With the starter on the bench I cleaned the terminals and set up a test. Anyone who has worked on a GM car for about the last 70 years has probably seen a Delco starter. Battery voltage goes to the big terminal. When 12V is applied to the “S” terminal the solenoid engages and the motor runs.
So I got out my trusty 12V power supply (sometimes called a battery charger) and set up a test.
12V on the big terminal, ground on the frame, and a jumper wire on the “S” terminal. When I touched the jumper to power this happened:
So the starter is good. I cleaned up the hardware and put it back in.
I was still kind of hoping the solenoid had been stuck and it would work now. But no joy. So I pulled the spark plugs and ran a lead from the “S” terminal to the battery. The engine cranked strong and fast. So we have a wiring problem. Dammit.