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Circuit/System Testing: Notes

NOTE:

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

  • Gateway expansion high speed GMLAN devices and terminating resistors the vehicle is equipped with
  • Device locations on the gateway expansion high speed GMLAN serial data circuits
  • Each device's ground, B+, ignition and gateway expansion high speed GMLAN serial data circuit terminals

Some devices with an internal terminating resistor have a loop in the harness that connects the internal terminating resistor to the serial data circuit. When wired this way, test these loop circuits for the appropriate failure mode short to voltage, short to ground or open/high resistance prior to replacing the device for each of the following tests.

  1. Ignition/Vehicle OFF, disconnect the harness connectors with the gateway expansion high speed GMLAN serial data circuits at an easily accessible device that is not communicating. Refer to Data Link References to determine which devices are on the gateway expansion high speed GMLAN bus.
  2. Ignition ON/Vehicle In Service Mode.
  3. Test for less than 4.5 V between each gateway expansion high speed GMLAN serial data circuit at the device 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 2 min for all vehicle systems to power down.
  5. Test for greater than 100 Ω between each gateway expansion high speed GMLAN serial data circuit at the device 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 gateway expansion high speed GMLAN serial data circuits at the device 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. Refer to Testing the Device Circuits.
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