How does it work ? The solenoid is energised by 12V supplied thru
the skinny wire to the spade connector terminal. The solenoid plunger
pulls in and does two things... (a) it kicks the starter pinion gear out
and into engagement with the starter ring gear, and (b) it closes the
contacts that energise the starter motor itself.
So the solenoid kicks the pinion gear out, but it will not spin unless
the starter motor itself is energised and spinning. If you have made
sure the brushes are making good contact with the commutator, and
the commutator insulators are trimmed down to below the surface of
the copper, the starter should turn when it gets solid 12V from the
fat wire. The starter grounds thru its frame, so it has to be bolted to
the engine, and there has to be a good ground connection from the
engine to the frame, and from the frame to the battery negative.
If all that is good and it don't turn, then your starter is toast
PS: Check the contact points in the solenoid before you chuck the
starter out. Arcing at the contacts can build up crusty crud and stop
the starter motor getting enough current. You should be able to clean
them up, or replace them.