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Trailer running lights not working...WTH

63K views 9 replies 3 participants last post by  BlueSLT05 
#1 ·
Ever since I bought the D, the running lights for the trailer haven't worked, but all the lights on the D work. I have the factory installed harness that comes back from the underhood fuse blocks. Here's what I have checked so far with no failures:
-20A fuse, outside of the blocks underhood
-every fuse underhood has been tested with a light on both sides of the fuse
-all fuses are there that should be
-kickpanel fuses

I have no power coming through the pin designated for the running lights on the 7pin connector. There is a relay tucked under the rear bumper by the left taillamp that is wrapped in black cloth tape. I unhooked it thinking it may be the culprit, called my local dealer and asked about the relay with no luck- they said they would have to sell me the whole wiring harness. I took the relay out, brought it to Robbins Auto Parts and tried to match it up / look it up. They found one that appeared to be similar, but told me the relay wouldn't cause this issue and assured me it's a bad wire somewhere. I looked at the wiring in a few points and it is all wrapped in the black tubing and some points are wrapped in black cloth tape also- everything APPEARS fine.

What wire would I probe to check for power prior to this relay?
Any other ideas? I need to tow a trailer this weekend and it will be dark coming home.
Thanks in advance.:worthy:
 

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#2 ·
you probadly have a 4 pin plug,too.Could be tucked up behind the bumper area,look for it and it should have a boot on it(to cover the pins) check it out for corrosion(it will short out the other pins)
 
#4 · (Edited)
I don't have a 4 pin plug anywhere, just that stupid relay box. I have a wire that appears to be running lights power- I'm partially color blind, but it looks like a thin white wire with a green stripe. There is only power to that wire when the running lights are on and I tested it all the way to that relay, where it apparently changes wire color from there. After the relay, I have no power for the running lights so I'm going to try to replace it, or bypass it.

I took the relay out and tried testing the turn signals and running lights, the turn signals still worked, so I think this is just a relay designed to piss me off and regulate running lights. There are 4 spades, but 5 metal female points on the relay, not so sure why that would be either.

I'll keep the thread going until I can figure this out.
 
#5 ·
I just got a new relay from the towing section of the parts store but it didn't fix the issue, still no power coming out the transition in the relay. I'm going to test the spades on the piece the relay plugs into and hope that maybe the issue is with that pice. If that doesn't solve it, I may have to try and run a wire direct and eliminate the 85 to 86 terminal- I'm running out of options.
 
#6 ·
I had the same problem with my D after a friend shorted two wires trying to wire a trailer with it still plugged into the truck, my problem was a 20amp red fuse under the hood but it sounds like you have checked all those.
 
#8 ·
I checked all the fuses I could find. I checked for voltage after the solo red 20A fuse and it was good. It's frustrating because there's voltage going to the "relay" but I can't get anything beyond that.:wedgie:
 
#7 ·
I ran a wire with a 20A fuse between the two terminals and it blew the 10A fuse under the hood. I'm stumped. I don't want to put a bigger fuse up front for fear of ruining something.
 
#10 ·
So I got tired of trying to figure out what was going on with that damn relay, so I did the smart thing- bought a $10 trailer wiring kit with the LED indicators on the 4-pin flat connector. ( I only tow utility trailers, so I'll only ever use the 4 pin connector) I spliced the wires into the existing trailer wires before the relay and according to the LED's on the new 4 way flat, everything is working as it should. Why didn't I do this sooner.....:hide:
 
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