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Old 03-22-2014, 11:10 AM
DSK DSK is offline
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Angry engine won't stop and tach not working

Started up my 1987 Prostar 190 after 2 year storage. Ran fine but engine won't stop when I turn off the ignition! I have to disconnect the purple wire at the alternator to get it to stop. Also the tach is not working. I notice some sort of electronic module mounted on the back of the tach? Not sure what it's for. Any thoughts?
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Old 03-22-2014, 01:45 PM
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The not turning off issue would either be the starter solenoid or the ignition switch.
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Old 03-22-2014, 02:03 PM
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Unplug the tach completely and see if it will shut off properly. The faulty tach could possibly be sending voltage back to the coil.

I'm not exactly sure how the tach would do that, but I would still disconnect it and see what results.
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Last edited by Table Rocker; 03-22-2014 at 02:14 PM.
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Old 03-22-2014, 10:40 PM
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Thanks for the ideas guys. Dismantling the tach is where I plan to start. Ignition switch also seems like a possibility although that seems like a bizarre way for it to fail. Don't really see how the solenoid could do it since it's only in the circuit when the starter is engaged. If anyone has any other thoughts I'm all ears.
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Old 03-22-2014, 10:45 PM
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I'm with Table Rocker on unplugging the tach. A worn ignition switch usually fails the other way, but I have seen them fail to not shut the engine off. Had that happen on my little Toro snowblower this year. Turned out the contacts were loose and causing weird things to happen.
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Old 03-24-2014, 11:53 AM
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Question Update

Wierd! Completely disconnected tach AND the ignition switch while running and she keeps on running. The only way I can stop it is to disconnect the purple (exciter?) wire at the alternator. I'm thinking purple wire must be shorted to something in the wiring harness??? Or is the alternator not supposed to allow current to flow back through the purple exciter wire? I've looked at the Indmar 351 diagram that's been circulated on the forum but still can't make any sense of this. HELP
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Old 03-24-2014, 12:49 PM
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Per the diagrams on here, the purple wire at the alternator should be the exciter.

I wouldn't think that the alternator should be producing voltage on the exciter wire. There should be two terminals, one "voltage sensing" and one "exciter". If you are on the correct post, I bet there is a diode in the alternator that isn't doing it's job.

Is there any chance the purple wire is on the voltage sensing post? With the ignition in the run position, what happens when you disconnect the purple wire? If it keeps running, turn the switch to off and see if it shuts down.

However, I am far from an expert on alternator wiring.
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Old 03-24-2014, 12:51 PM
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Quote:
Originally Posted by Table Rocker View Post
Per the diagrams on here, the purple wire at the alternator should be the exciter.

I wouldn't think that the alternator should be producing voltage on the exciter wire. There should be two terminals, one "voltage sensing" and one "exciter". If you are on the correct post, I bet there is a diode in the alternator that isn't doing it's job.

Is there any chance the purple wire is on the voltage sensing post? With the ignition in the run position, what happens when you disconnect the purple wire? If it keeps running, turn the switch to off and see if it shuts down.

However, I am far from an expert on alternator wiring.
I think you are on the right track with this.
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Old 03-24-2014, 01:11 PM
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Thanks Table Rocker

I'll have a closer look at the connections on the alternator. By the way it does keep running with the ignition on and the purple wire disconnected. And in this case I do measure alternator voltage (slightly higher than battery voltage) at that "post". IT's actually not a post but rather a purple wire exiting the alternator with a connector about 3 inches from the alternator. I too was suspicious of the diodes but I had the alternator checked and was told they were fine? It does seem that if I put a diode in that purple wire so that current cannot flow back through it the problem would be solved. I may try that if I find no other explanation. Thanks again to all for putting your brains to this.
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Old 03-24-2014, 02:01 PM
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I wouldn't try to put an band-aid fix on this, but dig deep enough to find the actual cause. Have you checked that all the ground wires are clean and connected? This is likely from something that you did recently.

I had a problem last year where I attached a hot wire to the ground side inadvertently. Took out the solenoid on my new Arco starter from a backfeed. That reminds me, need to order a new solenoid.
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