LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Buick >> 2016 >> Cascada Base >> Repair and Diagnosis >> Engine Performance >> System >> Data Communication System >> Diagnostic Information and Procedures >> DTC U0074: Control Module Communication Bus B >> Circuit/System Testing
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Circuit/System Testing

NOTE:

Each device may need to be disconnected to isolate a circuit fault.

Use the schematic to identify the following:

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

Testing the Serial Data Circuits for a Short to Voltage

  1. Ignition OFF, disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at an easily accessible device, ignition ON.
  2. Test for greater than 4.5 V between each serial data circuit at the device connector that was just disconnected and ground.
    • If each serial data circuit is 4.5 V or less 
    1. Ignition OFF.
    2. Test for less than 10 Ω between each of the device's ground circuit terminals and ground.
      • If 10 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 10 Ω, replace the device that was disconnected.
    • If any serial data circuit is greater than 4.5 V 
  3. Ignition 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, ignition ON.
  4. Test for greater than 4.5 V between each serial data circuit at the device connector that was just disconnected and ground.
    • If each serial data circuit is 4.5 V or less 
    1. Ignition OFF.
    2. Test for less than 10 Ω between each of the device's ground circuit terminals and ground.
      • If 10 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 10 Ω, replace the device that was disconnected.
    • If any serial data circuit is greater than 4.5 V 
  5. Repeat step 3 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.

Testing the Serial Data Circuits for a Short to Ground

  1. Ignition 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 minutes for all vehicle systems to power down.
  2. Disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at an easily accessible device.
  3. Test for greater than 100 Ω between each serial data circuit at the device connector that was just disconnected and ground.
    • If each serial data circuit is 100 Ω or greater 

      Replace the device that was disconnected.

    • If any serial data circuit is less than 100 Ω 
  4. 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.
  5. Test for greater than 100 Ω between each serial data circuit at the device connector that was just disconnected and ground.
    • If both serial data circuits are 100 Ω or greater 

      Replace the device that was disconnected.

    • If any serial data circuit is less than 100 Ω 
  6. Repeat step 4 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.
    • A short to ground on the serial data circuit between the X84 Data Link Connector and the first device or splice pack.

Testing the Serial Data Circuits for a Short between the Circuits

  1. Ignition 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 minutes for all vehicle systems to power down.
  2. Disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at an easily accessible device that is not communicating.
  3. Test for greater than 110 Ω between each pair of serial data circuits at the device connector 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 
  4. Connect the harness connectors at the device that was disconnected.
  5. Disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at another device, in the direction of the circuit shorted together.
  6. Test for greater than 110 Ω between each pair of serial data circuits at the device connector 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 
  7. Repeat step 4 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.
    • Serial data circuits shorted together between the X84 Data Link Connector and the first device or splice pack.
    • A shorted terminating resistor.

Testing the Serial Data Circuits for an Open/High Resistance

  1. Ignition 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 minutes for all vehicle systems to power down.
  2. Disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at an easily accessible device that is not communicating.
  3. Test for less than 130 Ω between each pair of serial data circuits at the device connector 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 
  4. Connect the harness connectors at the device that was disconnected.
  5. 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.
  6. Test for less than 130 Ω between each pair of serial data circuits at the device connector 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 
  7. Repeat step 4 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.

Testing the Device Circuits

  1. Ignition 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 minutes for all vehicle systems to power down.
  2. Disconnect the harness connectors at an easily accessible 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 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 OFF.
    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 OFF.
    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.
  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 OFF.
    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 OFF.
    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 OFF, 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 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 minutes for all vehicle systems to power down.
  9. Test for less than 130 Ω between each pair of chassis high speed GMLAN serial data circuits at the device connector that was just disconnected.
    • If any pair of serial data circuits is greater than 130 Ω 

      Repair the open/high resistance in the serial data circuits between the disconnected device and the circuit splice in the serial data circuits.

    • If each pair of serial data circuits is 130 Ω or less 
  10. Replace the device that was disconnected.
scientia non olet · About LEMON Manuals