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Tool Talk Discussion Forum

Re: Engine Run-on after Shut-off

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07-24-2014 17:43:57

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Bill, does the engine have to be up to operating temperature for it to do this?

Or, can you start it cold, run it for 5 minutes or so and have it do it?

I had a mid 70's 350 Chevy engine that did this after only being run maybe two minutes after cold startup.

Eventually found the problem, was not carbon, was not idle speed, but electrical.

In my case the diagnosis was simple, I had a mechanic tell me to shut off all electrical accessories except the radio.

He then had me check to see if the radio still played while the engine was doing it's run-on thing, and sure enough it did.

Problem was in the alternator, somehow it was still supplying some power to the ignition even with the key off, but only while it was turning.

That was a common problem with some mid 70's GM products, seldom was diagnosed properly.

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07-24-2014 19:18:00

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 Re: Engine Run-on after Shut-off in reply to Milan, 07-24-2014 17:43:57  
Milan, That's good to know, Thanks! Do you remember the final fix, or did you just have to work around it?

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07-24-2014 19:27:09

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 Re: Engine Run-on after Shut-off in reply to ss55, 07-24-2014 19:18:00  
Something was wrong inside the alternator.

At least that was what I was told, somehow it was feeding power back into the accessory/ignition circuit, replacing the alternator fixed the problem.

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07-24-2014 19:38:28

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 Re: Engine Run-on after Shut-off in reply to Milan, 07-24-2014 19:27:09  
That prompted me to do a quick search. In Google, I typed in, "Can a faulty alternator cause engine run-on?"

One hit was

In which they said, "Engine Run On.

An engine that still runs after turning the ignition key off is a victim of engine run on. Engine run on can be caused by several things.

Faulty alternator/regulator. GM 10SI and 12SI alternators sometimes feedback voltage into the exciter wire even when the engine is shut off. This causes voltage to feed into the ignition circuit which keeps the coil hot with voltage and causes the engine to keep running. Remedy this issue by installing a diode #276-1661 from Radio Shack, inline with the alternator exciter wire. Make sure to install the diode with the line closest to the alternator. By installing this diode you have introduced a one way gate in which voltage can only flow to the alternator and not vice-a-versa on the exciter wire."

So, in reality, according to this, I may not have needed to replace the alternator.

In my application, it was an externally regulated alternator, with simply an "Idiot Light" indicator.

As quickly as I found that site with that quote, I would be willing to bet that there are many other sites explaining that as well.

Unfortunately, back in the mid 70's when I had that problem, not that many mechanics were aware of this situation.

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07-24-2014 23:23:34

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 Re: Engine Run-on after Shut-off in reply to Milan, 07-24-2014 19:38:28  
Thank you again!

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