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Home >> Cadillac >> 2015 >> XTS Base >> Repair and Diagnosis >> Engine Performance >> System >> Data Communication System >> Diagnostic Information and Procedures >> DTC U0028: MOST Bus >> Circuit/System Testing
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Circuit/System Testing

NOTE:

You must perform Circuit/System Verification before proceeding with Circuit/System Testing.

NOTE:

Use the Radio scan tool, schematics and specific vehicle build RPO codes to determine optional devices and to identify the following:

  • The MOST bus ring configuration
  • Each MOST device's MOST control and serial data circuit terminals
    NOTE:

    If the A11 Radio is the first node position in the "Node Locations of MOST Bus Communication Break" range, proceed to step 8.

  1. Ignition OFF, disconnect the harness connector containing the MOST circuits at the MOST device at the first Node position reported by Node Locations of MOST Bus Communication Break parameter value.
  2. Connect the appropriate connector from the EL-51578 MOST Bus Diagnostic Tool Kit to the harness connector of the MOST device to bypass this MOST device.
  3. Ignition ON.
    4. NOTE:

    MOST control (ECL) voltage will toggle from 9-13 V to 0 V while DTC U0028 is current.

  4. Test for 9-13 V between the ECL connector from the EL-51578 MOST Bus Diagnostic Tool Kit and ground.
    • If less than 9 V 
    1. Ignition OFF and all vehicle systems OFF, disconnect the harness connectors at the A11 Radio.
    2. Disconnect the EL-51578 MOST Bus Diagnostic Tool Kit from the harness connector of the first Node location MOST device.
    3. Test for less than 2 Ω in the MOST control circuit end to end between the MOST device that was just disconnected and the A11 Radio.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, replace the A11 Radio.
    • If greater than 13 V 
    1. Ignition OFF, disconnect the harness connectors at the A11 Radio and all other MOST devices.
    2. Disconnect the EL-51578 MOST Bus Diagnostic Tool Kit from the harness connector of the first Node location MOST device, ignition ON.
    3. Test for less than 1 V between the MOST control circuit and ground.
      • If 1 V or greater, repair the short to voltage in the circuit.
      • If less than 1 V
    4. Ignition OFF, reconnect the harness connectors at the first MOST device, ignition ON.
    5. Test for 9-13 V between the MOST control circuit and ground.
      • If greater than 13 V, replace the MOST device that was just connected.
      • If 13 V or less and there are more MOST devices to connect, connect the next MOST device and repeat step 4.4.
      • If 13 V or less and there are no more MOST devices to connect
    6. Replace the A11 Radio.
    • If between 9-13 V 
  5. Verify that DTC U0028 status remains current.
    • If the status of DTC U0028 changes from current to history 

      Replace the disconnected MOST device.

    • If the status of DTC U0028 remains current 
  6. Ignition OFF, disconnect the EL-51578 MOST Bus Diagnostic Tool Kit from the harness connector of the first Node location MOST device.
  7. Reconnect the harness connector containing the MOST circuits at the first Node location MOST device.
    8. NOTE:

    If the A11 Radio is the last node position in the "Node Locations of MOST Bus Communication Break" range, disconnect the harness connector containing MOST circuits at the A11 Radio and proceed to step 13.

  8. Disconnect the harness connector containing the MOST circuits at the last Node location MOST device of the fault location identified by Node Locations of MOST Bus Communication Break parameter value.
  9. Connect the appropriate connector from the EL-51578 MOST Bus Diagnostic Tool Kit to the harness connector of the last Node location MOST device to bypass this MOST device.
  10. Ignition ON.
  11. Verify that DTC U0028 status remains current.
    • If the status of DTC U0028 changes from current to history 

      Replace the disconnected MOST device.

    • If the status of DTC U0028 remains current 
  12. Ignition OFF, disconnect the EL-51578 MOST Bus Diagnostic Tool Kit from the harness connector of the last Node location MOST device.
  13. Disconnect the harness connector containing the MOST circuits at the first Node location MOST device. The following tests will be performed on the MOST serial data circuits between the two MOST devices that are disconnected.
  14. Ignition ON.
  15. Test for less than 1 V between each MOST serial data circuit and ground.
    • If any serial data circuits are 1 V or greater 

      Repair the short to voltage in the circuit.

    • If each serial data circuits are less than 1 V 
  16. Ignition OFF, all access doors closed, and all vehicle systems OFF.
  17. Test for infinite resistance between each MOST serial data circuit and ground.
    • If any serial data circuits are less than infinite resistance 

      Repair the short to ground in the circuit.

    • If each serial data circuits are infinite resistance 
  18. Test for infinite resistance between each pair of the MOST serial data circuits.
    • If any pair of serial data circuits are less than infinite resistance 

      Repair the short between the circuits.

    • If each pair of serial data circuits are infinite resistance 
  19. Test for less than 2 Ω in each MOST serial data circuit end to end.
    • If any serial data circuits are 2 Ω or greater 

      Repair the open/high resistance in the circuit between the two MOST devices.

    • If each serial data circuits are less than 2 Ω 
  20. Replace the A11 Radio.
  21. Ignition ON. Radio ON.
  22. Verify that DTC U0028 does not set.
    • If the DTC U0028 sets 

      Refer to Diagnostic Aids.

    • If the DTC U0028 does not set 
  23. All OK.
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