I’m having an issue with my 83 GSL [12A SP auto to manual converted]. So recently when the engine is hot and the car is running, the starter will begin spinning full tilt boogey. Not engaged with the flywheel, just spinning. I shut off the ignition and it keeps going, have to disconnect the battery to get it to stop. This has happened twice now.
I did put the starter to hard work tuning after the rebuild, but it starts easily now. Any starting point for diagnosing would be appreciated!

 

Possibility the solenoid, which is attached externally to the starter, is failing. This is what engages the starter into the flywheel.
Solenoids avai separately, tho complete rebuilt starters are on ebay all the time for $50+.
Starter easy to pull - and an auto electric shop can prob diagnose whole unit on the bench, prob for free.
You're going to need to pull it no matter what, so start there-
Stu A
80GS
AZ

 

I'm going to go with an internal short in the starter. Point being, you said the starter motor is spinning disengaged from the flywheel. The solenoid kicks the pinion gear forward into the ring gear on the flywheel, and THEN contact is made inside the starter to apply 12v to the starter motor. This sequential operation prevents you grinding down your ring or pinion gear by trying to get them to mate while one is in motion.

There are heavy copper contact bars inside the starter that complete the connection, and if you have a short across them, they will spin the starter motor without the Bendix being engaged.

You need a new starter.

 

It definitely sounds to me like there is either a short within the starter or the wiring to the starter itself. I never had a problem like this with my fb but I did with my truck! Check the wires to make sure they are going to the right place and aren't loose or worn through. If they are worn through, electric tape will hold for a while to keep it from shorting out but check to see what is causing the wear through. Maybe a pinched wire?