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Home >> Cadillac >> 2017 >> XT5 Base >> Repair and Diagnosis >> Engine Performance >> System >> Data Communication System - Diagnostic Information And Procedures >> Diagnostic Information and Procedures >> DTC U18A7: Lost Communication with Power Supply Transformer on Powertrain Expansion CAN Bus >> Circuit/System Testing
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Circuit/System Testing

NOTE:

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

  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 min for all vehicle systems to power down. Disconnect the harness connectors at the T19 Power Supply Transformer.
  2. 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 
  3. Ignition ON.
  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 T19 Power Supply Transformer.
    • If the test lamp illuminates 
  5. Ignition ON.
  6. If equipped, verify a test lamp illuminates between each ignition 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 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 T19 Power Supply Transformer.
    • If the test lamp illuminates 
  7. Ignition ON.
  8. If equipped, verify a test lamp illuminates between the accessory wakeup serial data circuit terminal and ground.
    • If the test lamp does not illuminate 
    1. Ignition OFF, disconnect the harness connectors at the K9 Body Control Module.
    2. Test for infinite resistance between the accessory wakeup serial data 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 accessory wakeup serial data circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, replace the K9 Body Control Module.
    • If the test lamp illuminates 
  9. Test for less than 4.5 V between each powertrain high speed GMLAN serial data circuit terminal and ground.
    • If 4.5 V or greater 

      Refer to DTC U0074  to test for a short to voltage.

    • If less than 4.5 V 
  10. 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 min for all vehicle systems to power down.
  11. Test for greater than 100 Ω between each powertrain high speed GMLAN serial data circuit terminal and ground.
    • If 100 Ω or less 

      Refer to DTC U0074  to test for a short to ground.

    • If greater than 100 Ω 
  12. Disconnect the harness connectors at the device setting the DTC.
  13. Test for less than 2 Ω in each powertrain high speed GMLAN serial data circuits end to end between the T19 Power Supply Transformer and the device setting the DTC.
    • If 2 Ω or greater 

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

    • If less than 2 Ω 
  14. Replace the T19 Power Supply Transformer.
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