engine keeps running with new MSD ignition

[grumpyvette]

"We've all experienced engines not starting... I'm having the opposite problem. I turn the key to off-lock (or even off) and it doesn't kill the engine. If I turn the key rapidly it eventually turns off.

I'm driving a 1972 Chevrolet Malibu with the original 350 (rebuilt) unfortunately the guy I bought the car from didn't know much about it. I can see it has a Holley electric fuel pump, msd ignition and some other upgrades. I replaced the ignition switch and that did not fix the problem. I'm honking they jumped a wire somewhere which is back-feeding electricity and keeping the engine running. Thoughts?"




Engine Run-On

If your engine continues to run even when the ignition is turned Off you are experiencing engine Run-On. This usually only occurs on older vehicles with an external voltage regulator. Because the MSD receives power directly from the battery, it does not require much current to keep the unit energized. If you are experiencing run-on, it is due to a small amount of voltage going through the charging lamp indicator and feeding the small Red wire even if the key is turned off.

Early Ford and GM: To solve the Run-On problem, a Diode is supplied with the MSD in the parts bag. By installing this Diode in-line of the wire that goes to the Charging indicator, the voltage is kept from entering the MSD. The diagram below shows the proper installation for early Ford
and GM vehicles.

Note: Diodes are used to allow voltage to flow only one way. Make sure the Diode is installed facing the proper direction.

Ford: Install the Diode inline to the wire going to the #1 terminal.
GM: Install the Diode in-line to the wire going to terminal #4.
GM 1973 - 1983 with Delcotron Alternators:
GM Delcotron Alternators use an internal voltage regulator. Install the Diode in-line on the smallest wire exiting the alternator. It is usually a Brown wire.
Most other applications: On other applications where engine Run-On is experienced, a Resistor can be put in-line to the MSD's small Red wire (See diagram below). This resistor will keep voltage from leaking through to the MSD unit.

http://www.msdignition.com/uploadedFile ... ctions.pdf

http://www.radioshack.com/product/index ... Id=2062579

http://www.radioshack.com/product/index ... Id=2062591

http://www.4secondsflat.com/CD1-MSD-Crane Comparison.htm

 

[mathd]

Yeah mine did that too with the MSD box, changed the diode and it fixed it.
Too long of a spark plug reach can cause this problem too, or a badly tuned engine.

What happen is, its the Gen light on the dash, one side is connected to the ignition (pink wire on GM) and the other side to the alternator brown wire.
That wire is used so when the alternator/generator is no longer charging, the brown wire is to ground making the GEN bulb light up on the dash.
But when the engine is running, there is +12v on this brown wire feeding a small current to the ignition wire/box through the GEN bulb.. keeping the unit energized as the MSD book say. The diode will block current so it can flow only the way we want it to.
Some car have a bleeding resistor on the GEN bulb circuit for this purpose. The diode is much better to my taste