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Trailer Brake and Turn Signals Not Working on One Vehicle But Working on the Other Vehicle

Question:

My boyfriend has a f250 super duty 6.7L and put of no where when he tried to pull his trailer the brake and turn signals do not work. He hooked his truck up to his second trailer and everything works fine. He said he checked the ground connection and ND it was good. Any idea as to what can be causing yhe lights not to work on one trailer and work ok n the other?

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Expert Reply:

The issue with your boyfriends trailer lights still sounds like a ground issue, but there are a few things he can check.

He needs to make sure that on both the connector on his truck and the trailer plug is clean and free of any corrosion. I recommend using a dielectric grease, # LT37534, once the plug and connecter have been cleaned and inspected. The grease will prevent corrosion and eliminate water contamination.

If the trailer plug and vehicle connecter have check out and are clean, then you will need to check the electrical signal coming out of his trucks trailer connecter. I have attached a picture that show the standard wiring of a vehicles 4-flat and 7-way trailer connectors. He will need to use a tester like # PTW2993 to test the output on the trailer and stop light pins in the connector. If the signal is strong on the truck side of the wiring, then the issue be on the trailer.

He will have to check the grounds on the trailer for physical damage or unclean rusty connection to the trailer frame. Even though the trailer worked on a different vehicle, electrical system draws are different from vehicle to vehicle. If his truck is drawing more than the other vehicle, he used to test the trailer, then the trailer light might not work.

I have also linked an article on trailer wiring that might help in your troubleshooting.

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