I recently bought a 2006 HD2500 Duramax. I used it Friday to pull my flat bed trailer to a friend who wanted to borrow it. When I got it hooked up I tested the lights. The signals and brake lights worked fine. So did the tail lights. But when I turn on a siginal or hit the brakes while the tail lights are on all of the lights on the trailer would go out. The lights on the truck still worked fine. I checked all the connections I could see in the dark. And I made sure the ground wires were connected. I never had any problem with my old truck nor has anyone who has borrowed the trailer. It worked fine behind my friends 05 GMC when he took the trailer. My truck has a seven pin connector. I bought a 7-4 adapter at Holiday Trailer.
If this is a problem with the trucks wiring I can take it to the dealer. It still has a full warranty as it only has 14,000 miles.
I plan on putting a brake controller on the truck and using it to pull a travel trailer or my boat also.
Does anyone have any idea what the problem could be.

hadude
this sounds like you are crossfeeding voltage thru one of the bulbs,and are loosing the ground to the trailer.you say you have the 4 wire plug on the trailer do you have the white wire on the trailer side grounded to the trailer frame.

Yes. When I bought the trailer it wasn't connected. I drilled a hole in the frame put an end on the wire and bolted it to the frame. I don't understand why all the lights would go out when the brakes are touched. I would think that if that were the problem when the signal blinked off the tail lights would come back on till the signal blinked on again. In 40 years of driving I have never encountered a problem like this. I have a spare trailer light that I am going to wire to a four prong flat plug to use to test and try to work this out. I will let you know if I discover anything. Let me know if you have any other ideas to try.
Thanks..........

hadude you might try another 7-4 adapter just in case your adapter is the problem.

Thanks Stan! I will give that a try. I am going to make up the test light tomorrow. If that doesn't work I will try another adapter.

Check the fuses in the fuse box under the hood. Your owners manual has the location of the fuses. There are fuses for the turn signal, brakes, ect for the trailer that are separate from the tail lights on the truck. That is why the tail lights stay on when the trailer lights go out. I have had to replace one before and spent hours checking connections on trailer when it was a fuse.