Pinpoint Test Ad: The Trailer Lamps Are On Continuously

WARNING: This page is about a different car, the 2005 Mercury Mountaineer and 2005 Ford Explorer. However, it is still accessible from the selected car via links, so may be relevant.

AD1 DETERMINE WHICH LAMP(S) ARE ON CONTINUOUSLY
- Key in ON position.
- Observe the trailer lamps.
- Are the trailer parking lamps illuminated?
- Yes : GO to AD4.
- No : If the trailer reversing lamps are illuminated, GO to AD2.
  If a trailer stop/turn lamp is illuminated, GO to AD6.
AD2 CHECK THE TRAILER TOW REVERSING LAMP RELAY
- Disconnect: Trailer Tow Reversing Lamps Relay C4023.
- Do the trailer reversing lamps continue to illuminate?
- Yes : GO to AD3.
- No : INSTALL a new trailer tow reversing lamp relay. TEST the system for normal operation.
AD3 CHECK CIRCUIT 963 (BK/LG) FOR SHORT TO POWER
- Disconnect: Trailer Tow C473.
- Measure the voltage between the trailer tow C473-7, circuit 963 (BK/LG), harness side and ground.
  
  Fig 1: Measuring Voltage Between Trailer Tow C473-7 And Ground
  Courtesy of FORD MOTOR CO.
- Is any voltage present?
- Yes : REPAIR the circuit. TEST the system for normal operation.
- No : The vehicle is operating correctly. SEND the trailer to an authorized camper/trailer repair facility.
AD4 CHECK THE TRAILER TOW PARKING LAMP RELAY
- Disconnect: Trailer Tow Parking Lamp Relay C4022.
- Do the trailer parking lamps continue to illuminate?
- Yes : GO to AD5.
- No : INSTALL a new trailer tow parking lamp relay. TEST the system for normal operation.
AD5 CHECK CIRCUIT 962 (BN/WH) FOR SHORT TO POWER
- Disconnect: Trailer Tow C439 (4-Pin Connector).
- Disconnect: Trailer Tow C473 (7-Pin Connector).
- Measure the voltage between the trailer tow C439-2, circuit 962 (BN/WH), harness side and ground (4-pin connector); and between the trailer tow C473-6, circuit 962 (BN/WH), harness side and ground (7-pin connector).
  
  Fig 2: Measuring Voltage Between Trailer Tow C439-2 And Ground
  Courtesy of FORD MOTOR CO.
- Is any voltage present?
- Yes : REPAIR the circuit. TEST the system for normal operation.
- No : The vehicle is operating correctly. SEND the trailer to an authorized camper/trailer repair facility.
AD6 CHECK THE TRAILER TOW RH TURN OR LH TURN RELAY
- Disconnect: Trailer Tow RH Turn Relay C1187 or LH Turn Relay C1188.
- Does the RH or LH trailer stop/turn lamp continue to illuminate?
- Yes : GO to AD7.
- No : INSTALL a new trailer tow turn lamp relay. TEST the system for normal operation.
AD7 CHECK CIRCUIT 52 (YE) OR 64 (DG) FOR SHORT TO POWER
- Disconnect: Trailer Tow C439 (4-Pin Connector).
- Disconnect: Trailer Tow C473 (7-Pin Connector).
- If the RH trailer stop/turn lamp is on continuously, measure the voltage between the trailer tow C439-4, circuit 64 (DG), harness side and ground (4-pin connector); and between the trailer tow C473-4, circuit 64 (DG), harness side and ground (7-pin connector).
  
  Fig 3: Measuring Voltage Between Trailer Tow C439-4 And Ground
  Courtesy of FORD MOTOR CO.
- If the LH trailer stop/turn lamp is on continuously, measure the voltage between the trailer tow C439-3, circuit 52 (YE), harness side and ground (4-pin connector); and between the trailer tow C473-1, circuit 52 (YE), harness side and ground (7-pin connector).
  
  Fig 4: Measuring Voltage Between Trailer Tow C439-3 And Ground
  Courtesy of FORD MOTOR CO.
- Is any voltage present?
- Yes : REPAIR circuit 64 (DG) (RH stop/turn lamp) or 52 (YE) (LH stop/turn lamp) as necessary. TEST the system for normal operation.
- No : The vehicle is operating correctly. SEND the trailer to an authorized camper/trailer repair facility.