Thump is right about cutting everything off, when cranking your big motor, especially, if your accessories are connected to the cranking battery....even if they're not connected to the cranking battery, it's just a good habit to get in to.

I blew up my on-board trolling motor battery charger, this spring, when I first got my Tahoe out of storage....both of my batteries are hooked in series, and I forgot to disconnect it, after charging the battery, and cranked up the big motor....burned the innerds to a crisp....called the mfr., and it was still under warranty, and they sent me a brand new charger.

Buck, if I had to guess, I'd say your DF wires are either loose or corroded, where they hook to the battery.