Pinpoint Test Aa: The Trailer Reversing Lamps Are Inoperative

WARNING: This page is about a different car, the 2014 Ford F-150. 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.

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) And Ground
  Courtesy 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 OEM WIRING DIAGRAMS to identify the possible causes of the circuit short.
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) And Ground
  Courtesy 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 .
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) And Ground
  Courtesy 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 .
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
  Courtesy 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.
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
  Courtesy 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.
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
  Courtesy 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.