Today I took a jumper wire (10 gauge) and made a connection between the big terminal on the starter and the small terminal where the start wire is attached. I get the same thing as when I try to start it with the key: A grinding, buzzing sound with the pinion gear spinning but not engaging the flywheel. So I believe I have eliminated the ignition switch and the wiring from the switch to the starter. I also followed the advice here and made a connection between the two big terminals on the starter to see if the starter would spin without the solenoid and it did.
So my next step is removing the starter again and taking it apart to see if there is wear on the fork mechanism and the contacts associated with it. I will report back with pictures.
