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Key off but engine keeps running

N

northern_lights

Member
I have see the many threads of guys who can't get their engine to run...but not to many on how stop them running. I am hoping its a simple answer...like a tired and worn out ignition switch needing to be replaced. I have twin 454's with upper and lower helm stations. I always start with keys at the lower helm. Yesterday...I fire up the engines and run them for 20 minutes. When I turn the key to off....engine kept running. I turned the Perko swich off....still running???? I finally had to reach down and turn the fuel valve off which still took it 5-minutes to quit. Original ignitions on this 1986 Tollycraft. So that will be the first thing I replace. Is their a bigger issue looming out there as well?
 
I'd bet on the alternator before the ignition switch. Some of the diodes in the excitation circuit have a failure mode where there is enough current provided that will enable the ignition system to still function.

You can safely stop the engine by jumping the - terminal to ground; just remove it promptly after the engine stops....much easier on the nerve endings than trying to liberate that coil wire.
 
I made the same bet and checked it first by pulling it off and having it bench tested at a reputable shop. Got to be wiring at the ignition switch or should I get second opinion on alternator???
 
Unless you told the shop what you were experiencing, they may NOT have performed a complete test on the alternator.

Why not do the testing to ID the culprit yourself?

Disconnect and tape the wires on the alternator. You only have to remove the excitation lead that is connected to the IGN feed from the switch. If you aren't sure which one it is, you can remove all the thinner wires - just make sure to insulate them from each other and the block.

Something is continuing to provide +12VDC to the coil when the switch is turned off. You may have to disconnect the upper helm control from the lower to isolate things. It shouldn't be too hard to find, just be methodical and consistent.
 
Yep, Volvo has had a similar issue with certain alternators.
The same "Ignition" circuit provides the excite power, so if the alternator back-feeds, your ignition remains powered.

As a temporary fix, an external diode in this excite circuit will correct the problem.
 
Another fix would be to go with one wire alternators and eliminate the voltage regulators completely.

Jeff
PS: If that happens again, ground the negative (NOT positive!) terminal on the coil to ground. That will kill the motor without damaging anything.
 
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Taking the belt off may be sufficient.....I've never tried doing it that way. more work than removing the wire.
 
I think the responses have this well covered, though there are a couple of things to watch out for: First off; don't use the "Perko switch" to kill an engine. It won't ever work and it might destroy the diodes in the alternator. It is usefull if the STARTER motor is stuck on. Also, if you run the engine without the alt belt, understand the coolant pump "wont", and limit the run to 30 seconds or less. The ignition switch test is easy; Turn the switch to on (not start), do the gauges spring to life (voltmeter)? If ok, turn to OFF. Does the voltmeter return to zero? If so, the switch is ok. Do not start the engine for this test.
As stated by all, you more likely have a regulator defect. In particular, a D+ to excite short. Without more detail on the actual alternator/regulator, it's hard to nail down the exact issue. It could even be a wiring short in the back of the alt, do you see a small yellow wire back there?
 
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