Trailer light issue

I have a 2015 F-150 that I use to pull a 2009 Brenderup Baron. I have had the truck for 3 1/2 years but just started hauling the trailer again last year after. I hauled the trailer I think, 3 times last year and I just plugged it in and went. All lights worked fine. The Brenderup does not have electric brakes so the only wire for brakes is for the brake lights.

After the winter lull, backed up to the trailer and have NO trailer lights. I tested the truck plug with an anemic little tester I had thrown in the trailer and got nothing from the truck plug. Stopped today and invested in a better tester and I had juice to all prongs.

What could be causing the issue? I checked the trailer plug and all contacts were good and I tightened them all. I stick the trailer plug under the frame so it should be protected. Again, all the contacts looked good. Replace the plug? Again, I have NO lights on the trailer. I would think something should work and have a hard time believing all the wiring went poof.

I am trying to do some pre-service diagnosis. Being a woman I get all kinds of BS from most service guys so I am trying to pare down the possibilities.


My first thought was a blown fuse there are Three fuse boxes on our F150… I believe the trail light fuse is in the fuse box under the front passenger seat… that had happened to us… but that was before reading all of your post. … so your problem might be a ground issue between the truck and the trailer

This happened to my 2011 B’up Baron LSL. It ended up being the connector on the trailer end.

Yes, I thought fuses but when I got a better tester, all the prongs were hot. The ground wire was
well attached in the trailer plug. I got under the truck but don’t really know what I am looking at.
My but there are a lot of wires under there and 3 different plugs on the truck. Not like back on in the old days.

I am taking the truck in for service and will have them check the truck and go from there.


The actual plug?


Yes. The actual plug needed to be replaced. My mechanic said it was super easy.

Yes, the plug is pretty easy to replace. I will try that after the truck is checked.


Check this out;

This is a link to somebody’s dropbox info on Brenderup Trailers Including wiring info;

Check all bulbs on the same circuit too.

I had this issue on a Sundowner I got for super cheap, had been sitting in the seller’s yard unused for years. I put on a new 7-pin plug/wiring harness, lights worked great again.

I actually have the manual…I am one of THOSE people (that keeps all those things).:hugs: thank you though. Always good to know where you can find it.

Given that it’s been sitting for a while and wasn’t used much, there could be rust on the ball/receiver and/or coupler that’s not allowing a good ground.

Disclaimer: A hitch ball ground isn’t sufficient and there should be an actual wired ground.

However a lot of times either the tow vehicle or trailer ground wires become rusted themselves and there is no longer good contact with a bare metal surface, so the hitch winds up becoming the default/backup ground.

Try squirting some PB Blaster/WD40 on the ball, receiver, and coupler and see if that helps. (Or if you happen to have a new painted ball you could scuff some paint off to allow a metal/metal connection with the coupler.) Then you know there’s an issue with the actual ground wire(s).

I’ve seen this happen a bunch of times when hooking up a trailer for the first trip of the season. The vehicle plug tests just fine but no lights. Often they magically come on as you’re driving to the shop (since the movement dislodges the rust). I learned the trick to squirt it with some WD40 from a trailer repair guy. :grinning:

Will do. I did change hitch pins last fall after I hauled her to a saddle fitting. This is a new to me truck and when I hauled it last fall, the pin I had, had it tipped toward the front a bit and since Ms Horse thinks the trailer is her private porta-potty, a lot of the pee (despite shavings) ran forward under the mats :thinking:. So, I did change the pin.

Thanks for the tip.


Ding, ding, ding. We have a winner. Well, mostly.

I did the WD-40 treatment and had brake lights and turn signals but still no running lights or tail lights. I am going to put on a new plug. The one on it, the casing isn’t very tight… the one held with a tiny screw so I am just going to replace the whole thing.

Thanks again for the help.


Well glad it gave you a place to start!

Electricity/wiring is like magic to me and I struggle to understand how it works (still really have very little idea), but what I’ve learned over the years that the majority of the time issues tend to be with the ground(s).

So there’s the vehicle ground, the trailer ground, and sometimes there are individual grounds on each light and the fun part is figuring out which it is. This is why I let my husband deal with it! :smile:

Unfortunately (?), I don’t have one of those :smiley:.


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