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Home >> Cadillac >> 2021 >> CT4 Sport, RWD >> Repair and Diagnosis >> External Pages >> Different car >> Section 191 (Data Communications - Diagnostic Information And Procedures (3 Of 3)) >> Diagnostic Information and Procedures >> Scan Tool Does Not Communicate with Device on CAN 2 >> Circuit/System Testing

Circuit/System Testing

WARNING: This page is about a different car, the 2022 Cadillac CT4. However, it is still accessible from the selected car via links, so may be relevant.
NOTE:

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

  • CAN 2 devices and terminating resistors the vehicle is equipped with
  • Device locations on the CAN 2 serial data circuits
  • Each device's ground, B+, ignition, and CAN 2 serial data circuit terminals

Serial Data Starting Point

    NOTE:

    Starting the test at a device in the middle of the CAN bus will divide the CAN bus in half and shorten the time to isolate the fault.

  1. Ignition/Vehicle OFF, disconnect the harness connectors with the CAN bus 2 serial data circuits at an easily accessible device that is not communicating.
  2. Ignition ON/Vehicle In Service Mode.
  3. Test for less than 4.5 V between each of the CAN bus 2 serial data circuit terminals 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  .

    • Go to next step: 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 10 min for all vehicle systems to power down.
  5. Test for greater than 100 Ω between each of the CAN bus 2 serial data circuit terminals 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  .

    • Go to next step: If greater than 100 Ω 
  6. Test for 110-130 Ω between each pair of the CAN bus 2 serial data circuit terminals 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  .

    • Go to next step: If between 110-130 Ω 
  7. Replace the device that was just disconnected.

Testing the Serial Data Circuits for a Short to Voltage

  1. Ignition/Vehicle OFF, disconnect the harness connectors with the CAN bus 2 serial data circuits at another device, in the direction of the circuit shorted to voltage.
  2. Ignition ON/Vehicle In Service Mode
  3. Test for greater than 4.5 V between each of the CAN bus 2 serial data circuits at the device connector that was just disconnected and ground.
    • If each serial data circuit is 4.5 V or less 

      Replace the device that was just disconnected.

    • Go to next step: If any serial data circuit is greater than 4.5 V 
  4. Repeat step 1 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/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 with the CAN bus 2 serial data circuits at another device, in the direction of the circuit shorted to ground.
  3. Test for greater than 100 Ω between each of the CAN bus 2 serial data circuits 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 just disconnected.

    • Go to next step: If any serial data circuit is less than 100 Ω 
  4. Repeat step 1 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.

Testing the Serial Data Circuits for a Short between the 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. Connect the harness connectors at the device that was previously disconnected.
  3. Disconnect the harness connectors with the CAN bus 2 serial data circuits at another device, in the direction of the circuit shorted together.
  4. Test for greater than 110 Ω between each pair of the CAN bus 2 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 just disconnected.

    • Go to next step: If any pair of serial data circuits is less than 110 Ω 
  5. Repeat step 1 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.
    • A shorted terminating resistor.

Testing the Serial Data Circuits 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.
  2. Connect the harness connectors at the device that was previously disconnected.
  3. Disconnect the harness connectors with the CAN bus 2 serial data circuits at another device, in the direction of the circuit with the open/high resistance.
  4. Test for less than 130 Ω between each pair of the CAN bus 2 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 just disconnected.

    • Go to next step: If any pair of serial data circuits is greater than 130 Ω 
  5. Repeat step 1 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/Vehicle OFF, disconnect the harness connectors at the CAN bus 2 device that is not communicating.
  2. Verify a test lamp illuminates between each of the ground circuit terminals and B+.
    • If the test lamp does not illuminate 

      Repair the open/high resistance in the ground circuit or ground connection.

    • Go to next step: If the test lamp illuminates 
  3. Ignition ON/Vehicle In Service Mode.
  4. If equipped, verify a test lamp illuminates between each of the B+ circuit terminals and ground.
    • If the test lamp does not illuminate and the circuit fuse is good 
    1. Test for an open/high resistance in the B+ circuit.
      • If an open/high resistance is found, repair the fault in the circuit.
      • Go to next step: If an open/high resistance is not found
    2. 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. Test for a short to ground in the B+ circuit.
      • If a short to ground is found, repair the fault in the circuit.
      • Go to next step: If a short to ground is not found
    2. Replace the disconnected device.
    • Go to next step: If the test lamp illuminates 
  5. If equipped, verify a test lamp illuminates between each of the ignition circuit terminals, which has a fuse in the circuit, and ground.
    • If the test lamp does not illuminate and the circuit fuse is good 
    1. Test for an open/high resistance in the ignition circuit.
      • If an open/high resistance is found, repair the fault in the circuit.
      • Go to next step: If an open/high resistance is not found
    2. 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. Test for a short to ground in the ignition circuit.
      • If a short to ground is found, repair the fault in the circuit.
      • Go to next step: If a short to ground is not found
    2. Replace the disconnected device.
    • Go to next step: If the test lamp illuminates 
  6. Test for less than 4.5 V between each of the CAN bus 2 serial data circuit terminals and ground.
    • If 4.5 V or greater 

      Refer to Testing the Serial Data Circuits for a Short to Voltage  .

    • Go to next step: If less than 4.5 V 
  7. 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.
  8. Test for greater than 100 Ω between each of the CAN bus 2 serial data circuit terminals and ground.
    • If 100 Ω or less 

      Refer to Testing the Serial Data Circuits for a Short to Ground  .

    • Go to next step: If greater than 100 Ω 
  9. Test for greater than 35 Ω between each pair of the CAN bus 2 serial data circuit terminals.
    • If 35 Ω or less 

      Refer to Testing the Serial Data Circuits for a Short between the Circuits  .

    • Go to next step: If greater than 35 Ω 
  10. Test for less than infinite resistance between each of the CAN bus 2 serial data circuit terminals and ground.
    • If infinite resistance 

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

    • Go to next step: If less than infinite resistance 
  11. Replace the non communicating device.
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