How to Find out what is grounding out my tractor?

I have a JD Sabre 15.5 38 Hydro, the tractor was running fine before now it blows the fuse when you turn the key on. I have traced the line all the way to the starter or the alternator, I am not sure where the short is. The alternator when a ohm meter is applied it comes out as an open circuit across the red and black I am not sure if this is how it is supposed to be or if there is supposed to be a resistance or not open at all. please advise