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  1. #1

    Default aux 12v power to factory trailer plug

    Hi, New to this forumand i have a question to start. I have a 2008 Chevy 3500HD crew cab long bed single rear wheel Duramax with Allison with factory brake box and towing package. I tow an 18ft gooseneck horse trailer with living quarters that has a coach battery that is used for the breakaway source as well. I realized when my wife used the trailer for a long weekend that the truck was not charging the coach battery that I do not have 12v at the rear plug of the truck. Upon further investigation I read in the owners manual that there is a wire that needs to be connected in the engine compartment. There are two studs at the front of the fuse box in the engine compartment. One (closest to the engine) has a wire connected to it. the second stud(closest to the fender) is empty. Is there a wire hidden somewhere along the fender well that needs to be connected to that stud? Does anyone know what I'm talking about?
    Any reply would be greatly appreciated, thanks ,Neal

  2. #2

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    Yes It should be in a wire bundle coming from the rear of the truck. Check under the brake booster, and around the fuse box.

    1996 Chevy Tahoe LT 5.7L V8 4X4 205,000+ miles. Built proudly at Janesville Assembly in Janesville, Wisconsin
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  3. #3
    Jr. Engineer Jaele's Avatar
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    Quote Originally Posted by Conlan Rose View Post
    Yes It should be in a wire bundle coming from the rear of the truck. Check under the brake booster, and around the fuse box.
    hes Correct its a red cable, you will connect to the stud that is towards the outside of the fuse box, you will need a fuse and a nut.
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  4. #4

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    Thanks for the quick response. The truck isnt here at the moment. Aren't the two studs already fused from inside the fuse box? I'm assuming the stud closest to the engine is powering the "in dash" break controller. Will be able to dig into this when I have the truck here. Thanks, Neal

  5. #5

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    I found the two wires, but the one between the fuse block and fender was marked 12v aux. when I hooked it to the stud I couldn't figure out where the wire went. I'm thinking that wire goes under the dash for an aftermarket brake box. I found the one under the master cylinder and hooked it up and Hooray! Power to the trailer plug. The only thing I don't like is that it's always hot. On my old truck I wired this line thru a continuous duty solonoid that was wired to a ign hot. That way my trailer, if being used, won't run the truck batteries dead when parked. Thanks for all of the great info, Neal

  6. #6

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    You could always break that circuit with a relay that is connected to an ignition switched source.
    csltrains96
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  7. #7

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    OK I got one for yeah.... I have a 2008 suburban hooked this up good and proper. Aux brake controller and all.

    Today I sold my old camper trailer to a couple who have a 2009 Yukon , never towed before and they just assumed that they would hook it up and all was good to go...haha , Well since i would not let the trailer go without confirming that they had brakes on the trailer. I went about and tested it first..no power to the brakes or trailer , pop the hood and discovered that both red wires were not connected to the proper studs,dug out the wires and connected them to the proper studs and then proceeded to verify again power to the trailer wiring. still nothing at the hitch. this vehicle had a factory brake controller so i presumed it was not necessary to crawl under the dash to connect it... So pulled the fuses for the studs and verified they were good, checked power on the stud . no power . not possible you say.. well you are right. after talking to the owner of the truck, i decided that i will clean the studs. well that seemed to have done the trick. i removed the connections cleaned the studs good, tested power at the studs again . All it took was a little coat of corrosion to break the contact to prevent power to the stud. so i tested again at the hitch to make sure i had power . >>> well no i am now stumped on this last problem . maybe some one can give me an idea on what to do next.

    I have power for my brakes now but not for my aux 12 V. at the hitch. both studs at the fuse box give me 12 V. since it is a factory installed brake controller and it seems to be working i am not worried about that, but since the trailer requires a battery connection for the breakaway switch to activate the brakes in case of emergency, as well as keeping the battery charged for the aux battery in the trailer i would like to get that aux 12 volt to work.
    Did i overlook a fuse somewhere else that needs to be connected or am i leaning more toward a broken wire somewhere along the way between the front and back of the truck?

    I am hoping to help these people avoid a trip to a garage if possible. I know it is not my responsibility to fix this problem..but who can not help a fellow GM owner.

  8. #8

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    Probably missing one of the J-case fuses. Check to see if there's a dummy plug (or no plug) in stud #1. That's usually the case for NBS trucks like mine ... and it's that dummy plug that needs to be removed and replaced with a 40 amp J-case fuse to supply +12v aux power to the charging pin (usually pin#4) of the 7-spade connector.

    Here's a good write-up for a NBS truck: http://tinyurl.com/c493z89

    NNBS trucks like yours or that Yukon may be similar... so perhaps it'll help. As I don't have a NNBS truck, myself, and have not been through this with one, I can only speculate and hope for you.

  9. #9

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    thanks for the tip but i have powwer on both studs now just not on the hitch wiring. just hoped there was another fuse somewhere that needed to be activated or replaced. thanks

  10. #10
    Jr. Engineer Jaele's Avatar
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    You are correct, its hidden under the dash for an aftermarket break controller.

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