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This week's WTF trailer issue...

MochaMike

MochaMike

All Whee Drive!
Joined
May 19, 2020
Member Number
206
Messages
301
Loc
Concord, CA
I have a 4x8 trailer (Dad got it in the 80s/90s, Harbor Freight special, had 2 foot walls welded up on the sides with plywood & covered by sheetmetal).

Anyways, both lights were cracked/not working, and plug was fubar from getting dragged. Basically it was ignored for 5+ years.

Last week I got a new pig tail plug (4 pin straight) and wired it it.
I discovered a grounding issue from tongue to back half from rust, so I grinded and added a jumper).
Respliced the light connections to harness as well.
All splices were crimps & then shrink wrapped with heat covering.

Tested everything out, worked great.

Used it the first time this weekend to move some stuff and discovered passenger light stays lit while connected to vehicle.
Truck can be running or off.

Truck tail lights are off, etc.

I'm guessing it's truck wiring: 04 Dodge 2500 Cummins
I have a the round multi-pin on bumper and the PO wired the 4 pin so it's underneath.
I have an updated truck in plug that has the dual combo round/straight that I'm planning on wiring in, but wanted to figure this out prior to that.
 
Tiha said:
Gotta be the truck. Voltage is coming from somewhere.

I had an rv to 4 pin adapter before that was made wrong from the factory, did something similar.
Click to expand...
Yeah, I'm going to use a test light & trace wires in the next day or two.
 
You probably have a wire crossed on your vehicle connection. There is a charge wire going to the trailer emergency break away battery. It is always hot.
 
This was going on in my 2010 power wagon last year. It was a tipm issue.

But the whole parking circuit on trailer was constant 12v, as well as the 12v constant thats supposed to be there.
 
I worked on trailers for about a year. Check all the grounds. Make sure they are perfect. Pretty much all weird lighting problems were from a bad ground somewhere.
 
get one of them plug in testers for the truck side to eliminate that.
always weld on a stud for the ground...sheet metal screws make shitty grounds
never use scotch locks
 
Tiha said:
Not sure what the local laws are there, but in Iowa where I am, if you can see the tail lights on the towing vehicle the trailer lights are not required to work.
Click to expand...
Same here. I hate it. It’s lazy and not hard at all to make trailer lights work properly.
 
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