Kubota 4050DT is my starter bad?

My late 1980's model 4x4 diesel tractor ran for 4 hours today and I parked it for an hour. It would not restart. I took at electrical meter and checked at where the large wire is going to the starter and it had 12.2 volts on it all the time. There is a little black wire also that connects to the starter very close to the battery wire. How do I check the starter? Can you send me an electrical diagram for a typical tractor starter and or my Kubota? Thanks