Troubleshooting Right Turn and Running Lights on Trailer Towed by 2007 Ford F150

Question:

Im connecting my 2007 Ford F-150 to a trailer both equiped with a 7 way connectors and only the left turn signal works. Where do I start?

asked by: Bob V

Expert Reply:

I would recommend starting at the 7-way connector at the rear bumper of your truck.

Use a circuit tester like part # PTW2993 and activate the right turn signal. Test the pin in the 3:00 position. The tester should flash with the turn signals. When the brake pedal is pressed, this pin should light up the tester as well. Repeat the test on the pin in the 9:00 position which is the left turn/brake and the pin in the 11:00 position, which is the running/ marker lights. Check the pins for excessive white or green corrosion. In extreme cases, the corrosion can become bad enough that it spreads to the interior of the connector and prevents the current from passing through. If large amounts of corrosion are found, replace the connector with part # HM40975. Make sure the connector on the truck is securely grounded to clean metal.

If you aren't getting current on your right turn and running light pin, you might have a blown fuse. The fuses that manage the towing circuits are usually located in the fuse box under the hood. Your owners manual should have additional information about fuse location. The underside of the fuse box should have a legend on it that shows which fuses protect which circuits.

If the trailer connector on the truck checks out OK, you know the wiring problem exists on the trailer itself, and not the truck. Check the connector on the trailer for corrosion. If excessive corrosion is found, replace the connector with a new 7-way trailer plug like part # A7WCB. Make sure the ground connection (usually the white wire) is securely fastened to the trailer frame and is not corroded. Trace the running lamp circuit (brown wire) and right turn/brake circuit (yellow wire) back from the connector to the lamps they serve. Make sure the insulation on the wire is in good shape, no cracks or worn areas that would allow the copper conductor within to short to the trailer frame. Make sure no wires are pinched between portions of the trailer frame. Check that the connections at each lamp are secure.

Diagnosing your problem might take some work, but with using a careful process of elimination, you should be able to locate and correct the problem.

We also have an excellent trailer wiring FAQ page that might be of assistance, click on the provided link.