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Circuit/System Testing

NOTE:

The MOST devices on both sides of the MOST ring break point will need to be disconnected to isolate a circuit fault.

Use the schematic to identify the following:

  • The MOST bus configuration and MOST device locations on the MOST serial data circuits
  • Each MOST device's electronic control line and serial data circuit terminals
  1. Ignition OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 meters (9.8 feet) away from vehicle. Disconnect the harness connectors at the MOST device upstream from the surrogate MOST Master device where the MOST ring break occurs. It may take up to 2 minutes for all vehicle systems to power down.
  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 disconnected device.
    • 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 disconnected device.
    • If the test lamp illuminates 
  7. Ignition ON.
  8. Test for 9-13 V between the MOST control circuit at the MOST device connectors that was just disconnected and ground.
    • If less than 9 V 
    1. Ignition OFF, disconnect the harness connectors at the A11 Radio.
    2. 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.
    2. 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, replace the A11 Radio.
    • If between 9-13 V 
  9. Ignition OFF, disconnect the harness connectors at the surrogate MOST Master device, ignition ON. The following tests will be performed on the MOST serial data circuits between the two MOST devices that were just disconnected.
  10. Test for less than 1 V between each MOST serial data circuit and ground.
    • If any serial data circuit are 1 V or greater 

      Repair the short to voltage in the circuit.

    • If each serial data circuit are less than 1 V 
  11. Ignition OFF, test for infinite resistance between each MOST serial data circuit and ground.
    • If any serial data circuit are less than infinite resistance 

      Repair the short to ground in the circuit.

    • If each serial data circuit are infinite resistance 
  12. 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 a short together in the circuit.

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

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

    • If each serial data circuit are less than 2 Ω 
  14. Replace the MOST device upstream from the surrogate MOST Master device where the MOST ring break occurs.
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