Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Electrical Confusion!

This question has been posted for days on a car forum and no one has replied yet. Help me Click & Clack. My Dad forced me to listen to you guys since I was a kid and I’m 30 now and still listen to you all the time. :slight_smile:

I have a car and a tear drop trailer and I keep loosing the lights on the trailer. Here’s what I have:

- 2005 Subaru Legacy GT, Turbo, Manual Transmission, Sedan

- Tear drop trailer - SignaTour (

- Tow Ready Wiring Harness from E-Trailer (http://www…d=20059928)

I installed the wire-harness and everything seemed to work fine for a while (I think!) then all of the sudden I’m blowing fuses (20 amp) that feed from the battery to trailer lights. I had regular incandescent lights on the trailer and swapped them over to LED thinking that the older lights were just junk and maybe pulling too many amps. I rewired everything ensuring not grounds. Still the same problem exists and I keep blowing fuses, but in a very strange way…

Here’s what I did today. I checked the ohms reading to ground on all four wires on the car side & on the trailer side with no abnormal readings. Next I hooked up the car and trailer together placed a clamp on amp meter on the wire to the lights from the battery and started the car. Lights work fine. I pressed the brake, hazards, individual turn signals; turned the lights on and off nothing abnormal. I even rev’d up the engine several times thinking there may be voltage spike from a poor voltage regulator. No problems. The highest amp reading I read was .9 amps. Everything seemed normal, but then…

As soon as I would place the car in gear (manual transmission) and start to creep forward the fuse would pop. One point I think it jumped to 40amps. I recorded it with my video camera and the meter only showed 7amps and it popped, but I think it was a function of the meter not being able to display as fast.

The same thing happens when I place the car in the reverse gear. When I renew the fuse and keep the car in neutral all the lights work perfectly. I can’t figure it out.

I tried disconnecting the trailer ground as each light on the trailer has its own ground, but that didn’t make a difference. There seems to be some sort of ground within the car when the gears mesh. I just can’t understand what is going on. Is there something wrong with the wiring harness? I don’t know what is inside that little black box off the wiring harness. I have no other electrical problems in the car just this.

Please help! I’m supposed to be taking my girlfriend on a surprise camping trip this weekend. Can I put a current resistor in series to help “absorb” some of this stray current if this is only a momentary surge when the gears mesh? If so what size? Thanks for all your support!


An in line resister is not going to solve the problem. You clearly have a dead short being created when the trailer is connected and moving.

If I’m correct in assuming that everything works fine without the trailer lights being plugged in, than the only thing I can suggest is to rewire the lights. You might be able to find the appropriate wire by putting an ohmmeter on the lines and jiggling the wire bundles, but sometimes rewiring the lights is the easiest solution.

Rent an RV for the surprise camping trip. Unless your GF is the “hands-on” type and would enjoy spending the weekend in the woods searching for an intermittant short with you.

Your link to the harness is not working. Here is a pic of the one from autozone. It is not a straight wire to wire adapter, You may need one with some specialized brains designed specially for your make model and year.

Yes, rewire the trailer lights so that they all use a common ground wire, and not ground them through the trailer frame to the car frame, that is probably what is causing the issue.


Yeah I thought a ground wire issue after reading this…Also please verify that your system works fine with no trailer plugged in. The just go thru your wires one by one making sure you got them connected properly. This is a fairly ez job and you seem more than qualified to do it.

Is iy possible that you got the wrong type of conversion brain for the lights on the trailer? Check into that not likely, but check. I believe they use these modules when you have shared lights on the car…like same bulb for turn signal and brakes…or something like this. It seems the vehicle is sending voltage up a wire that is connected to ground creating a dead short. Go thru it again and take it slow. Get a VERY GOOD ground established…even if you need to use a jumper cable for a test ground wire…I always run a separate ground wire when rigging trailers as the ground usually goes bad when I use the trailer body for ground. I run a hard ground from the vehicle to the trailers lights…not using the trailer frame for ground… In fact I do both methods at the same time…Grounding is super important in a trailer esp since it isnt tied into your electrical system the way everything else is…you must provide the grounding for the trailer and it needs to be a GOOD RELIABLE GROUND…very important

I think you may have the polarity reversed somehow on the trailer such that the positive is going to the metal frame of the trailer. So it works fine until you push or pull the trailer, at which point the hitch ball moves and makes contact with the hitch, completing a circuit with the frame of the car, and causing a direct short to ground.

I think you have a pinched wire on the trailer somewhere and when you start it moving the short to ground takes place. Look for a pinched wire or where a screw may have penetrated the insulation on the wire to the lights. One way to test for the short is place an ohmmeter across the running light connection and ground and then pull on suspected trouble areas to see if the reading shows a short condition which would be less than a couple of ohms on the meter.