ill get to the point.1968 camaro 12.1 302 auto .msd 6al msd distributor . new ignition switch . new ice cube relay between switch and msd . new optima battery. car will start fine , if you run it for a short time it will cut off with switch. if youtake it for a test drive over a few miles it will not shut off with the switch. any help would be greatly appreciated . thanks in advance

 

Your topic says run on. Does the engine keep running and running after the ignition key is switched off, or does it just actually "run-on", meaning only for a couple of seconds?
Also, are you running an electric fan? If you are and it's wired direct (not relay), it can act as a generator when the engine is shut down, and current feedback can make the engine run on for a few seconds.

 

i am running a flexalite electric fan .the engine will not shut off with the key. i also installed the one way diode per msd instructions.the fan is wired with a relay.

 

I think the ice cube relay might be your problem. If you're using the kind that I think you're using, it might not be heavy duty enough. Is it one of those clear box-cover, plug-in types that's used on electronic boards? Those come rated anywhere from the 5-15 amp range. What's yours rated at? If it isn't rated high enough, the contacts could be welding themselves together as it stays on and gets hot. That could explain why it works for a short term, but not for a long stretch. And are you using the same relay to power more than one thing? If so, I'm fairly positive it's an overload issue. Using a relay for the ignition isn't an absolute necessity. But if you still want to, I would use either a starter relay, or a different heavy duty type that can take the heat.

 

its a 60 amp relay . you may have helped solve the problem with your first responce.the fans may be the problem. when the fans are not running it will shut off with the switch. going to the garage now to try a new relay on the fans . thanks for your help dude. if this works ill let you know.

 

Wait, I got the impression that yours keeps on running, and more than a couple of seconds? Which is your engine doing?
About three years ago, I put on an electric fan that I picked up at a swap meet, and hot-wired it to a switch. That was only a temporary hook-up that I never got around to making permanent yet! In the garage, I have a nice dual fan set-up with a relay and thermostat controls that I'm going to put in instead. I also have the MSD 6A-L ignition. When I switch the car off with the fan still turned on, the fan feeds power back as it's free wheeling, and the engine keeps running for a couple of seconds, then chugs. I do try to remember to turn the fan off first and let it wind down, then shut the car off and it doesn't run on. This is like clock-work, so I know for certain what's going on with mine. The way MSD builds voltage, I swear it could run the engine off of static electricity if it had to (exaggeration)! It is kind of embarrassing if I go to a car cruise and forget to turn the fan off first.
I'm surprised that your fan would be causing it though, since the relay should be isolating the fan from the rest of the electrical system as soon as power to it is cut. Playing with your fan, and killing power to it before switching off the engine will tell you if that's what is causing the run on. Do you have anything else wired that could be giving feedback?