Everyones favourite subject! Trailer lights

Need some help with this one

Last week I discovered the running lights werent working on my trailer. Meaning at night no sidelights or illumination of the rear lights. Brakes and turn signals worked fine. Didnt matter if the lights were set to "auto" or I had manually turned lights on.

I had a brand new set of LED lights at home so I decided this was the time to switch them out. Installed full new wiring harness with the lights. Everything. Drilled new holes for the grounds. Everything!

.....and the results are the same. Still no running lights. I checked the voltage coming out of the brown wire (ground) on the vehicle side and it is fine. I have one of those little trailer testers that plugs it and it lights up that bulb too.

Im stumped. Any ideas?

Try more than one truck?

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Originally Posted by SongDog

Try more than one truck?

Sent from my SM-G960W using Tapatalk

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Not an easy option. Im going to try a different trailer tomorrow.

If you have a multimeter you can check the ground on the vehicle side. I've had trouble with this before and the trailer lights would work after a few bounces down the road it would ground through the hitch but the vehicle ground through the connector was corroded where bolted to the frame. That 30 amp fuse is likely for a brake controller if you had it as an option. Or a 12 volt supply for 7 pin connector.

I had weird things happening with my trailer lights too, to a point of replacing the entire wiring harness needlessly ! Turned out to be a bad ground on the trailer frame, caused by corrosion. Now I have 2 sets of wiring on the trailer!
Put a good clean grind to the trailer frame, re-attach the ground wire and try again. Good luck ..

To eliminate the ground.... Use boaster cables to ground the truck directly to the trailer.... So boaster cable on a solid frame part of the truck to a solid frame part of the trailer.... That will make sure the ground is there....
Then it is easy..... If power is coming out of the clearance/parking connection (prong) which is the brown coloured wire, it must work....
Trailer wiring is easy if you think about current being water..... If it is coming out of the truck prong, it must go along that wire to the light and if the trailer is truly grounded it must work.....
Right is green
Yellow is left
Brown is clearance/parking
Black is ground...
I wish I was closer, I'd come out and get it done for you...

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Originally Posted by fratri

To eliminate the ground.... Use boaster cables to ground the truck directly to the trailer.... So boaster cable on a solid frame part of the truck to a solid frame part of the trailer.... That will make sure the ground is there....
Then it is easy..... If power is coming out of the clearance/parking connection (prong) which is the brown coloured wire, it must work....
Trailer wiring is easy if you think about current being water..... If it is coming out of the truck prong, it must go along that wire to the light and if the trailer is truly grounded it must work.....
Right is green
Yellow is left
Brown is clearance/parking
Black is ground...
I wish I was closer, I'd come out and get it done for you...

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That's a slick trick, I'll store that one in my brain for later:thumbup:

I had a similar issue with my trailer. It turned out to be a blown fuse in the truck.....some of my trailer lights worked, others didn’t ( clearance lights etc.) don’t ask me why, but the dealer checked my truck fuses and one was blown. (2014 Ford F-150)

Update - not sure exactly what happened the first go around but I double checked the vehicle end of the connection and I'm only getting 6-7v on the running light connection. Soaked in vinegar and a polish with a brass brush in a drill didnt do anything so I guess I'm replacing that plug tomorrow.

Hopefully that solves the issue!

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Originally Posted by blasted_saber

Update - not sure exactly what happened the first go around but I double checked the vehicle end of the connection and I'm only getting 6-7v on the running light connection. Soaked in vinegar and a polish with a brass brush in a drill didnt do anything so I guess I'm replacing that plug tomorrow.

Hopefully that solves the issue!

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Bypass the plug, I mean if you are thinking of changing it, what will be the harm, this will only confirm........... Right behind the plug on the truck, find the wire that is bringing power to that prong, cut and strip it then run a bare wire right (bypassing the truck plug)to the trailer plug end.... This will confirm your suspicions ..