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No 12 volt charge to 7pin tralier plug

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I have a 2002 gmc sierra 2500hd 8.1 with 7pin trailer plugs in the bed and at the bumper. There is no 12volt power to either plug for top off of trailer batteries or 12 volt trailer accessories...ie winch. I know that gmc has 2 12volt posts at the fuse box and 40 amp fuses to those corresponding posts ( left post controlling the 12v trailer charging  and the right post controlling trailer brakes). I have power at both posts, both fuses are good and the trailer brakes work but still no 12v power at either plug. Any one have any suggestions?

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You have a square 12 volt fuseable link missing. Many trucks never got these from the factory as they are not needed for trailering most trailers. Under the hood Drivers side, lift off the cover, the one missing is close to the engine. Its a 4 pin plug in.

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Trailer plug is wired exactly as the diagram shown. As is the Male 7pin on the truck and the Male 7pin in the bed of the truck. I get full voltage to tail and running lights, turn signals when activated and have trailer brakes when trailer is hooked up. But no power to the auxiliary 12volt charge 

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