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Trailer running lights out- Its my truck. Ideas?

Texoma LSV

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As the tittle states the running lights on my trailer will not come on with my truck. They work with my wife's SUV but not my F-150.

I have checked the fuse/s and everything is ok.

Took it to the dealer when I did an oil change and asked if they might be able to trouble shoot. Service rep. took a few min. To visually inspect the fuses and the plug adapter and said it looked good to him. They would need to keep it a few hours if I wanted a real mechanic to look at it (and I am sure a few hundred $).

Any ideas on next steps? Thank you in advance.


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Not sure which version F-150 you have, mine has a square block type fuse under the hood in a fuse box located near the radiator that controls the trailer lights. I can go look if you need a better description.

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7 round plug? Or 4 flat? Or 5 flat? Is it OEM or aftermarket?

Whatever plug it is, Google the pin outs & get a test light out & go thru each pin to see if any of them are not working. Then start tracing back the wires to wherever they go.

Edited by Bill_AirJunky
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Did the F150 used to get the running lights to work?

Cheap solution is to get a test light and test the power output at the trailer plug.

Does the boat trailer use a dedicated ground, or are you using the ball to ground?

Are you using the same drawbar & ball for the SUV as the F150?

If all else fails, you can upgrade to a RAM. :)

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Thanks guys. I did check fuses and all appear to be ok. The service guy told me that a blown fuse would be obvious and I would not need to remove it to check. Yes at one time the lights did work with this truck. I like the idea of getting tester to plug in and then follow that wire back. Upgrading to a Dodge may not be a bad idea either, though a more expensive solution than I was looking for.

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Take a multi meter back to your truck's harness and verify you are getting 12v at the plug terminals?

Also, humor me, spray down both sides of the connection plug (truck's female side and the trailer's male side) with Wd40 and take a wire brush to all the metal contacts. After you scrub both sides with the wire brush, rinse it with a wd40 spray down.

FYI, in Oklahoma I had to do this often. Hard mineralized water will coat the contacts and keep them from being conductive. When you trailer... Does your wiring harness get wet at the ramp?

Edited by nyryan2001
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  • 2 weeks later...

just bought and 2005 F-150 to pull the boat a few miles in and out each year, and it has a brake controller however i have same issue, brake lights work, turn signals work, however running lights do NOT work as well as the breaking locks up in reverse "not fun at the ramp when you have to unplug/plug in, pull fwd, reverse etc.. took it into the dealer to have them troubleshoot.

supercrew 3.73 9200 lbs tow capacity so i thought i'd be ok with the 2013 23lsv, but man is it me or do stores not sell class 4 hitch/balls anymore? online is only place around here seems crazy to me...

soooo maybe this is why the previous owner sold it.....hmm.. haha..

i'll post what they find out in a day or two as it might be the same issue for you.

Edited by The Hulk
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tried everything different plugs/adaptors, even trailers no luck and there is short or bad connection somewhere or the break controller is broken

First off, the brake controller has nothing to do with anything unless you have electric brakes.

Are you running 7 pin or 5 pin on your trailer?

Fwiw, the hundreds of threads that come up when you search trailer running lights not working f150 mostly say it is a relay under the hood.

Edited by oldjeep
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Do you have a voltmeter? If not, get one and check the power to the poles on the wiring harness on the truck. If you don't have power, there is obviously a problem with the truck and I would start with the relay.

The most common problem that I have seen with trailer lights is a bad ground.

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Yes i used meter and no power on the light one . Dealer found what they said was a shorted grounded wire and a blown fuse however I did check all fuses in fuse box before I took it in... so unless there was another fuse elsewhere who knows but it's fixed now. Perhaps she ment something else.. lady who was on phone didn't elaborate much nor did she know much just said it was fixed and ready.

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  • 3 months later...

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