Pinpoint Test Aa: The Trailer Lamps Are Inoperative - LH 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.

AA1 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 C439-3, 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 1: Measuring Voltage Between Trailer Tow C439-3 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 AA2.
AA2 CHECK CIRCUIT 52 (YE) FOR AN OPEN
- Disconnect: Trailer Tow LH Turn Relay C1188.
- Measure the resistance between the trailer tow LH turn relay C1188-5, circuit 52 (YE), harness side and the trailer tow C439-3, circuit 52 (YE) (4-pin connector), harness side, or the trailer tow C473-1, circuit 52 (YE) (7-pin connector), harness side.
  
  Fig 2: Measuring Resistance Between Trailer Tow LH Turn Relay C1188-5 And Trailer Tow C439-3
  Courtesy of FORD MOTOR CO.
- Is the resistance less than 5 ohms?
- Yes : GO to AA3.
- No : REPAIR the circuit. TEST the system for normal operation.
AA3 CHECK THE TRAILER TOW LH TURN RELAY
- Carry out the trailer tow LH turn relay component test. Refer to COMPONENT TESTING .
- Is the relay OK?
- Yes : GO to AA4.
- No : INSTALL a new trailer tow LH turn relay. TEST the system for normal operation.
AA4 CHECK FOR VOLTAGE ON CIRCUIT 175 (BK/YE)
- Measure the voltage between the trailer tow LH turn relay C1188-3, 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 AA5.
- No : REPAIR the circuit. TEST the system for normal operation.
AA5 CHECK FOR VOLTAGE ON CIRCUIT 374 (DB/WH)
- While applying the brake pedal, measure the voltage between the trailer tow LH turn relay C1188-1, circuit 374 (DB/WH), 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 374 (DB/WH). TEST the system for normal operation.