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Ignition running on after key is removed?

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Bret

Bret

Yoda
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Refresh my memory. Why would an engine keep running after the key is completely removed? I’m not talking about dieseling, the engine is actually continues running normally after I turn off the ignition. It is totally intermittent and happens when the car is hot or cold.
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Anyway I know this is familiar territory, but for the life of me I can’t seem to remember the details. Seems that there is something about this happening with Later MGBs but I can’t recall where I saw or heard this before. I will go over my manual and notes but if anybody had any ideas feel free to share?

Thanks,
Bret
 
Bret: I have experienced this with my MGB and also some Jaguars. My memory is a little dim due to time but I think it had to do with the excitation voltage to the alternator that is sourced to switched ignition. There must be a diode in line to prevent the alternator voltage from feeding into the switched circuitry, IE, the primary ignition circuitry that keeps the car running. If there isnt a functional (it can become shorted) diode from preventing the alternator excitation to feed the ignition, this takes place. That is one source and can be corrected by a diode in line or replacing it in the alternator. Hope this helps. Jack
 
I was leaning towards the run-on circuit. So I checked the run-on fuse and it was fine. Next I checked the voltage across the run-on valve – Nothing? Doing a little deeper investigation I checked the whole circuit. Turned out that the connection to the running on control valve oil pressure switch had a weak connection. All I did for now was use some contact cleaner and a little scotch bright to get things working correctly again. But this weekend I will go over it a little more in-depth.

But it looks like I’m on to the problem. I’ll update later.
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I'm not sure how the running-on valve, and circuit could have much to do with anything except dieseling. The actual ignition isn't controlled by the valve. I'd be curious to know if concentrating on the valve actually cures the problem though!

I have had the alternator diode problem John described though when I did a gen/alt conversion on a VW engine and the alternator circuit didn't have a diode. Everything would work normally until time to kill the ignition, then the engine would run just fine with the key out of the ignition.
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by aerog:
I'm not sure how the running-on valve, and circuit could have much to do with anything except dieseling. The actual ignition isn't controlled by the valve. I'd be curious to know if concentrating on the valve actually cures the problem though!
<<SNIP>><hr></blockquote>

While I ain’t an expert on MGs, I am wise enough to know there is a good possibility that indeed something else could be going on.

However I believe that the run-on valve circuit is an integral part (designed in) to help shut down the engine buy opening the vacuum line to intake. While this will obviously help to prevent dieseling it also insures that the engine comes to a stop under normal conditions too by basically interrupting fuel getting into the engine.

That said I could still have an issue with the Alternator but I replaced it with a new unit a little over two years ago. But new doesn’t mean good so I will look into this. Also I’ve heard of this problem raising it’s ugly head on cars that have had "upgrades" to more powerful alternators. And a separate diode in line with the ignition light will usually take care of that problem in those cases. While my alternator is a stock replacement it might be stronger or more efficient. So it might not be a bad idea to go ahead and install the diode in my car too just to be safe. Eiter way, it shouldn't hurt.

I’ll refer you to the web page that good old Jeff (buegey58) listed above for more details on the diode issue.

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Bret
 
Bret - & when it does that, if you disconnect the battery, what happens?
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by tony barnhill:
Bret - & when it does that, if you disconnect the battery, what happens?<hr></blockquote>

Hey Tony,

I didn't actually try that, but I would think that it would die or start dieseling.

Prior to working on the anti-run-on circuit when I turned off the ignition key switch it would continue to run normally and would even accelerate or fall off smoothly when I pushed down or let off of the accelerator.

Bret
 
Bret - it should die unless you have alternator problems like a diode...I'm not an electrical guy but I've heard of some cars continuing to run if the alt's innards are fried
 
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