What you discovered is part of a charging system test,,
That you left the key in the on position,, and in doing so,, you in fact tested the regulator, and it's working,, the alt brushes are contacting the rotor,, and the alternator rotor doesn't have an open circuit.. and if the alt is not charging,, that it's likely a faulty rectifier (diodes) causing the prob..
The rotor is a giant electromagnet when powered up..
This is a field test I do without meters to determine if an alt or reg is faulty..
With the key off,, there would be no (or little) magnetizm at the rotor.
Key on,, - no magnetism at the rotor (either end),, fault is regulator, alt brushes, or wiring connections..
Want to know if the regs working,, key-on,, check for magnetism..
hope it helps
