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Home >> Ford >> 2012 >> Mustang Base, 2D Coupe, Automatic Trans >> Repair and Diagnosis >> External Pages >> Different car >> Section 139 (Exterior Lighting System - F150) >> Diagnosis And Testing >> Trailer Lamps >> Pinpoint Tests >> Pinpoint Test AA: The Trailer Reversing Lamps Are Inoperative >> Pinpoint Test Aa: The Trailer Reversing Lamps Are Inoperative

Pinpoint Test Aa: The Trailer Reversing Lamps Are Inoperative

WARNING: This page is about a different car, the 2012 Ford Pickup. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: Use the correct probe adapter(s) when making measurements. Failure to use the correct probe adapter(s) may damage the connector.
NOTE: Before beginning diagnostics, visually inspect the trailer tow connector mounted on the bumper for signs of corrosion or damage. Repair or install a new trailer tow connector as required.
  1. AA1 CHECK FOR VOLTAGE THROUGH THE TRAILER TOW CONNECTOR 
    • Apply the parking brake.
    • Ignition ON.
    • Select REVERSE.
    • Measure the voltage between the trailer tow connector reversing lamp voltage supply pin, circuit CAT16 (GY/BN), component side and ground.
      Fig 1: Measuring Voltage Between Trailer Tow Connector Reversing Lamp Voltage Supply Pin, Circuit CAT16 (GY/BN), Component Side And Ground
      G06065336Courtesy of FORD MOTOR CO.
    • Is the voltage greater than 10 volts? 
      • Yes  : The vehicle is operating correctly. SEND the trailer to an authorized camper/trailer repair facility.
      • No  : VERIFY the BJB fuse 43 (15A) is OK. If OK, GO to AA2. If not OK, REFER to the Wiring Diagrams information to identify the possible causes of the circuit short.
  2. AA2 CHECK FOR VOLTAGE TO THE TRAILER TOW CONNECTOR (VEHICLE HARNESS) 
    • Ignition OFF.
    • Disconnect: Trailer Tow C439A.
    • Ignition ON.
    • Select REVERSE.
    • Measure the voltage between the trailer tow C439A-7, circuit CAT16 (GY/BN), harness side and ground.
      Fig 2: Measuring Voltage Between Trailer Tow C439A-7, Circuit CAT16 (GY/BN), Harness Side And Ground
      G06065337Courtesy of FORD MOTOR CO.
    • Is the voltage greater than 10 volts? 
      • Yes  : INSTALL a new trailer tow connector. TEST the system for normal operation.
      • No  : GO to AA3.
  3. AA3 CHECK THE TRAILER TOW REVERSING LAMP RELAY 
    • Ignition OFF.
    • Disconnect: Trailer Tow Parking Lamp Relay.
    • Substitute a known good relay.
    • Ignition ON.
    • Select REVERSE.
    • Measure the voltage between the trailer tow C439A-7, circuit CAT16 (GY/BN), harness side and ground.
      Fig 3: Measuring Voltage Between Trailer Tow C439A-7, Circuit CAT16 (GY/BN), Harness Side And Ground
      G06065337Courtesy of FORD MOTOR CO.
    • Is the voltage greater than 10 volts? 
      • Yes  : REMOVE the known good relay. INSTALL a new trailer tow reversing lamp relay. TEST the system for normal operation.
      • No  : REMOVE the known good relay. GO to AA4.
  4. AA4 CHECK FOR VOLTAGE TO THE TRAILER TOW REVERSING LAMP RELAY COIL 
    • Select REVERSE.
    • Measure the voltage between the trailer tow reversing lamp relay pin 1, circuit CLS28 (BU/WH), BJB face side and ground.
      Fig 4: Measuring Voltage Between Trailer Tow Reversing Lamp Relay Pin 1, Circuit CLS28 (BU/WH), BJB Face Side And Ground
      G06141841Courtesy of FORD MOTOR CO.
    • Is the voltage greater than 10 volts? 
      • Yes  : GO to AA5.
      • No  : REPAIR circuit CLS28 (BU/WH) for an open. TEST the system for normal operation.
  5. AA5 CHECK THE TRAILER TOW REVERSING LAMP RELAY BJB VOLTAGE SUPPLY 
    • Select PARK.
    • Ignition OFF.
    • Measure the voltage between the trailer tow reversing lamp relay pin 3, circuit SBB43 (BU/RD), BJB face side and ground.
      Fig 5: Measuring Voltage Between Trailer Tow Reversing Lamp Relay Pin 3, Circuit SBB43 (BU/RD), BJB Face Side And Ground
      G06141842Courtesy of FORD MOTOR CO.
    • Is the voltage greater than 10 volts? 
      • Yes  : GO to AA6.
      • No  : REPAIR circuit SBB43 (BU/RD) for an open. TEST the system for normal operation.
  6. AA6 CHECK THE TRAILER TOW REVERSING LAMP RELAY GROUND CIRCUIT FOR AN OPEN 
    • Measure the voltage between the trailer tow reversing lamp relay pin 3, circuit SBB43 (BU/RD), BJB face side and the trailer tow reversing lamp relay pin 2, circuit GD123 (BK/GY), BJB face side.
      Fig 6: Checking Trailer Tow Reversing Lamp Relay Ground Circuit For Open
      G07409899Courtesy of FORD MOTOR CO.
    • Is the voltage greater than 10 volts? 
      • Yes  : REPAIR circuit CAT16 (GY/BN) for an open. TEST the system for normal operation.
      • No  : REPAIR circuit GD123 (BK/GY) for an open. TEST the system for normal operation.
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