Pinpoint Test Z: The Trailer Stop/Turn Lamp Is Inoperative

WARNING: This page is about a different car, the 2010 Lincoln MKX and 2010 Ford Edge. 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.

Z1 CHECK FOR VOLTAGE TO THE TRAILER TOW CONNECTOR
- Ignition OFF.
- For the LH trailer stop/turn lamp, while applying the brake pedal, measure the voltage between the trailer tow C439-2, circuit CAT06 (YE), harness side and ground.
  
  Fig 1: Measuring Voltage Between Trailer Tow C439-2, Circuit CAT06 (YE) And Ground
  Courtesy of FORD MOTOR CO.
- For the RH trailer stop/turn lamp, while applying the brake pedal, measure the voltage between the trailer tow C439-1, circuit CAT09 (GN), harness side and ground.
  
  Fig 2: Measuring Voltage Between Trailer Tow C439-1, Circuit CAT09 (GN) 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 24 (15A) (LH stop/turn relay) or fuse 46 (15A) (RH stop/turn relay) is OK. If OK, GO to Z2 . If not OK, REFER to SYSTEM WIRING DIAGRAMS for Edge or SYSTEM WIRING DIAGRAMS for MKX to identify the possible causes of the circuit short.
Z2 CHECK THE TRAILER TOW STOP/TURN RELAY
- Disconnect: Suspect Trailer Tow Stop/Turn Relay.
- Substitute a known good relay.
- For the LH trailer stop/turn lamp, while applying the brake pedal, measure the voltage between the trailer tow C439-2, circuit CAT06 (YE), harness side and ground.
  
  Fig 3: Measuring Voltage Between Trailer Tow C439-2, Circuit CAT06 (YE) And Ground
  Courtesy of FORD MOTOR CO.
- For the RH trailer stop/turn lamp, while applying the brake pedal, measure the voltage between the trailer tow C439-1, circuit CAT09 (GN), harness side and ground.
  
  Fig 4: Measuring Voltage Between Trailer Tow C439-1, Circuit CAT09 (GN) 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 stop/turn relay. TEST the system for normal operation.
- No: REMOVE the known good relay. GO to Z3 .
Z3 CHECK FOR VOLTAGE TO THE STOP/TURN LAMP RELAY COIL
- While applying the brake pedal, measure the voltage between the trailer tow LH stop/turn relay pin 1, circuit CLS18 (GY/BN), BJB face side and ground; or between the trailer tow RH stop/turn relay pin 2, circuit CLS19 (VT/OG), BJB face side and ground.
  
  Fig 5: Measuring Voltage Between Trailer Tow LH Stop/Turn Relay Pin 1, Circuit CLS18 (GY/BN) BJB Face Side And Ground
  Courtesy of FORD MOTOR CO.
- Is the voltage greater than 10 volts?
- Yes: GO to Z4 .
- No: REPAIR circuit CLS18 (GY/BN) (LH stop/turn) or circuit CLS19 (VT/OG) (RH stop/turn) for an open. TEST the system for normal operation.
Z4 CHECK THE BJB
- Measure the voltage between the trailer tow LH stop/turn relay pin 3, BJB face side and ground; or between the trailer tow RH stop/turn relay pin 5, BJB face side and ground.
  
  Fig 6: Measuring Voltage Between Trailer Tow LH Stop/Turn Relay Pin 3 BJB Face Side And Ground
  Courtesy of FORD MOTOR CO.
- Is the voltage greater than 10 volts?
- Yes: GO to Z5 .
- No: INSTALL a new BJB . TEST the system for normal operation.
Z5 CHECK FOR VOLTAGE TO THE STOP/TURN LAMP RELAY USING THE RELAY GROUND PIN
- Measure the voltage between the trailer tow LH stop/turn relay pin 3, BJB face side and the trailer tow LH stop/turn relay pin 2, circuit GD120 (BK/GN), BJB face side.
  
  Fig 7: Measuring Voltage Between Trailer Tow LH Stop/Turn Relay Pin 3 BJB Face Side And Pin 2, Circuit GD120 (BK/GN)
  Courtesy of FORD MOTOR CO.
- Measure the voltage between the trailer tow RH stop/turn relay pin 5, BJB face side and the trailer tow RH stop/turn relay pin 1, circuit GD120 (BK/GN), BJB face side.
  
  Fig 8: Measuring Voltage Between Trailer Tow RH Stop/Turn Relay Pin 5, BJB Face Side And Pin 1, Circuit GD120 (BK/GN)
  Courtesy of FORD MOTOR CO.
- Is the voltage greater than 10 volts?
- Yes: REPAIR circuit CAT06 (YE) (LH trailer stop/turn) or circuit CAT09 (GN) (RH trailer stop/turn) for an open. TEST the system for normal operation.
- No: REPAIR circuit GD120 (BK/GN) for an open. TEST the system for normal operation.