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Troubleshooting Non-Functioning Trailer Lights Powered by 2009 Ford F-150

Question:

i got problems with trailer lights, check relays are ok #37,38,#10, check wiring looks ok what can i do to fix the problem.

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Expert Reply:

Troubleshooting trailer wiring is easier when you disconnect the trailer from the towing vehicle and test each one individually.

Since you have already checked the truck's towing-related relays I suggest focusing on the trailer. The best way to test the trailer lights is to directly apply +12V power from the F-150 battery (or a well-charged spare battery) to the contacts on the trailer's plug. You can refer to the linked photo and article on trailer wiring that shows you which contact on the plug feeds each different lighting circuit. A set of jumper cables and two short lengths of wire will make this testing easier.

Apply the battery negative terminal to the ground contact on the plug and then one at a time touch the positive wire to the contact for each of the light circuits. If these functions work with this direct power then you know the problem is on the truck side. If the trailer light functions still do not work then you could still have an issue, such as a loose ground wire or corrosion in the plug. Both the truck and trailer main ground wires must make solid contact to clean bare metal on the frames. A weak ground can cause all sorts of problems with lights. Good clean ground connections can be protected from corrosion and dirt by applying dielectric grease like # 11755.

If you have a circuit tester like # 40376 you can test the socket on the truck. Have a helper activate each light circuit on the vehicle while you apply the tester to the appropriate contacts. If signals are present at the truck socket as they should be then this tends to indicate the problem is on the trailer.

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