OEM Factory Towing Wiring.

[Grubbworm]

I have a 2004 GMC Sierra 2500 HD 4X4 Ext Cab. I am having an issue with the factory trailer towing connector. When the trailer connector is hooked to a trailer, both of the trailer lights flash when I use the Left Turn Signal. Only the Right Flashes when the Right Turn Signal is used. I am perplexed as to what is causing this and where to find the problem in order to fix it. I have checked all of the wire colors and other functions and they seem to be working correctly. I am looking for some help or to be pointed in the right direction so I can fix this issue. Thanks.

[Jsdirt]

You probably have a bad ground on the trailer - I'd bet there's nothing wrong with the tow vehicle's wiring.

 

Check both grounding points on both rear lights first, and if that checks out, make sure you have a perfect ground from the white wire on the trailer plug to the trailer frame. Also, inspect the filaments of both bulbs CLOSELY - sometimes one can burn out and bridge the other, so when you hit the turn signal it will find a path to ground through the running light filament.

[Grubbworm]

18 hours ago, Jsdirt said:

You probably have a bad ground on the trailer - I'd bet there's nothing wrong with the tow vehicle's wiring.

 

Check both grounding points on both rear lights first, and if that checks out, make sure you have a perfect ground from the white wire on the trailer plug to the trailer frame. Also, inspect the filaments of both bulbs CLOSELY - sometimes one can burn out and bridge the other, so when you hit the turn signal it will find a path to ground through the running light filament.

Thank you. I will check all of the Grounds out the first chance I get. I just replaced both rear turn signal sockets with new ones. However the problem was there before and after. I will check the left bulb as the right one is new.

I do have another question about the grounds though. If there were issues with the turn signal grounds, wouldn't that affect the vehicle turn signals also?

[Jsdirt]

No, just the trailer's. The truck has its own grounds, and electricity takes the path of least resistance. The power to the trailer lights is in parallel to the vehicle's lights, and the trailer's lights path to ground is through the trailer, so they won't be affected.

Edited November 5, 2019 by Jsdirt

[Doug_Scott]

Take a good look at the left side bulbs and sockets.  Try swapping the bulbs side to side.  Sounds stupid, but, it is amazing was a simple incandescent bulb can do.

[Grubbworm]

OK, Saturday I took a good look at the wiring, changed to new turn signal bulbs on both sides, added new grounds to both rear turn signals at the lights. None of it helped. I am considering splicing in a new ground to the black wire on the plug itself. More help and things to look at would be appreciated.

[Jsdirt]

I should've told you to do this first, but that's what I get for assuming!

 

You'll have to test at the truck's connector to be sure everything is working correctly. Can use a test light, but a multimeter would be better for checking the grounds. If you're missing power on the problem wire, check for any aftermarket connectors or connections to the factory wiring. Lots of people, and even quite a few garages, will use those cheap auto parts store connectors, or Scotchlok connectors that do nothing but corrode wherever there's road salt in winter. 9 times out of 10, whenever I have a truck-side trailer lighting problem, it's one of those turned green inside.

 

Now, if all the lights work, take your ground end of the test light and try and connect it to the plug's ground, without bridging any other connections, and test everything again. If things suddenly don't work, you've got a bad ground there. In that case, running an extra wire will help.

Edited November 11, 2019 by Jsdirt

[Grubbworm]

1 hour ago, Jsdirt said:

I should've told you to do this first, but that's what I get for assuming!

 

You'll have to test at the truck's connector to be sure everything is working correctly. Can use a test light, but a multimeter would be better for checking the grounds. If you're missing power on the problem wire, check for any aftermarket connectors or connections to the factory wiring. Lots of people, and even quite a few garages, will use those cheap auto parts store connectors, or Scotchlok connectors that do nothing but corrode wherever there's road salt in winter. 9 times out of 10, whenever I have a truck-side trailer lighting problem, it's one of those turned green inside.

 

Now, if all the lights work, take your ground end of the test light and try and connect it to the plug's ground, without bridging any other connections, and test everything again. If things suddenly don't work, you've got a bad ground there. In that case, running an extra wire will help.

Thank you. I will try that as soon as I can. I have an aftermarket plug that I added to my last truck. If I have to use it and rewire this truck I will do it as a last resort. But, I hope I do not have to.

[Grubbworm]

I have tried all that has been suggested. I have had no luck in finding an answer. I am afraid that I may have to rewire this plug as a last resort. I have an aftermarket plug on my other truck. I may just have to cut it off and wire it to this truck. This is not something that I am looking forward to doing, but will if I have to.

[Jsdirt]

So everything works at the truck end? If so, then the trailer is still the problem.

 

Any neighbors or friends have a trailer you could plug into? If their lights work and yours still don't, then it's 100% confirmed a trailer wiring issue.

Edited November 18, 2019 by Jsdirt

[Grubbworm]

No, the issue is in the truck. All 3 of my trailers do the same thing. While testing the plug on the truck, when I turn the right turn signal on - the Right Turn Signal connection on the plug flashes the test light. When I turn the Left Turn Signal on both the LT and the RT connections on the plug flash the test light. Which in turn will flash both lights on the trailer when hooked up. 

If I cut the Right Turn Signal wire on the plug and splice it in to the Right Turn Signal wire near the back of the truck right light would that possibly eliminate the issue? Or what if I did both?

[Jsdirt]

Ahh, ok - got it now. There has to be a short between the 2 wires somewhere in the harness. I can't explain why it works one way but not the other, though - that is strange.

 

If it were mine, I'd just run a wire straight off the left turn signal wire from the tail light (since it's close by) to the plug. This does have the potential to overload the circuit, depending on how many incandescent lights you have on your trailer (ie - big trailers with incandescent bulbs). Before you cut the wire permanently, try stripping back a quarter inch of insulation off it, and jump a wire from there to the left turn signal, and see if everything works normally again.

[Doug_Scott]

If this only happens when a trailor is connected, and  it works normal without a trailor connected, try connecting the lights without connecting the trailor to the hitch. 

[Doug_Scott]

You may also try twisting and pulling the wiring at both sides of the 7 wire plug and socket.