I have a 2017 F150. Trailer running lights will not work when connected to the truck. All other lights work so I don't believe it would be a fuse or relay. They do work if I connect trailer directly to a battery. The pin voltage at the truck trailer socket is only 3.3v. Any suggestions?

 

First thing to do is clean everything. Road grit is slightly conductive and can create shorts that drop voltage as well as separate contacts, increasing resistance.

Clean and lube the 7-pin and trailer connector, the connector at the backside of the 7-pin behind the bumper, and the connector that's a bit down the wires from that connector. Then remove the ground connection back there, clean it up, and use the same dielectric grease between the frame and lug to slow future corrosion.

Test your voltages again to see if this fixes the issue.

 

There are separate fuses for the trailer running lights/marker lights and for the stop/turn lights. Check your Owners Manual under "Fuses" and leaf through the section. It's several pages in length with a couple of charts and diagrams.

That said, the fact that you're seeing any voltage (which pin? though) means that whatever fuse is in that circuit isn't blown.

 

I had issues with my 2017 F-250 trailer light connector. After a couple trips to the dealer, I took the connections underneath apart and sprayed them really good with CRC Electric - Clean and greased them really good. Never had another problem. Give that a try and let us know how you make out. Good luck, boomer

 

Did you check the plug under the dash that some disconnect to disable ***?

 

Old thread but I found it searching and wanted to add some info Incase it helps others who search.

I had a similar issue, no running lights on the trailer. It was a rental trailer and my camper always worked good behind my 2019 f150 with factory trailer hitch. I checked the running light pin on the truck while it’s running, 3.3v. Thought that was low but ruled out a fuse as I had some voltage.

I opened up the junction box on the trailer and 2 wired had pulled out of their connectors. So I figured that’s my problem. Put in a new end and still no lights. Used a spare battery and jumped a wire to the terminal and I had lights.

now to figure out why I only had 3.3v while running and only .3 while the key was off. Pulled the fuse under the hood and sure enough it was blown. Replaced and all good now. I think the 3.3v comes from another source as the truck checks the function of the circuit to tell us if there is a fault.