Trailer Running Lights Do Not Work After Adding 4-Way to 7-Way Adapter on 1991 Ford F-350  

Question:

Helpful Expert Reply:

The first thing you are going to want to do is unplug the 4-Way from the 7 and 4-Way so you can test it. You will need a circuit tester # PTW2993 if you do not already have one. The pin right next to the ground pin is the running light circuit. Have someone sit in the truck and operate the running lights while you test with the circuit tester.

If the running light circuit does not work, then you will want to check for a tow package running light fuse or relay. Use the owner's manual and/or legend on the fuse box to find the locations. If that does not work, then there is a wiring issue somewhere between the 4-Way and where it connects to the truck you will need to track down.

If it does work on the existing 4-Way but stops working when you add the 7-Way, then that would indicate a problem with the 7-Way. You will want to open it up and check for loose or missing connections. Also make sure that the unit is properly grounded to the truck frame. It will need to ground to a clean and corrosion free bare metal surface.

If everything works on the truck side but not the trailer, then make sure that the trailer side connector is clean and free of corrosion. Then make sure the main connector ground and all of the light grounds are attached to clean and corrosion free bare metal surfaces.

There are some features on certain trailers that can cause similar issues. If the trailer has an aluminum frame, tilt bed, or folding tongue then running jumper grounds from the lights to the main trailer connector ground can help. Also, if you recently added a plastic license plate frame between a light ground and the frame, that, too, can interfere with a good ground connection.

I have included a couple of links that should help you.

expert reply by: Michael H

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