I reciently added an MSD 6AL to my 1987 f150 302 and ran into quite the problem.
This is what happened:
I followed the instructions from the site and once the MSD 6AL was installed the truck it fired right up! I let it idle for a bit and it sounded great - shut it down went in for lunch. The problem came when I came back to the truck an hour later to take her out for a spin. I turned on the ignition and the RPM's shot straight to the roof, this gave me a little scare so I turned the key off and the engine slowly died off (about 2-3 seconds after it was turned off). The worst part came when I realized that the starter had not stopped turning her over -even with the keys out. I had to pop the hood and pull the battery cable off to stop it. Needless to say this got me quite worked up - I thought I had killed my truck. So I removed the MSD box from the system, hooking it back up the way it was before and everything has been fine since.
Any ideas how this could have happened? I sent that exact paragraph to MSD techs, along with questions as to where I could hook up the diode to fix this, and their reply was:
"The diode supplied should take care of this problem. "
Thats it.. gee thanks! I just don't see how it could make the starter run - its as if something got latched in the on position.
Any help would be great!

 

What diagram are you using? Here is a link to one for a Ford TFI ignition system.

http://www.msdignition.com/pdf/pdf18.pdf

I don't know if I am looking at the correct system, but it sounds like you are getting feedback from the ignition box to the starter system. And the starter system is usually commonly linked to the ignition system to either; give a hotter spark for starting, and or, used by the ignition box to signal it to retard the timing till the engine starts. Retarding the timing during starting helps the engine turn over easier.

So you want 12 volts to flow from the starter relay wire to the ignition box, but you don't want 12 volts flowing from the ignition box to the starter relay, making it crank all the time. A diode is like a check valve. It will only let the current flow one way.

See if you can find your diagram on MSD's website and we can look at it and see if we can figure it out.

 

It sounds like you nailed it man! That diagram is the one I printed out to wire this baby up - so your on the money there too!
Could this be solved simply by putting the diode on the red wire that goes to the harness - like you said, so that voltage can only go into the msd box?
Thanks again!!

 

Yes, I think I would try that. The diode probably has a stripe or band on one end? If so, put the band side of the diode toward the ignition box. This should let 12 volts flow toward the box, but block it if it wants to flow out of the box.