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Home >> Cadillac >> 2016 >> CTS Base, AWD >> Repair and Diagnosis >> Engine Performance >> System >> Data Communication System >> Diagnostic Information and Procedures >> DTC U1804: Lost Communication with Inside Rearview Mirror Control Module >> 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+ 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. Disconnect all the harness connectors at the A10 Inside Rearview Mirror. 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. 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 A10 Inside Rearview Mirror.
    • If the test lamp illuminates 
  5. Ignition OFF, all access doors closed, all vehicle systems OFF and all keys at least 3 m (9.8 ft) away from vehicle. Disconnect the harness connectors at the device setting the DTC. It may take up to 2 minutes for all vehicle systems to power down.
  6. Test for less than 2 Ω in each of the serial data circuits end to end between the A10 Inside Rearview Mirror and the device setting the DTC.
    • If 2 Ω or greater 

      Repair the open/high resistance in the serial data circuit between the A10 Inside Rearview Mirror and the device setting the DTC or a serial data splice pack.

    • If less than 2 Ω 
  7. Replace the A10 Inside Rearview Mirror.
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