I rewired my little trailer I have and replaced the lights. When I hooked it up to my truck to check it everything worked as it should except for when I have the lights on on the truck and hit the brake the trailer lights go out. I checked the plug w/ a meter and everything appears to be correct.

I sure could use some help on this as I am having a heck of a time figuring it out

Thanks in advance.

Ground good?

Try putting a temp ground from the trailer to the truck, jumper cables would work if you can get a good connection.

If you probe the plug on the truck is works the right way? Can you plug it in but leave enough of the connectors exposed you can check it with the meter when hooked to the trailer?

I am pretty sure it has a good ground but I will try the temp out and see if that is what the problem is.

Plug on the truck all works as it should and I did check it w/ it connected but pulled slightly apart and everything was good.

I thought it was odd that they worked when the truck lights were off but don't when the truck lights are on.

Are you using a factory wiring harness or did you wire in a plug to a modern vehicle? Alot of newer(late 90s and up) vehicles use some kind of power regulaing computer and when you tap in, it screws it up. It thinks that there is a short or something. I had to buy a $100 power module and harness for my wifes minivan when I added the reciever. Hope that is not your problem.

I am using a 4 prong plug that plugs into the factory harness on my '98 Chevy Z71 4x4. My 16' trailer always worked w/ no problems and so did this one. Then it started blowing my rear tail light and dash light fuse when I used this small trailer so I replaced the lights and rewired the trailer w/ a new harness when I saw the wire insulation was rotted and cracked.

So did the temp ground fix it?

If not then disconnect the lights and check with a meter at each light for function maybe one of the new lights is bad?

Haven't had a chance yet. It was raining earlier and now it's dark.

I wouldn't think the light is bad since it works for brakes, running lights and turn signals. The only time they don't work is when the truck headlights are on and I hit the brake pedal, then they go out.

I know a bad ground on some of my trailers make them act funny most of the time they just blink real fast on the turn signal and the tail lite is dim, I usually don't hook up a ground and let the ball and hitch do the grounding so the rust that builds up between use has to be worn down a little before it all starts to work right again heh

I had the same problem,rang up a auto sparks mate of mine,& he said it was definately a bad earth. After checking & cleaning etc it worked,& I owed him a beer. Baz
Reason is because,due to bad earth, the current then travels along to next path of least resistance,being your adjacent circuits.

Redoing the grounds will be the first thing I do.

Well?

Find the problem?

Also check your dual filament light bulbs that one isn't all the way twisted and may be shorting out the dim and the bright filament.

Or incase a single filament bulb is in a socket for a dual filament. Just a hunch.


You do see bright and dim operating properly right? Lights on should illuminate the dim filament only. Hazards or turn signal should be brighter than the running lights. Sort of stating the obvious so I apologize.

If you have a floating ground your symptoms are explanable. Floating ground is a ground that is not connected (someplace). If you have this the lights can operate in odd manor as you have noticed.

But, a tell tail sign of this is both the Bright and Dim filaments will illuminate. When only one should. This is because the circuit will complete
through the other filament to its signal source. Ie. running lights on and you have both filaments glowing. You will need to pull the lens covers off.

But, when both filaments are supposed to be on they both have the same +12V potential and no complete circuit to ground. Thus they will not glow.

Does this make sense?

Good stuff MassOdy!

A few years back I built a trailer light tester.

This goes in line and have a few features.

There 12 lights that illuminate like the rear lights are suppose to.
You can measure teh voltage.

And Current draw. For this I had to use a switch to be abel to switch the meter into the circuit path. Not perfect since its the ground I open up. Ground is also provided via the ball and hitch.

But, this has 12 Volt inputs as well. So I can test the lights without a vehichle. With it a current draw reading is useful.

This thing does nothing more than organize teh trailer light wires/signals. Any volt meter can do the same.

But, if you think it will help you. Let me know.

If I were to build a new one of these. It would be different. Same principals different design.

Sorry guy but I have not had a chance to address this problem yet.

Thanks for the great info Massody.

I will be tinkiering w/ it this week sometime and will post up what I find.