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Circuit/System Testing

NOTE:

Each device may need to be disconnected to isolate a circuit fault. Use the schematic to identify the following:

  • All low speed GMLAN devices the vehicle is equipped with
  • Low speed GMLAN device and splice pack locations on the low speed GMLAN serial data circuit
  • The low speed GMLAN serial data circuit terminals for each device or splice pack

Testing the Serial Data Circuits for a Short to Voltage

  1. Ignition/Vehicle OFF, disconnect the appropriate harness connectors at all low speed GMLAN serial data splice packs, ignition ON/Vehicle In Service Mode.
  2. Test for less than 4.5 V between the serial data circuit terminal 1 at the X84 Data Link Connector and ground.
    • If 4.5 V or greater 

      Repair the short to voltage on the serial data circuit.

    • If less than 4.5 V 
  3. Test for less than 4.5 V between each low speed GMLAN serial data circuit at a splice pack and ground.
    • If any serial data circuit is greater than 4.5 V 
    1. Ignition/Vehicle OFF, disconnect all devices on the failed serial data circuit, ignition ON/Vehicle In Service Mode.
    2. Test for less than 1 V between each section of the failed serial data circuit and ground.
      • If 1 V or greater, repair the short to voltage in the circuit.
      • If less than 1 V
    3. Connect the splice pack and connect the first device on the failed serial data circuit, ignition ON/Vehicle In Service Mode.
    4. Verify the scan tool communicates or not with the low speed GMLAN serial data circuit.
      • If the scan tool does not communicate, replace the device that was just connected.
      • If the scan tool communicates and there are more devices to connect, connect the next device and repeat step 3.4.
      • If the scan tool communicates and there are no more devices to connect
    5. All OK.
    • If all serial data circuits are less than 4.5 V 
  4. All OK.

Testing the Serial Data Circuits for a Short to Ground

  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. Disconnect the appropriate harness connectors at all low speed GMLAN serial data splice packs.
  2. Test for infinite resistance between the serial data circuit terminal 1 at the X84 Data Link Connector and ground.
    • If less than infinite resistance 

      Repair the short to ground on the serial data circuit.

    • If infinite resistance 
  3. Test for greater than 100 Ω between each low speed GMLAN serial data circuit at a splice pack and ground.
    • If any serial data circuit is 100 Ω or less 
    1. Disconnect all devices on the failed serial data circuit.
    2. Test for infinite resistance between each section of the failed serial data circuit and ground.
      • If less than infinite resistance, repair the short to ground in the circuit.
      • If infinite resistance
    3. Connect the splice pack and connect the first device on the failed serial data circuit, ignition ON/Vehicle In Service Mode.
    4. Verify the scan tool communicates or not with the low speed GMLAN serial data circuit.
      • If the scan tool does not communicate, replace the device that was just connected.
      • If the scan tool communicates and there are more devices to connect, connect the next device and repeat step 3.4.
      • If the scan tool communicates and there are no more devices to connect
    5. All OK.
    • If all serial data circuits are greater than 100 Ω 
  4. All OK.

Testing the Serial Data Circuit for an Open/High Resistance

  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. Disconnect the splice pack containing the devices that are not communicating on the low speed GMLAN serial data circuit.
  2. Test for less than 2 Ω between the X84 Data Link Connector terminal 1 and the disconnected splice pack.
    • If 2 Ω or greater 

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

    • If less than 2 Ω 
  3. Disconnect all devices on the failed serial data circuit.
  4. Test for less than 2 Ω between each section of the failed serial data circuit end to end.
    • If 2 Ω or greater 

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

    • If less than 2 Ω 
  5. Connect the splice pack and connect the first device on the failed serial data circuit.
  6. Verify if the device communicates or not with the scan tool.
    • If the device does not communicates 

      Replace the device.

    • If the device communicates and there are more devices to connect 

      Connect the next device on the failed serial data circuit and repeat step 6.

    • If all devices are connected and communicating 
  7. All OK.

Testing the Device Circuits

  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 at a low speed GMLAN device that is not communicating.
  3. Test for less than 10 Ω between each ground circuit terminal 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 
  4. If equipped, verify a test lamp illuminates between each B+ circuit terminal 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.
    • 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. Ignition ON/Vehicle In Service Mode.
  6. If equipped, verify a test lamp illuminates between each ignition circuit terminal, 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.
    • 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 
  7. If equipped, verify a test lamp illuminates between each ignition circuit terminal, 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.
    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, replace the control module that controls the ignition circuit.
    • If the test lamp illuminates 
  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 less than 2 Ω in the low speed GMLAN serial data circuit end to end between the device harness connector and the X84 Data Link Connector terminal 1.
    • If 2 Ω or greater 

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

    • If less than 2 Ω 
  10. Replace the device that was disconnected.
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