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Troubleshooting Trailer Lighting # 118551 Wiring Harness Installed on 2009 Mecury Mariner


2009 Mercury Mariner I installed the Curt wiring harness and tested it with my trailer and it worked fine. Today was my first day using the trailer and when I hooked it up, the passenger side tail light was lit solid, but not the driver side. I then tested the turn signals and the passenger worked, but the driver side was only faintly lit and seemed to even flicker. I verified that all the connections are secure and that the fuse is good. Any suggestions would be appreciated.


Expert Reply:

Locating your problem will require some troubleshooting.

You will first need to determine if the problem exists with the vehicle wiring or with the trailer wiring. Typically, the problem occurs on the trailer. To rule out the vehicle, start by using a circuit tester like part # 40376 on the trailer connector on the vehicle, as each function is being operated. Have a helper activate the left turn signal, then test the pin on the connector that corresponds with the yellow wire. Activate the right turn, and test the pin that corresponds with the green wire. Next, turn on the head lights, and test the pin that corresponds to the brown wire. If any of the pins does not show a good signal, check the points where the wiring harness attaches behind the tail lights. Make sure that the white ground wire with the ring terminal on the end of it is firmly attached to clean metal on the vehicle.

Next, closely examine the connector on the trailer. Make sure it is not corroded or damaged in any way. Check the ground connection, which is usually located on the trailer tongue. Make sure this connection is tight, and not corroded. A good sized majority of trailer wiring problems can be traced to the ground connection. Trace each circuit down the drivers side of the trailer (since the problem seems to exist with the circuits on the drivers side of the trailer), looking for wires that are pinched or have cracked or missing insulation. Make sure that the lamps are securely attached to the trailer, and that the connection point where the lamp mounts to the trailer is not corroded.

expert reply by:
Mike L

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