Hey Guest!
The starter spins but does not engage the fly wheel. As it does using the key switch.the solenoid has three terminals. IN from Battery / OUT to starter/ OUT to other wiring. It's a switch so, In from battery post and out to starter (the two biggeast lugs) . Also, it is a switch so it doesn't matter which get connected to which.
View attachment 82392
That was the problem. Thanksthat's weird. I suspect then that there might not be enough current flowing through to spin the bendix gear. Try taking a big screwdriver and touching it to both terminals and see what happens. (wear gloves)