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Trailer LED Lights Stay on After 2016 Ford F-150 is Turned Off

Question:

Hello, I have a utility trailer that I just refurbished and completely re-did the lights and wiring on it as well. The trailer has two STL72RB Optronics tail lights and two MCL55RB marker lights on it in medal protector boxes. The wiring is the optronics “wishbone” 4 way with the running lights wire brown on each side. The trailer has 3 grounds: 1 off the trailer side 4 way plug-in that goes to frame, and 1 off each of the rear 3-prong plugs for the STL72RB. The MCL55RB side marker light grounds are tied into the same one as the STL72RB tail lights. My problem is when I cut the truck off and leave the trailer plugged into the truck, there is a very small LED light lit up inside the lights on the trailer. It doesn’t shine as bright as if the running lights are actually on, in fact they won’t even run the truck battery down if left plugged in all night and the following day, but they are on so I sense there is a problem somewhere. I have seen y’all post remedy’s about this elsewhere and I tried all the other suggestions: there is no power coming off the truck when the key is off and the light switch is off. The truck is a 2016 Ford F-150, I have also hooked it up to my wife’s 2013 Expedition and the trailer does the same thing, Expedtion also has zero volts coming though trailer harness when key and engine off. I personally believe the trailer is drawing some sort of power from the truck through the grounds, or “backfeeding”. All functions stop, turn, running lamps work correctly on trailer. And I verified the grounds all work by plugging it in to my truck then unhooking each ground while I had the functions turned on and yes the trailer was not hooked up or touching the ball so it didn’t ground through that way either. What do you think would be causing this? I have also cleaned all the “pins” on both the trucks and trailer sides. I have no idea what is going on. Let me add too that I have another utility trailer that is wired up identical triple grounds, connections soldered, etc only it has incandescent bulb and it doesn’t do this at all. Please help!

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Expert Reply:

Since you have checked all of your grounds and tried hooking up your trailers to different vehicles you already went through all of the troubleshooting steps that I would have recommended for you to go through.

If your connectors are clean, your grounds are secure and connected to a clean, bare metal surface, and your wiring looks good, then there really is only one explanation. The LED lights on your trailer don't require very much power to turn on at all. Even though you have checked for power and found none, there is a very small amount that is still going through the wiring. It isn't enough to light the lights up completely, as you noted, but it does put through just enough that the LEDs are getting some signal.

Just as you have found this isn't going to really drain your battery since it is such a low draw. However, if you plan on having your trailer coupled to your F-150 for a couple of days then I recommend unplugging your trailer when you park for extended periods of time.

expert reply by:
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Jon G

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