Trailer light advice needed

Dwight Harley

Dwight Harley

Well-known member
I keep my boat trailer out of the weather in the garage. I just started having problems with the wiring harness fuse (on the vehicle end) blowing out as soon as I hook up the trailer and switch the car lights on. I have pulled this trailer for two years without problems until now. Do you guys think the problem is a short in the trailer wiring or a bad wiring harness in the Subaru? Its an aftermarket harness I installed shortly after buying the car. I would rather replace the wiring harness than have to trouble shoot some gremlin in the trailer wiring. Thanks for any suggestions
 
A wise man I used to fish with told me that 90% of trailer light problems are related to a bad ground or ground contact. Many times I remove the ground, sanded it, put it back and my problems were fixed. So I always start with that, and end with that since that is all I know to do.


Pat McManus always figured trailer lights were not REALLY meant to work.
 
In this order:


1) If it's the aftermarket plug in system disconnect it, blow it out, spray on brake cleaner, let dry, coat with dyslectic grease & reconnect.

2) Do you have your ground wires isolated to the trailer or are you running grounds back to the white wire on the plug?

If isolated to trailer, undo screws/bolts, clean contacts & reattach.

If your not running wires to front, I would strongly suggest you do so.

This is normally the #1 issue for trailer light issues.

3) short in the trailer wire

Something may have rubbed the sheathing off the wire & its grounding out, shorting out the fuse.

Replace all wiring
 
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I put my money on a short in the lights a/o trailer harness.
Can be really frustrating to track down, almost easier to run to Academy and grab a new harness.
 
Harbor freight has a plug that checks harness, cheap. I would hook up to a friends trailer first. If you have changed a bulb lately, make sure it's in correctly. If not sure pull both bulbs out and plug harness in. If it doesn't blow fuse, put one at a time in.
A meter is help full if you now how to use it. Look for grounded wires...
 
Don't worry about a bad ground this time, you do have a dead short which means a bare wire touching metal. Blown fuses mean a dead short but an open circuit such as corroded connections won't turn on the lights. There are only 2 types of electrical problems. I would recommend running a white wire from the plug all the way to the LED taillights, that won't fix your problem it will just save you from troubleshooting trailer lights when you should be hunting or spending shell money on a ticket. OR getting rear ended. Good luck
After reading the original post you should look at the brown wire (taillight) first.
 
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Thanks everyone for your suggestions. Trailer lights are truly one of the black arts. I'll post again when I figure it out
 
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All good, sound, advice.

If it's shorting out almost immediately, chances are great, it's a direct short. Bare wire touching metal somewhere.

If I was a bettin' man...

Get ahold of a multi meter and test continuity between the frame and each prong on the plug. That'll get ya started.

Jon
 
Don't worry about a bad ground this time, you do have a dead short which means a bare wire touching metal. Blown fuses mean a dead short but an open circuit such as corroded connections won't turn on the lights. There are only 2 types of electrical problems. I would recommend running a white wire from the plug all the way to the LED taillights, that won't fix your problem it will just save you from troubleshooting trailer lights when you should be hunting or spending shell money on a ticket. OR getting rear ended. Good luck
After reading the original post you should look at the brown wire (taillight) first.
Click to expand...

Tom gave me this advice years ago and I finally ran ground wires back to all the lights and ran it through the plug. Have not had one issue since and that was years ago. It is so nice to hook up the tailer for the first time of the year to check out the lights and have everyone of them work every time.

Mark W
 
Had the same problem with my Subaru. Factory installed harness and I blew an in line fuse. Didn't know where it was and the mechanic at /Baruch showed me where it was.....might have never found it, it was hidden under the back deck.
Anyway I assumed I had a short and would blow another fuse. I doubled up the ground wire and have not had a problem since. Doesn't make sense to me but if it ain't broke......
 
What I know about trailer lights is that they never work at 4:00 am.....Seriously
I have found that most of the time it is the ground.
 
Chris H, you got that right about they never work at 4 AM. That's what happened to me a few days ago. I was patting myself on the back for having the car and boat loaded the night before, coffee in the thermos, lunch packed and ready to go. Lights worked fine last week but as soon as I hooked up the trailer the fuse blew....replaced it and crossed my fingers...blew that fuse too. No trip to the river that day for me.
 
Use a jumper wire and touch each probe one at a time.
You'll need to hook a ground up first
you do this without plug connected

good luck
 
A common problem with trailer light going to dead short is they use the metal clips to hold the lights against the trailer. I would start by removing all the clips, keeps the wires off the trailer. If that stops the dead short you need to find which clips and stop the live wire to ground.
 
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