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Verifying the Serial Data Circuits using Digital Multimeter

  1. Ignition ON/Vehicle In Service Mode.
  2. Attempt to communicate with all devices on the low speed GMLAN serial data circuit. Refer to Data Link References  to determine how many devices should be communicating on the low speed GMLAN bus.
  3. Verify which devices are communicating on the low speed GMLAN serial data circuit.
    • If only one device is not communicating 

      Diagnose that device only. Refer to DTC U0100-U02FF .

    • If one or more devices are communicating but not all 

      Refer to Circuit/System Testing - Testing the Serial Data Circuit for an Open/High Resistance.

    • If none of the devices are communicating 
  4. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, 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 scan tool from the X84 Data Link Connector. The following tests will be done at the X84 Data Link Connector.
  5. Test for less than 10 Ω between the ground circuit terminal 5 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 
  6. Ignition ON/Vehicle In Service Mode.
  7. Test for less than 4.5 V between the serial data circuit terminal 1 and ground.
    • If 4.5 V or greater 

      Refer to Circuit/System Testing - Testing the Serial Data Circuits for a Short to Voltage.

    • If less than 4.5 V 
  8. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, 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 the serial data circuit terminal 1 and ground.
    • If 100 Ω or less 

      Refer to Circuit/System Testing - Testing the Serial Data Circuits for a Short to Ground.

    • If greater than 100 Ω 
  10. Disconnect the appropriate harness connector at the first splice pack closest in the circuit to the X84 Data Link Connector.
  11. Test for less than 2 Ω between the X84 Data Link Connector's serial data circuit terminal 1 and the splice pack harness connector's serial data input terminal.
    • If 2 Ω or greater 

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

    • If less than 2 Ω 
  12. Replace the splice pack.
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