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2004 gmc
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I have checked the receptacle ground on the car...
How, precisely did you do that? Your problem smells totally like a floating (high resistance) ground. Troubleshooting that is relatively simple if you use one trick - take your jumper cables, and ground the trailer frame to an unpainted part of the TB frame. If you used an ohmmeter to check out the connector ground, those aren't too accurate below an Ohm, and you really want that ground to be below 0.1 Ohm or better. If you used the meter in voltage mode, even a high resistance ground would read zero volts.

Another trick if you have a good meter and can get the lead onto the ground wire in the connector is to measure it while the truck's lights are on and you have the problem. See if it's near zero volts or bounces around to 6-10V.
 

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I just did a continuity check between the trailer plug on the vehiicle and the screw that holds the ground in place on the frame and that seemed ok but. I will give your suggestion a try tomorrow. I think I will also scrap the paint off the frame where the ground is connected to see if that helps. I will keep you posted. Thanks for the advice...
 

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Back in action

Well, all of my lights seem to be working properly once again. I never performed the resistance tests using the multimeter but I did check every ground and thoroughly cleaned each trailer receptacle contact on my vehicle. I was able to eliminate the trailer as the culprit when I plugged into another tailer and saw the same symptoms. I am not completely sure what the initial issue was, maybe corosion in the plug under the rear bumper. Regardless of what it was I am legal again and thank you all for your suggestions.
 

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OK, the front wire that is disconnected by the underhood fusebox is used for a constant +12V only!


I had this problem with my TB two years ago, this is how you fix it:

Open the underseat fusebox (not the underhood one, the underseat one) and check fuse #42. It will be labeled TR PARK for trailer parking lights.

That should be the burnt fuse, replace with the proper amp fuse from the end of the fusebox where the extra fuses are stored and voila!
Thanks, I was pulling my hair out trying to fix my trailer running lights, and I found this post, #42 fuse was the problem
 

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You know, (right?)... that the test lights in the device you're using are LEDs... and they can operate in extremely poor conditions that will never emulate the current load requirement of an incandescent system.

Even with a bad ground, LEDs in you tester might still operate, but the trailer lights need better current to operate at all.

Another problem that I run into is GM has a sequence in the wiring in the 7-pin that is in compliance with the RVIA wiring, while CHYRSLER or Ford tend(ed) to use their own patterns.

Then there's the users who can really mess up a wiring scheme and you'll pull your hair trying to figure why one trailer works correctly and another one is screwed up.

Enter electric brakes.... and I've seen running lights on the trailer getting voltage from them and completely messing up the tow vehicle lights whenever you apply the brakes.

There are too many screwed up variables to even consider over the internet.

I would start by totally believe GM's wiring and work from there, correcting the trailer to make IT comply.
  • This method assumes that the GM-side hasn't been played with by trying to make a poorly wired trailer work... in desperation, at midnight, in the pouring rain, during a zombie breakout.

.
 
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