I just wired up a 4 wire harness for my little tire trailer. It's behaving kinda funny and sometimes blowing fuses. The left light is somewhat intermittent in working Sometimes it won't turn on when braking and I think it didn't turn on the turn signal once. But other times it's just fine. Also, it's not just the trailer light, it's the car's left tail light as well. If one works, they both work. If one doesn't work, neither work. The right side brakes and turn signal work just fine. I'm getting ready to just say screw it to that and deal with it being funky.

The other problem I'm seeing is that I'm blowing the 20amp tail light fuse every time I turn on the running light or head lights. I don't understand why the brake lights can work, but only the running lights would blow a fuse. They both have 20 amp fuses and I would think that the brake lights would actually draw more power.

I hooked up a multimeter to a ground and to the light side of the tail light fuse holder (with no fuse installed) and learned that despite having a degree in computer engineering, I still can't figure out crap about analog circuits. With nothing turned on but the parking lights, and the tail light electrical plug (at the back of the car) unplugged, I'm seeing 0.7 ohms. That implies a 17 amp draw with the tail lights completely unplugged. Now if I turn on the headlights, the resistance drops to 21.4 ohms. Why would turning on the headlights effect the draw across the tail light circuit? Especially when the tail lights are unplugged??? If I turn the ignition on, then the that effects the readings as well. Plugging the lights in makes the resistance drop (potential load goes up) which makes sense, but why wouldn't there be no load if the lights are unplugged? This stupid car makes no sense to me. Or is it, this car makes no sense to stupid me.

FWIW, despite the ohm readings that I can't make heads or tails of, the car does NOT blow fuses when the trailer is not attached.

In the end, I'm left wondering if the trailer is just drawing too much current and that 20 amp fuse can't keep up. I know it's normally a no-no... but what about just trying a larger fuse? 25 or 30 amp? If I can't get this going, I have no clue how I'm going to get power out to my trailer.

Someone please save me before I rip out what's left of my hair.

Mine was blowing fuses on one trip as well. It turned out that there was a part of my wiring underneath the trailer that had got caught on something and ate through the sheathing (sp?). The wire was shorting and not making things happy. I ended up buying a new trailer wiring kit and re-doing the whole trailer. It was then that I discovered the bare wire I was careful to route the wiring correctly and I never had the problem again.

You might just see if you can get under your trailer and inspect the wiring before you go experimenting with different fuses.

I took a pretty quick look underneath and didn't find anything with the wire that seems to be the problem (brown one). I did find a small tear in the insulation of the yellow wire (left turn/brake) and taped that up. I'll take some time to do a more complete inspection, but using my multimeter, and even wiggling wires around, I'm not seeing the brown wire (running lights) and white wire (ground) go to 0 ohms.

I looked at all the wiring as best I could (not easy to see some spots) and couldn't find any more areas with issues in the insulation.

I think my next step is to pull all the light bulbs out (6 total I think) and plug the trailer in like that. If it still blows a fuse, then I have to believe there's a short somewhere. If that doesn't blow a fuse, then I can try plugging the lights in one at a time and see if one of them is causing a problem itself. If none of those blow a fuse, then I can try the lights in combination (increasing the load on the whole system) until something blows. If I can get all the lights back in and working, then that still implies a short in the trailer somewhere but that's it's not actually shorting at that moment.

Ok, enough thinking "aloud" and back to it.

I pulled all 6 lights. The two brake/turn lines have no connection to ground (infinite ohms measuring from yellow/green to white). The brown line is still showing 0.9 ohms going from it to ground... With no lights installed, that's telling me it's a short (if I'm right, nice call TxAgZ28). Haven't had time to try and plug it back into the car, I'm almost out of 20 amp fuses (and I didn't have any spares to begin with!) and I need to get going to church... so hopefully I'll get it nailed down later tonight or tomorrow morning.

Blowing fuses means too much current, so, as Nick said, you have way too much current flow for some reason, and that (almost always!) means short.

Also, are you using a trailer converter box for your lights? You should have amber turn signals on your car so you should use the "box of diodes" isolator to isolate the turn signals from the tail lights.

Justin

The Brown line is usually Running lights. and Yes it should not read anything going to ground......... So that is the circuit that is causeing you the issue. If its doing this with the car unplugged someone on that circuit is shorted to ground!!!! That will always blow fuses. You are just going to have to trace the wires to see what is hooked up wrong. Maybe at the Lights themselves or a mis wired pigtail. These are the common stuff that comes through my doors.

John

Kevin - based on the a similar conversation you had here, I learned that the 5 to 4 wire box I have wasn't what I needed. I did try to find a straight 4 wire harness with diodes, but was unable to do so. The guy at the autopart store where I got my 4 wire harness said that the only possible downside to not having diodes was that my blinkers would flash rapidly if I didn't have a "heavy duty" blinker unit. Aparently when I replaced my blinker module a few years ago, I got a heavy duty one because it does blink normally when it's all plugged in.

John - I'm about to start pulling the wires off the trailer so I can more closely inspect them. If that doesn't pan out, I figured my next step would be to start cutting the wire to narrow down where the short is. Is there an easier way than all that cutting and resplicing to find it? My last ditch effort will be to rewire the whole thing, which I'd rather not have to do.

Thanks to everyone for the help and input.

Justin we do work on trailers here at work. So my first advise would be to check the Lights assembly's and the pigtail connector(mainly if some one has spliced into it). Is the wiring exposed or run through the frame.

Now you say this is a small trailer. How big is it and does it have a break-away kit and electric brakes?

John

John - There are spots that the wiring has to go through the frame, but there's no other way around it. At least all of those points are painted and not too awfully sharp. It's a 4' by 4' utility trailer. One of those cheapo Harbor Freight ones.

The good news, however, is that I found the problem. The guy I bought the trailer from added a bar in the back that would allow it to be tipped up on its back edge for storing it vertically. It looks like when he tightened one of the bolts, he caught a wire and smashed it. That broke the insulation open and the bolt was tied straight to the frame, allowing it to ground out. I got the wire unpinched, taped it up, and now the trailer is working fine.

I've run into another unrelated problem that I think I've had with my car's brake lights for a long time, but I'll post that up in a new thread.

Thanks for the help guys, I'm glad it was an easy fix.

It's always the smallest, dumbest things that cause us the most frustration Glad to hear you got it sorted out!

Hate to say I was thinking of this when I was reading the thread...but I was thinking of this when I was reading the thread. I have done this myself, so it was my first thought.

One thing about trailer wiring is that it really isn't that tough. I have a very strong electrical and electronics background, and whenever I have had blown fuses or funky lights, I almost never used my ohm meter unless I am tracing connectors and adapters to the tow vehicle. I simply used my 12V test light and start looking for power. When blowing fuses, I look for the obvious in the obvious places, which are pinched or chaffed wires and I always start at the lights themselves. Broken wires can force odd ground paths, so when trouble shooting and you are fairly convinced you have no power path to ground at frame pass throughs or lights, next check connectors or junctions.

Bad grounds can create havoc and are probably the biggest problem in reliable trailer lighting. Many make the mistake of trusting the ground connection between the trailer ball and the trailer hitch. Don't do it, as corrosion and dirt will compromise the conductor path. Always make sure your ground wire runs through your trailer connector to a good ground on tow vehicle and the trailer.

Finally, disconnecting the trailer from the vehicle and the problem going away on the vehicle tells you right away your problem is behind the trailer connector.

Mike