Pinpoint Test Z: The Trailer Lamps Are Inoperative - RH Trailer Turn/STOPLAMP

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.

Z1 CHECK VOLTAGE TO TRAILER TOW CONNECTOR
- Disconnect: Trailer Tow C439 (4-Pin Connector).
- Disconnect: Trailer Tow C473 (7-Pin Connector).
- While applying the brake pedal, measure the voltage between the trailer tow C439-4, circuit 64 (DG), harness side and ground (4-pin connector); and between the trailer tow 3473-4, circuit 64 (DG), harness side and ground (7-pin connector).
  
  Fig 1: Measuring Voltage Between Trailer Tow C439-4 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 : GO to Z2.
Z2 CHECK CIRCUIT 64 (DG) FOR AN OPEN
- Disconnect: Trailer Tow RH Turn Relay C1187.
- Measure the resistance between the trailer RH turn relay C1187-5, circuit 64 (DG), harness side and the trailer tow C439-4, circuit 64 (DG) (4-pin connector), harness side, or the trailer tow C473-4, circuit 64 (DG) (7-pin connector), harness side.
  
  Fig 2: Measuring Resistance Between Trailer RH Turn Relay C1187-5 And Trailer Tow C439-4
  Courtesy of FORD MOTOR CO.
- Is the resistance less than 5 ohms?
- Yes : GO to Z3.
- No : REPAIR the circuit. TEST the system for normal operation.
Z3 CHECK THE TRAILER TOW RH TURN RELAY
- Carry out the trailer tow RH turn relay component test. Refer to COMPONENT TESTING article.
- Is the relay OK?
- Yes : GO to Z4.
- No : INSTALL a new trailer tow RH turn relay. TEST the system for normal operation.
Z4 CHECK FOR VOLTAGE ON CIRCUIT 175 (BK/YE)
- Measure the voltage between the trailer tow RH turn relay C1187-3, circuit 175 (BK/YE), harness side and ground.
  
  Fig 3: Checking Relay Power Supply
  Courtesy of FORD MOTOR CO.
- Is the voltage greater than 10 volts?
- Yes : GO to Z5.
- No : REPAIR the circuit. TEST the system for normal operation.
Z5 CHECK FOR VOLTAGE ON CIRCUIT 378 (BN/YE)
- While applying the brake pedal, measure the voltage between the trailer tow RH turn relay C1187-1, circuit 378 (BN/YE), harness side and ground.
  
  Fig 4: Measuring Voltage Between Relay Connector And Ground
  Courtesy of FORD MOTOR CO.
- Is the voltage greater than 10 volts?
- Yes : REPAIR circuit 1205 (BK). TEST the system for normal operation.
- No : REPAIR circuit 378 (BN/YE). TEST the system for normal operation.