Engine run-on with MSD box

[JCRVETTE]

Hi. I have my battery in the trunk with the rear mounted cut off switch. I just put a one-wire alternator on it and it doesn't shut off when you turn the rear cut off switch to off. I am using a ford type remote seloniod on the firewall and wired the alt. directly to it (the terminal that leads back to the switch). MSD said to use a diode as described in the msd owners manual, but it seems to apply more to stock type wiring set-ups and not what I have. What should I do? Thanks.

[Sparky]

What you have created by wiring the Alternator to the Solenoid, is a feedback issue. Although you’re killing the power to the car at the Battery Switch, the Alternator is still feeding the rest of the car, hence running on.
You have to run the Alternator wire all the way back to the Battery, it must be hooked up to the Battery side of the switch, which means the Alternator will be energized at all times, so be careful when working on the car and unhooking the Alternator.
There is a heavy duty 4 post Battery On/Off switch you could utilize to kill the power to the alternator completely. This switch offers an isolated circuit for the Alternator and when the Battery switch is shut off, it shuts the Alternator off too.

On another note, I don’t think you will find a Diode big enough to support an Alternator’s output. The idea for this is with an OE style Alternator and you would be installing the Diode on the exciter side of the Alternator.

I hope this helps,
Sparky

[JCRVETTE]

Thanks for the fast reply. And yes it did help, you are a great help to all of us. I will change it tomorrow. Thanks again.