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About TB2015

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  1. Aux switches

    Check the big plug on the back of the upfitter switch bank. If you don't have harness wires in position numbers 10, 11, 12, 13 you will have to add them to the harness. Since I don't have those wires, or the relay circuitry in the underhood box, or the cut pigtails in the underhood harness for upfitter's connections in my 2015, I'm in the process of building my own aux relay box to mount on the second battery tray. That way I only have to add 4 wires to one harness plug and run them to where I want them rather than deal with GM's idea of how things should be done. Seems it would be more cost effective to use one fuse box and harness set for all the trucks in a model series rather than a bunchton of different variants. Probably a bean counter involved somewhere.
  2. Wiring diagram looks like it shows the fuses are on the output side of the upfitter's relays, so no help there without a jumper to bypass where the relay should be, or the Aux switches and install a relay. But... Depending on the year/model, the whole upfitter's add-on provisions may not actually be wired in the fuse box or to the output harness. Just the sockets in the plastic panel where the relays and fuses should go. Use a bright flashlight to look down into the sockets and see if the little metal tabs are there. Or pull the fuse box and look at the underside, but that is a pain. Currently in the process of building a waterproof fuse/relay box that I can mount on the second battery tray to support additional lighting and 2-way radio circuits that are not OEM on my 2015. No telling why GM didn't do a one size fits all electrical system instead of maintaining an inventory of multiple versions of the same fuse box or wiring harness. Would make retrofits a lot easier if everything was plug and play instead of having to rewire and reprogram. UI Bulletin_110j.pdf

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