Trailer Lights Not Working

seethesun · Mar 8, 2015

Hi,

I have read tons of articles and posts on troubleshooting trailer wiring, plus watched several videos. I am still stuck.

So I bought an old, rusty utility trailer about a month ago. The guy I bought it from had wires duct taped and draped all over. When I picked it up, the wiring worked. The next time I tried to use it, they didn't. So here is what I have done so far:

Sanded and painted the trailer
New tow vehicle harness
Tested connector by putting gator clip of continuity tester on ground & probing each pin. Without turning on function (brake, etc) each lit up. I am not very knowledgeable with electricity so I followed an installation video, which didn't mention turning on the function as you test. Just said it wouldn't light up if it didn't work. Articles/videos since have said to turn on the function. So I tested it with the function on and the probe lights up and blinks. The only difference between on/off is when the blinkers were off, the prob wouldn't blink. I am confused why it would light up with the function on or off. Though, it doesn't blink with blinkers off.
Bought one of those trailer testers just to double verify (after hours of banging my head) and nothing lit up. I don't know if there is something wrong with the wiring or if the tester is defective. I attached the continuity tester to it and the lights of the tester came on for each pin.
New trailer harness
I am working with one light right now to test. I ran a ground from one of the mounting bolts to one of the bolts on the trailer jack.
Checked to make sure the light I'm using for test was working by probing the light socket and putting the gator clip on the pin. It lit up.
The trailer connector side is grounded to a different bolt on the trailer jack.
I tested to see if there was a ground there, by putting the gator clip on the bolt of wiring harness (trailer side) ground and touched the probe to the bolt with the light ground. It lit up. Given my experience (very little), this may mean nothing, but seems if there is a flow there, then the ground should be fine on the trailer and that light, right?
On the tow vehicle I checked the ground wire (per installation video instructions I grounded it behind the tail light assembly and even went so far as to sand to a shiny surface). I put the gator clip on it and probed the pins. Lights all came on. Again, this was despite the function not being activated.

Any ideas would be greatly appreciated. Everything says to determine which part is having the issues first, but after the continuity tester and trailer tester saying different things, I have no idea which side has the problem.

Thanks,

David

uglyboywith11fingers · Mar 8, 2015

A lot depends on the tow vehicle... Did you add the trailer wiring adapter to your tow vehicle ? Was it the same vehicle that delivered the trailer ?
Many newer vehicles have the brake & signal lights activated by a computer controller, and as such they are ground enabled. What I mean to say is that a hot fused wire goes to the bulb, the other side of the bulb is connected to vehicle ground thru the computer activation... neither side of the bulb is connected directly to vehicle ground.
So, maybe you need to investigate if your tow vehicle needs a special trailer wiring converter/adapter thingy.

Hope this makes sense.

Pete

beckmurph · Mar 8, 2015

Have you checked the fuse/fuses on your tow vehicle. On my Fords there is a fuse box in the engine compartment.
The trailer marker, brake, and turn signal fuses are there. This has caused me a great deal of aggravation. Trailer light problems, but actually in the truck. :banghead:

oldballs · Mar 8, 2015

I had similar problems that turned out to be in the trailer to truck connection. There was corrosion in the connectors...............and it seem to work only when I tied some bailing twine to the bumper to hold the connection in place.....that is, even after cleaning each side of the connection.
Cheers,
Mike

oosik · Mar 8, 2015

The two major causes I've seen are - The trailer or truck plug is corroded and does not make good contact & the wiring on the trailer depends upon the trailer hitch for ground

DFB · Mar 8, 2015

Ya bad vehicle to trailer grounds cause all kinds of problems.

I am slightly confused on the original troubleshooting description.

s219 · Mar 8, 2015

Sometimes with an old trailer, it's quicker to just rewire it than to chase down problems. Been there, done that.

Patrickdozer · Mar 9, 2015

Trying to follow original post, sounds like new wiring was added and trailer painted. If everything was hooked up correctly and connections are good then I would be leaning towards the grounding problem like DFB stated. I know first hand all kinds of goofy stuff will happen with a bad ground. You will have lights lit that aren't supposed to be lit and stuff like that.

Diamondpilot · Mar 9, 2015

s219 said:
Sometimes with an old trailer, it's quicker to just rewire it than to chase down problems. Been there, done that.

I agree.

Chris

seethesun · Mar 9, 2015

Thanks everyone. I really appreciate the input. This is one of those things where experience would have really paid off and it would have taken me 2 minutes to solve instead of hours over weeks. Short version. It turned out to be the fuse.

So I called my brother, who works a lot on race cars and their trailers. First, I learned the difference between a continuity tester and a circuit tester. Continuity tester just tests the integrity of the wire, so it has battery to provide power. A circuit tester, well, it's what I should have been using. :laughing: It has no battery and tests to see if the circuit is complete.

I thought about it providing it's own power and causing itself to light up after I tried the trailer tester and it didn't come on. The thing that made me pass over that idea was when the blinker came on the light inside the continuity tester did blink.

So my brother had me try my multimeter and it showed no voltage coming out. He said, well, it could be the fuse. Sure enough.

After spending so much time trying to chase down the issue, you can sure bet if I ever have an electrical issue again, the fuse is the first thing I will check!