Ok, take the solenoid apart and check it. The copper terminal that the disc goes against is probably pitted and needs turned half way around in the hole so the disc can start making connection on the new side. Use a file or emery cloth and then sand the disc down and make sure all dust and chaff is cleaned out of there and it should work. You take the plastic top off and you'll see in there what I'm talking about. Fairly simple fix if it is still a delco starter and what I remember. Been awhile since I done one.