Weird Trailer Wiring Problem

My '05 1500 ext cab Silverado has the factory trailer package and my wife pulls a horse trailer around when the weather is nice. Last weekend when she got back she said the left trailer turn light was on and when she turned on the right blinker both sides blinked. The truck lights are normal. I checked the trailer plug and it was tarnished so I replaced it. No change with the problem. I pulled the plug off the back of the 7 pin jack and it had green corrosion so I bought a new jack and cleaned the plug and it's still not fixed. I checked the Lt Rt trailer fuses in the cab and with the plug removed from the jack in back the Lt fuse is hot without having the left blinker selected. (Hot as in 12v to it.) I figure the plug must be bad and since I didn't want to spend $170 for the whole harness I bought and new one piece jack and wired it into the plug wires and guess what...no change. Now I'm out of options...you guys have any ideas?
:crying:

I would test the plug with a test light... start at the point where the trailer harness begins (not sure where on your exact vehicle) follow it back toward the trailer until the signal gets screwy... then replace the section in between and you should be good.

good luck

That sounds like a plan but I don't know where the trailer harness starts...anybody know?

That sounds like a plan but I don't know where the trailer harness starts...anybody know?

with problems like you have it is best to check the trailer wiring out first... the way I do this is apply a fused 12 volt supply to each of the trailer wires..then see if the proper lamps light. remove the trailer bulbs and inspect also any wires that may have been chewed up....

That sounds like a plan but I don't know where the trailer harness starts...anybody know?

Hello Sonny01
You trailer harness starts at the plug where you plug it into truck.
Start testing at the truck plug in with a test light.
As J Cat said test out trailer Wiring.
And always confirm good ground wiring.

Check all quick connector on wiring for lose or bad/green connection.
Best to solder and seal all splice in wiring.
pull both plug-ins to truck and trailer apart and check for lose or shorted wiring inside.

Sorry for the wrong terminology...I meant where the harness starts up under the front of the truck so I guess that means where it ends. I cut the plug off at the jack in the back and still had 12v to the LT pin so the short must be up towards the front of the truck. I used butt connectors and triple checked that all wires went to the correct pins. Then I tested the new jack with a test plug and everything is correct except for the LT being on all the time. So what I need is the location of where the front of the truck connector is.

When I say the LT signal is on I mean on steady...not blinking.

Sorry for the wrong terminology...I meant where the harness starts up under the front of the truck so I guess that means where it ends. I cut the plug off at the jack in the back and still had 12v to the LT pin so the short must be up towards the front of the truck. I used butt connectors and triple checked that all wires went to the correct pins. Then I tested the new jack with a test plug and everything is correct except for the LT being on all the time. So what I need is the location of where the front of the truck connector is.

When I say the LT signal is on I mean on steady...not blinking.

Looks like it starts in the underhood fuse box.
Some of the tralier wiring stuff is on A seperate trailer light fuse.
Catch your wire color at the trailer plug and it should be sme color going to under hood fuse block.
May have to pull fuse block apart and turn over.

Is the truck signals working good 4W and regular turn signals.

One strange thing the trailer plug terminal marked LT is backup lights.

Make sure you have a good picture of trailer plug terminals.

If that all does not help post back for more help.

---------------TOP OF 7 PIN PLUG--------------------


------------------------------0-------------------------------
--------------- ---- white ground------------------------

---------0--------------------------------------0---------------------
--black tail lights--------brown marker light-----------

------------------------------0-------------------------------
------------------ blue -auxilery--------------------------

---------0--------------------------------------0---------------
------ yellow left hand turn---green right turn---

--------------------------------0-------------------------------
----------------------- red - stop lights---------------------

WONDERS NEVER CEASE
I got to thinking about where the wiring goes and I thought of the flasher unit behind the knee bolster on the left side of the steering column. I pulled it out and the LT light was out on my 7 way tester. I took it apart, moved the contacts as i I sprayed it with contact cleaner that I had from Radio Shack. I put it back in and checked my tester plug and no more LT light. I cycled all of the flashers, lights and brakes and the light is still out so I think I got it.
Thanks guys for all of your help...I really appreciate it. You got me going on the right track.

WONDERS NEVER CEASE
I got to thinking about where the wiring goes and I thought of the flasher unit behind the knee bolster on the left side of the steering column. I pulled it out and the LT light was out on my 7 way tester. I took it apart, moved the contacts as i I sprayed it with contact cleaner that I had from Radio Shack. I put it back in and checked my tester plug and no more LT light. I cycled all of the flashers, lights and brakes and the light is still out so I think I got it.
Thanks guys for all of your help...I really appreciate it. You got me going on the right track.

You are welcome.
Yes that 4W flasher is a problem child on them.
Strange it on held trailer turn signal on.
Usually knocks or mess up truck lights.
I will have to remember that one.
Glad you got it fixed.
Good Luck

noone botered to tell him if he doesnt replace the blinker it eventually WILL happen again, contact cleaner is good for a tremporary fix, the contacts when hot will most likely solder themselves shut again, if it was dust inside that made it stick, it will still eventually happen again, due to loss of lubrication. you should replace that blinker relay in the future it could casue more sever problems with your wiring, even possibly casue issues with the lighting on the truck.

I agree the blinker should be replaced...I'm just happy to have found the problem. I had visions of going to the dealer and replacing the entire harness plus the non factory jack I just installed and hard wired into the harness. Hundreds of $ wasted but now I get to shell out $33 for a new blinker. :-)

I agree the blinker should be replaced...I'm just happy to have found the problem. I had visions of going to the dealer and replacing the entire harness plus the non factory jack I just installed and hard wired into the harness. Hundreds of $ wasted but now I get to shell out $33 for a new blinker. :-)


If the truck lights/ directionals work properly with no trailer hook up why do you think the truck is the problem ?

did you use the 12 volt fused feed to check trailer wiring/lighting components? what did you find ?

I have a 7 way tester that told me the same things that were faulty on the trailer which was left turn light on steady. It tested all of the 12v lines. The 12v and LT leds were lit on the tester and to me that indicated a short somewhere. I removed the flasher and the LT led went out so the short had to be at the flasher or possibly before the lines got to the flasher. I cleaned the relays and the light went out so I'm convinced that the flasher was the problem. I plugged the trailer in last night and it's back to normal.