I have a 5th wheel horse trailer with some kind of light/wiring issue and I'm hoping someone can help me debug this.On an earlier trip, we didn't have the rear running lights showing on the back of the trailer. A friendly officer gave us a warning, and we bought some LED headlamps from WalMart, duct taped them on, and got home.We discovered a blown fuse, which made sense. We replaced it, and went on another trip a month later, again, finding the rear running lights not working. Fuse blown, so replaced.We thought we had truck wiring issues, so we took to a mechanic with our other horse trailer (not camper). They didn't find any issues, and we used that trailer for a few months every week without issues.Fast forward to last week. We think the 5th wheel camper has the issue because the truck works with the other trailer. We were worried about some corrosion in the plug, so we rewired that with a new plug. Tested it out and find this:- brake lights work- both turn signals work- reverse lights work- running lights on side (top and bottom) of trailer work- trailer brakes work- rear running lights do not work. Yet it appears the entire LED panel lights up for the brake lights.I'm not sure where to go, and I'd prefer not to pay someone $$$$ to go searching around. Any ideas out there are appreciated.

I have fixed many a light set. First thing to do is take the red reflector off both sides of your tail lights so you can see the bulb itself. Now have someone go into the truck, and turn on the right signal and look at the bulb. If the thin filament is turning on, then you have a ground issue.Try the other side and repeat the process. If the thin filament is turning on, then you have a ground issue. Check the ground issue by connecting your negative jumper cable to the negative battery terminal, and then to a wire that you touch to the side of the socket. If that fixes it and makes things work correctly, that is the socket that needs to have the connection to ground from the socket repaired.What actually happens goes like this. You send power to the brake filaments, and normally that power returns on the ground. However when the ground comes loose, the power goes through the thick bulb filament, then the only way out is through the thin bulb filament in reverse. The thin one glows brighter when they are in series. The power then goes over to the other side and goes through the thin one on that side and out on the working ground.So, you think you have brake lights, but actually you have weak running lights. Hope this helps.

Thanks, but these are LEDs. Exposed resistors and bulbs here. Both the left and right signals work.Brakes work.Side running lights work.Rear running lights don't.

Check for a ground issue. Attach a long wire from the tow vehicle frame ground or battery negative terminal. Use it to check the 12V on the trailer. + should be 12V and - should be 0V or very close. When you find the problem you may also find the reason for the short.However intermittent shorts are not easy to find. Could be a chaffed wire that touches ground, etc.

Well, even then, take the negative from the battery to the LED negative side and try it. Because your brake LED is supposed to be brighter, take positive and negative to the led and see what the bulb is supposed to to. I still suspect your real running lights are not returning on the ground, and the current then tries to go through the brake light LED's and you end up with a series circuit that is not strong enough to light anything.

way0utwest1 wrote:Thanks, but these are LEDs. Exposed resistors and bulbs here.Resistors? Did you install lower voltage bulbs and resistors to reduce the voltage? If so why?

Trailer Running Light Issue | Good Sam Community

24 Replies

Artum_Snowbird
Explorer
Aug 02, 2019
Well, even then, take the negative from the battery to the LED negative side and try it.

Because your brake LED is supposed to be brighter, take positive and negative to the led and see what the bulb is supposed to to. I still suspect your real running lights are not returning on the ground, and the current then tries to go through the brake light LED's and you end up with a series circuit that is not strong enough to light anything.
CA_Traveler
Explorer III
Aug 02, 2019
Check for a ground issue. Attach a long wire from the tow vehicle frame ground or battery negative terminal. Use it to check the 12V on the trailer. + should be 12V and - should be 0V or very close. When you find the problem you may also find the reason for the short.

However intermittent shorts are not easy to find. Could be a chaffed wire that touches ground, etc.
way0utwest1
Explorer
Aug 02, 2019
Thanks, but these are LEDs. Exposed resistors and bulbs here. Both the left and right signals work.
Brakes work.
Side running lights work.
Rear running lights don't.
Artum_Snowbird
Explorer
Aug 02, 2019
I have fixed many a light set. First thing to do is take the red reflector off both sides of your tail lights so you can see the bulb itself.

Now have someone go into the truck, and turn on the right signal and look at the bulb. If the thin filament is turning on, then you have a ground issue.

Try the other side and repeat the process. If the thin filament is turning on, then you have a ground issue.

Check the ground issue by connecting your negative jumper cable to the negative battery terminal, and then to a wire that you touch to the side of the socket. If that fixes it and makes things work correctly, that is the socket that needs to have the connection to ground from the socket repaired.

What actually happens goes like this. You send power to the brake filaments, and normally that power returns on the ground. However when the ground comes loose, the power goes through the thick bulb filament, then the only way out is through the thin bulb filament in reverse. The thin one glows brighter when they are in series. The power then goes over to the other side and goes through the thin one on that side and out on the working ground.

So, you think you have brake lights, but actually you have weak running lights.

Hope this helps.

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Trailer Running Light Issue

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