Engine won't turn off...Opinions?
 

Last weekend after replacing my port side engines water pump, coming back from the test run, my starboard motor decided it wasn't done and continued to run even with the key out of the ignition. Installed a new key switch, didn't solve the problem. Had someone look at it and they said it was a bad alternator. I replaced the alternator last night. Will test it out today. I'm just curious if anyone else has had this problem. Could it be anything else, am I missing something, should I look anywhere else? All thought and feeling are welcomed.

I had this happen on an old jet boat after I switched to HEI ignition.. The alternator was back feeding the ignition after it was charging. You could turn it off but the engine would continue to run. However it doesnt sound like you changed anything that should have caused this. Is it possible you might have a wire switched on the back of the alternator?

Jerry.

I never switched the ignition or alternator. If ran it for two months as is, and then last weekend it just decided to keep running. I didn't touch a thing, it just started doing it on it's own.

Sometimes a bad alternator regulator will cause it to run on. My father recently had the same issue and it ended up being the plug on the back of the alternator. I believe inside of this plug, there was a wire that ran between the two prongs that had broken and caused a run-on situation.

How did he come to the conclusion it was the plug?

I spoke with him and he said he tested the continuity on the old one and when the key was shut off there was voltage running strait though the plug. After replacing the plug the voltage went in and not out. He said there was actually a diode in the plug that goes bad. I pretty sure he found this by accident..Not sure if this is true on yours but it could be worth a shot. I have seen on cars when the regulator is bad it will do the same.

The alternators do have diods in them. Basically a one way valve for electricity. If one went bad it could back feed and keep the ignition alive. Disconnect the alternator and see if it shuts off.

keep boating till you run out of gas!!!

right answer :)

I had same problem on a brand new engine as they were servicing it getting ready for delivery. Mercury Racing had mechanic hang a new computer, relays, engine wiring harness, and it ended up being a bad wire in alternator. Just unplug it and try shutting off engine before doing all the hard expensive stuff