View Full Version : truck side trailer connection

mike lane lawn care
05-21-2011, 11:07 PM
Hey guys, have a dillema with my truck, a 2005 GMC 2500HD that I can't figure out.

On my trailer, the right tail light is always lit up, even when the truck is off. I took a multi-meter to the 7-way to 4-way adapter and found that it is in fact getting power to that light. I then tested the actual plug on the truck, and the connector that plugs into that with all the wires from the truck, and got the same result, the truck is sending power to the right trailer stop/turn light. I remoed the fuse for it and of course, it went off. but then I also discovered that when I turn the right turn signal on, the right light flashes correctly, but when I turn the left turn signal on, both left and right lights flash, any ideas on what to check or do before I bring it to the dealer? could there be a relay stuck open somewhere?

thanks guys

05-21-2011, 11:49 PM
sounds like a harness or turn signal switch problem :confused:

knox gsl
05-21-2011, 11:54 PM
Is the truck or trailer new to the equation or have the both work together before?

mike lane lawn care
05-22-2011, 12:18 AM
both the truck and trailer have worked flawlessly for years now, the trailer has all new wiring as of last year, but has worked fine, and works fine on other trucks.

knox gsl
05-22-2011, 12:41 AM
That blew my guess then.
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05-22-2011, 08:50 AM
do you have a wire diagram? If so start from the fuse and go back. I know I can not say all relays work alike but if a relay closes them it provides a ground to power the item. If it was open then no ground and no light you could have a stuck closed relay not a open one. Look for one that functions both lights. Next and it would not be good but you could have a chafed wire in the harness.
I am looking at this from my background in aircraft maintenance. A good spark chase gets paid well.

05-22-2011, 12:39 PM
Those trucks seem to have an unusual trailer wiring system. When my '05 Silverado was fairly new, I was having a problem with the right brake light on the trailer not working. The right turn signal and the 4-ways worked fine. All the lights on the truck worked fine too. I took it to the dealer (under warranty) and it had a bad flasher relay. I thought this was strange because the flashing lights all worked fine, it was just the one brake light that was out. You may possibly have a similar issue.

05-22-2011, 12:57 PM
Are you in a snow area where they use salt? The right turn wire and the battery wire could be touching inside the 7 way or in a harness do to the salt. See if anything is green/white. Try using a electronic cleaner then some dielectric grease.
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