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Circuit/System Testing

NOTE:

Some control modules 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 control module for each of the following tests.

Testing the DLC Serial Data Circuits for an Open

  1. Ignition OFF, disconnect the harness connectors at the body control module (BCM).
  2. Test for less than 2 Ω on each serial data circuit between the DLC and the BCM.
    • If greater than the specified range, test the circuit for an open/high resistance.
  3. Test for less than 2 Ω on each serial data circuit between the BCM and the first splice or splice pack.
    • If greater than the specified range, test the circuit for an open/high resistance.
  4. If the circuits test normal, replace the BCM.

Testing the Serial Data Circuits for a Short to Voltage

  1. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module.
  2. Ignition ON, test for greater than 4.5 V between each serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are greater than 4.5 V.
    • If all serial data circuits are less than the specified range, replace the control module that was disconnected.
  3. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to voltage.
  4. Ignition ON, test for greater than 4.5 V between the serial data circuits at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are greater than 4.5 V.
    • If all serial data circuits are less than the specified range, replace the control module that was just disconnected.
  5. Repeat step 3 until one of the follow conditions are isolated:
    • A short to voltage on the serial data circuit between 2 control modules or splice packs, if equipped.
    • A short to voltage on the serial data circuit between a control module and a terminating resistor.

Testing the Serial Data Circuits for a Short to Ground

  1. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module.
  2. Ignition OFF and all vehicle systems OFF, key out of the ignition switch, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 1k Ω between each serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are less than 1k Ω.
    • If all serial data circuits are greater than the specified range, replace the control module that was disconnected.
  3. Disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to ground.
  4. Ignition OFF and all vehicle systems OFF, key out of the ignition switch, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 1k Ω between the serial data circuits at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are less than 1k Ω.
    • If greater than the specified range for all serial data circuits, replace the control module that was just disconnected.
  5. Repeat step 3 until one of the follow conditions are isolated:
    • A short to ground on the serial data circuit between 2 control modules or splice packs, if equipped.
    • A short to ground on the serial data circuit between a control module and a terminating resistor.
    • A short to ground on the serial data circuit between the DLC and the first control module or splice pack.

Testing the Serial Data Circuits for a Short between the Circuits

  1. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
  2. Ignition OFF and all vehicle systems OFF, key out of the ignition switch, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 110 Ω between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are less than 110 Ω.
    • If each pair of serial data circuits is greater than the specified range, replace the control module that was disconnected.
  3. Connect the harness connectors at the control module that was disconnected.
  4. Disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module, in the direction of the circuits shorted together.
  5. Ignition OFF and all vehicle systems OFF, key out of the ignition switch, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 110 Ω between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are less than 110 Ω.
    • If each pair of serial data circuits is greater than the specified range, replace the control module that was just disconnected.
  6. Repeat step 3 until one of the following conditions are isolated:
    • Serial data circuits shorted together between 2 control modules or splice packs, if equipped.
    • Serial data circuits shorted together between a control module and a terminating resistor.
    • Serial data circuits shorted together between the DLC and the first control module or splice pack.
    • A shorted terminating resistor.

Testing the Serial Data Circuits for an Open/High Resistance

    NOTE:

    A control module that has only one pair of high speed GMLAN serial data circuits and no internal terminating resistor should not be disconnected during this diagnostic test. A control module with a single pair of serial data circuits and no internal terminating resistor that has an open/high resistance would be diagnosed in the single control module not communicating diagnostic.

    The serial data circuits directly between a control module and the DLC do not need to be tested.

  1. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module that is not communicating and has 2 or more pairs of serial data circuits or an internal terminating resistor.
  2. Ignition OFF and all vehicle systems OFF, key out of the ignition switch, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 130 Ω between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are greater than 130 Ω.
    • If each pair of serial data circuits is less than the specified range, replace the control module that was disconnected.
  3. Connect the harness connectors at the control module that was disconnected.
  4. Disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module which has 2 or more pairs of serial data circuits or an internal terminating resistor, in the direction of the open circuit.
  5. Ignition OFF and all vehicle systems OFF, key out of the ignition switch, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 130 Ω between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are greater than 130 Ω.
    • If each pair of serial data circuits is less than the specified range, replace the control module that was just disconnected.
  6. Repeat step 3 until one of the following conditions are isolated:
    • An open/high resistance on the serial data circuit between 2 control modules or splice packs, if equipped.
    • An open/high resistance on the serial data circuit between a control module and a terminating resistor.
    • An open/high resistance terminating resistor.
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