Pinpoint Test AA: The Trailer Lamps Are Inoperative - LH Trailer Turn/Stoplamp

WARNING: This page is about a different car, the 2004 Mercury Mountaineer and 2004 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: Checking Voltage To Trailer Tow Connector
  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-oin connector), harness side.
  
  Fig 2: Checking Circuit 52 (YE) For An Open
  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 for schematic and connector information.
- 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.
Yes

GO to AA5.

No

REPAIR the circuit. TEST the system for normal operation.

Fig 3: Checking For Voltage On Circuit 175 (Bk/Ye)
Courtesy of FORD MOTOR CO.
- Is the voltage greater than 10 volts?
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: Checking For Voltage On Circuit 374 (DB/WH)
  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.