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Home >> Cadillac >> 2021 >> CT4 Sport, 4WD >> Repair and Diagnosis >> External Pages >> Different car >> Section 162 (Data Communications - Diagnostic Information And Procedures (2 Of 3)) >> Diagnostic Information and Procedures >> DTC U1600 >> 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:

Use the schematics and connector end views to identify the device's ground, B+, ignition, and serial data circuit terminals.

  1. Ignition/Vehicle OFF, disconnect all the harness connectors at the K71 Transmission Control Module.
  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 K71 Transmission Control Module.
    • 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 K71 Transmission Control Module.
    • Go to next step: If the test lamp illuminates 
  6. Ignition/Vehicle OFF, disconnect all LIN devices that share the Engine Control Module LIN Bus 1 circuit.
  7. Ignition ON/Vehicle In Service Mode.
    8. NOTE:

    For accurate voltage reading, disconnect the battery charger prior to performing the following test step.

  8. Test for 3.5-12 V between the Engine Control Module LIN Bus 1 circuit terminal (circuit 4621) and ground.
    • If less than 3.5 V 
    1. Ignition/Vehicle OFF, disconnect the harness connectors at the K20 Engine Control Module.
    2. Test for infinite resistance between the Engine Control Module LIN Bus 1 circuit terminal and ground.
      • If less than infinite resistance, repair the short to ground in the circuit.
      • Go to next step: If infinite resistance
    3. Test for less than 2 Ω in the Engine Control Module LIN Bus 1 circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • Go to next step: If less than 2 Ω
    4. Replace the K20 Engine Control Module.
    • If greater than 12 V 
    1. Ignition/Vehicle OFF, disconnect the harness connectors at the K20 Engine Control Module.
    2. Ignition ON/Vehicle In Service Mode.
    3. Test for less than 1 V between the Engine Control Module LIN Bus 1 circuit terminal and ground.
      • If 1 V or greater, repair the short to voltage in the circuit.
      • Go to next step: If less than 1 V
    4. Replace the K20 Engine Control Module.
    • Go to next step: If between 3.5-12 V 
  9. Verify how many LIN devices are connected to this LIN circuit.
    • If there is no other LIN device besides the K71 Transmission Control Module 
    1. Ignition/Vehicle OFF, inspect both harness and component connectors for contamination, corrosion, and terminal tension.
    2. Reconnect the harness connectors at the K71 Transmission Control Module. Verify all connectors/terminals are fully seated.
    3. Ignition ON/Vehicle In Service Mode.
    4. Verify the DTC is not set again.
      • If the DTC is set, replace the K71 Transmission Control Module.
      • Go to next step: If the DTC is not set
    5. A fault is currently not present and may be an intermittent condition.
    • Go to next step: If there are more LIN devices besides the K71 Transmission Control Module 
  10. Ignition/Vehicle OFF, disconnect the harness connectors at the K20 Engine Control Module.
  11. Reconnect the harness connectors at the K71 Transmission Control Module.
  12. Ignition ON/Vehicle In Service Mode.
  13. Test for 3.5-12 V between the LIN serial data circuit terminal at the K20 Engine Control Module and ground.
    • If less than 3.5 V or greater than 12 V 

      Replace the K71 Transmission Control Module.

    • Go to next step: If between 3.5-12 V 
  14. Ignition/Vehicle OFF, reconnect the harness connectors at another LIN device.
  15. Ignition ON/Vehicle In Service Mode.
  16. Test for 3.5-12 V between the LIN serial data circuit terminal at the K20 Engine Control Module and ground.
    • If less than 3.5 V or greater than 12 V 

      Replace the LIN device that was just connected.

    • Go to next step: If between 3.5-12 V 
  17. Repeat steps 14-16 for the remaining LIN devices until a faulty LIN device is identified. If there is no faulty LIN device, replace the K71 Transmission Control Module.
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