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Author Topic:   Trailer Lighting Load Blows Fuse in Vehicle
Dauntless_14_TX posted 02-02-2009 12:01 PM ET (US)   Profile for Dauntless_14_TX   Send Email to Dauntless_14_TX  
I'm new to this forum, and hoping someone may have an insight into a peculiar problem I'm having with my trailer lights.

First off a little background. Problem started with no right turn signal or brake light and diagnosed (using a small test plug adapter) as a ground problem and had trouble trying to find a good ground on my old (2000) galvanized trailer. After a bit of messing with it, the running lights and the right turn/stop lights were not working. Solution was then to upgrade and switch to LED lights and rewire the trailer.

Now to the current problem. Rewired the trailer with no problems, easier job than I thought. Plugged my test light plug into the truck wire harness just for a test and had all turn, hazard, brake, and running lights. So I hook up the trailer and have hazard, brake, and turn signals... no running lights still. I unplug the boat and test the truck wire harness and no running lights... blown fuse.

I know the fault is in the trailer, but I can't seem to find it. I know the wire is good as I just ran all new wires and lights. Since its in the running, next step is to disconnect all side marker lights and over 80" light bar and see if it still blows fuse. If not then reconnect one at a time until fuse blows to isolate problem.

I don't know what else to do and thought someone may have an ingenious idea on how to remedy the problem without having to do a lot of wire snipping and heat shrink tubing work.

Bella con23 posted 02-02-2009 12:31 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
Welcome to the forum.
I would check the rating of the fuse verses the load of the trailer lighting first. Then I would look into the white wiring connection.
This is the trailer's ground wire and could be wired to one of the positive lighting connection on the tow vehicle.

Read Jim's referance on trailer lighting and let us now how you make out with it.

Chuck Tribolet posted 02-02-2009 08:38 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
It wasn't blowing fuses before, and he's gone from incandescent
to LED, so I doubt it's a real overload problem. I'm guessing
that the running lights are shorted to ground somewhere in the
trailer. That would be pinched wire, abraded wire (unlikely
on a new install) or a failed shorted light.

But first a sanity check: turn off the truck lights. Plug
the trailer into the truck. Verify that the colors match.

Next, double check that all the wiring is the right for the
color code, though I can't offhand think of a mis-wiring of
LEDs that would blow fuses. Light doesn't work, yes, blown
fuse now.

Next step is to unbolt all the lights and let them hang loose.
Inspect each for any pinched wires. Now plug it back in and
turn on the truck lights.
Best if you do it in the dark with enough buddies to watch
all the lights.
If the fuse blows, I suspect you are looking at a light that's
failed shorted and you will have to check each one. If you
had buddies watching, it's the light that DIDN'T briefly
light when you turned on the truck lights. It takes a little
while for a fuse to blow, and LEDs come on NOW (which is why
I have them on my truck and trailer). The shorted one won't
come on.


Dauntless_14_TX posted 02-04-2009 08:21 AM ET (US)     Profile for Dauntless_14_TX  Send Email to Dauntless_14_TX     
I appreciate the feedback, and will tackle it again this weekend. Really want to get this figured out as its keeping me off the water, lol. Good suggestion on letting the lights hang to look for a possible pinched wire. If I have any luck I'll be sure to post the eventual solution to help share the knowledge.
Dauntless_14_TX posted 02-09-2009 08:25 AM ET (US)     Profile for Dauntless_14_TX  Send Email to Dauntless_14_TX     
I tackled the problem Friday night. First thing was use my test light to make sure the truck trailer harness was working and it was. Next I took the marker and over 80" light out of the circuit and tested the running lights and voila, tail lights. Knowing it was then one of the 3 lights causing the short I hooked up the over 80" light and still had good running lights. Next I hooked up the passenger side marker light and checked the running lights and they all worked. Now it was just the driver side clearance light. I hooked that one back in and tested the lights and blew the fuse. I removed the light and tested it with a 12v battery and wouldn't work. I'm guessing the brand new LED light was bad when I bought it. Anyone ever hear of a bad LED? Anyway problem was fixed, did a dance for joy, said a few words of praise to the trailer light Gods and was up bright and early to spend a great day on the water Saturday. Thanks to all who helped with the problem.
Chuck Tribolet posted 02-09-2009 08:27 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Diodes can fail closed as they melt together. If not
protected by fuses, they very shortly thereafter fail open
when the innards go up in smoke. I'll bet it's now failed
open after you tested it on a 12V battery with no fuse.


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