At wits end with a 1950 8N. The starter seems to be grounded sometimes but sometimes not. I've pulled the starter and bench tested it and it spins great. Installed it, and it cranked a few dozen times, and then just stopped. Now it won't crank at all. Solenoid clicks but no starter. Jumper from my truck will spark at the starter housing, but nothing at the stud. It seems like the grounding path has been interrupted somehow, but I don't understand how it could change while it's still bolted to the tractor.