min for all vehicle systems to power down.
2. Test for less than 10 Ω between each of the ground circuit terminals and ground.
• If 10 Ω or greater
1. Ignition/Vehicle OFF.
2. Test for less than 2 Ω in the ground circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, repair the open/high resistance in the ground connection.
• If less than 10 Ω
3. Ignition ON/Vehicle In Service Mode
4. If equipped, verify a test lamp illuminates between each of the B+ circuit terminals and ground.
• If the test lamp does not illuminate and the circuit fuse is good
1. Ignition/Vehicle OFF, remove the test lamp.
2. Test for less than 2 Ω in the B+ circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, verify the fuse is not open and there is voltage at the fuse. Refer to Power Mode Mismatch.
• If the test lamp does not illuminate and the circuit fuse is open
1. Ignition/Vehicle OFF, remove the test lamp.
2. Test for infinite resistance between the B+ circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance, replace the disconnected device.
• If the test lamp illuminates
5. If equipped, verify a test lamp illuminates between each of the ignition circuit terminals, which has a fuse in the circuit, and ground.
• If the test lamp does not illuminate and the circuit fuse is good
1. Ignition/Vehicle OFF, remove the test lamp.
2. Test for less than 2 Ω in the ignition circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, verify the fuse is OK and there is voltage at the fuse. Refer to Power Mode Mismatch.
• If the test lamp does not illuminate and the circuit fuse is open
1. Ignition/Vehicle OFF, remove the test lamp.
2. Test for infinite resistance between the ignition circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance, replace the disconnected device.
• If the test lamp illuminates
6. If equipped, verify a test lamp illuminates between each of the ignition circuit terminals, which is controlled by a control module, and ground.
• If the test lamp does not illuminate
1. Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the control module that controls the ignition circuit and all devices that share the same ignition circuit.
2. Test for infinite resistance between the ignition circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 Ω in the ignition circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω
4. Reconnect the control module that controls the ignition circuit, ignition ON/Vehicle In Service Mode.
5. Verify a test lamp illuminates between the ignition circuit terminal and ground.
• If the test lamp does not illuminate, replace the control module that controls the ignition circuit.
• If the test lamp illuminates
6. Ignition/Vehicle OFF, reconnect the first device that shares the same ignition circuit, ignition ON/Vehicle In Service Mode.
7. Verify a test lamp illuminates between the ignition circuit terminal and ground.
• If the test lamp does not illuminate, replace the device that was just connected.
• If the test lamp illuminates and there are more devices to connect, connect the next device and repeat step 6.7 until there are no more devices to connect.
• If the test lamp illuminates
7. If equipped, verify a test lamp illuminates between the accessory wakeup serial data circuit terminal and ground.
• If the test lamp does not illuminate
1. Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the K9 Body Control Module.
2. Test for less than 2 Ω in the accessory wakeup serial data circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, replace the K9 Body Control Module.
• If the test lamp illuminates
8. If equipped, verify a test lamp illuminates between the serial data communication enable circuit terminal and ground.
• If the test lamp does not illuminate
1. Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the K9 Body Control Module.
2. Test for less than 2 Ω in the serial data communication enable circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, replace the K9 Body Control Module.
• If the test lamp illuminates
9. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
10. Disconnect the harness connectors at a chassis high speed GMLAN device that is still communicating and closest to the non communicating device.
11. Test for less than 2 Ω in each of the chassis high speed GMLAN serial data circuits end to end between the non communicating device and the device that was just disconnected.
• If 2 Ω or greater

  Repair the open/high resistance in the serial data circuit.

• If less than 2 Ω
12. Replace the non communicating device.

Perform the Diagnostic Repair Verification after completing the repair: Diagnostic Repair Verification.

For control module replacement, programming, and setup, refer to: Control Module References.