Starter motor won't stop turning engine over

I have a 1981 South wind with a 351. When I try to start the engine, the starter motor engages and turns the engine over but it will not stop trying to turn the engine over even with the key turned off. I have to disconnect the battery to get it to stop. When I touch the battery cable to the battery post it starts to turn over. I put in a new key switch but that didn't help. I don't think it's a stuck solenoid. Any suggestions?

With your ignition switch off, have someone with a voltmeter set to measure the voltage at the solenoid ignition post as you briefly touch the battery cable to the battery post. If it's 12V, something is closing your start circuit. If it's 0V and the starter still spins, your solenoid is sending 12V to the starter when it shouldn't.

Low battery voltage can cause the solenoid to get stuck. Also if the brushes are worn down too small that can cause more voltage draw and cause it to get stuck.

There was a problem for a long time with the starter relays. Some folks call it a "solenoid," which isn't technically correct. The solenoid is the magnetic actuator on the starter itself that throws the drive gear into the flywheel when energized. The relay is the electrically-operated "switch" that connects the + battery cable to the starter.

Sometimes a weak battery will cause the relay to stick. Next time it happens, just open the box and smack the gray plastic cover that's over the relay with your fist. It will cause the relay to unstick. I would suggest that you NOT use one from the auto parts store. It is not Coast Guard approved, and if you ever had a fire your insurance company would have an easy out. I know, I know, they look the same, but the marine ones have spark arrestors in them. Look for the words "Marine SAE J1171 stamped or printed on the base. Your insurance adjustor will look for it. SkiDim has the proper ones. Yes, they cost more.
Art

Art