A long time ago, I was helping a fellow that had a Volvo
engine. I donít remember which model. We wanted to start the engine with the alternator physically mounted, but disconnected electrically. No go. After an hour of going through the diagrams, we found that there was a starter relay that was GROUNDED via a pin on the alternator. Without that connection the engine wouldnít crank. My guess was that by connecting the start relay to that point, the relay was grounded when the motor
was NOT running, but was at +12v once the engine started. That would keep you from trying to engage the starter when the engine was running.
Something like that would match your problem description.