I'm pretty sure some of you guys will know why the key wont switch off the engine when I say it's in regards to the external voltage regulator, warning light and alternator wiring, but I am stumped.. I know of the symptom, I read it here very recently, but I have failed to use the correct key words to find it again in a forum search. ..so here I am asking..

The problems I am experiencing..
* Popping through the exhaust, (I'm sure an ignition misfire. -MSD 6AL drawing it's power from the horn relay's bus-bar. I can't see a problem, but it could be worth mentioning. The relay is good as the horns work, key warning works and other power from the bus-bar appears good (such as elec fuel pump). All other 6AL wiring is as per MSD install instructions.)
* Alternator wasn't charging, then found that the blue & white wires were swapped. However, with blue & white wires incorrectly connected, the engine shut off was fine but with some intermittent popping through exhaust.
* Swapped blue and white wires (blue being Field as per this diagram http://farm4.static.flickr.com/3190/2427479011_b2da388c8c_o.jpg) and the alternator is now charging but.. with very difficult starting, backfiring through carb while cranking, lots more popping through the exhaust and the key won't shut of the engine..

I have checked and rechecked my wiring. It is all as per the 69 AIM
It's late here, I'm going to bed.. I can't seem to get my head around what could be quite obviously wrong.
Any ideas? :)

Just found this over at MSD.. (post #2) It may answer my popping issue as I do have my magnetic pick-up and coil wires wrapped up together in the same loom.
http://www.msdignition.com/forum/showthread.php?t=4952&highlight=backfire+exhaust

I'll assume you converted your alternator from external to internal or you wouldn't have this problem.

In your package of stuff that came with the MSD-6AL you'll find a little diode. Wire that between your alternator #1 connection and the wire that goes to it. The silver side of the diode goes to the alternator.

All this is referenced with the instructions that came with the 6AL.

Remember, the wire from the ignition switch goes to the #1 connection and the #2 is the sense wire that CAN go to the BATT terminal on the alternator but really should be routed to the horn relay buss.

For the car not shutting off, did you install a diode inline w/ a the altenator??

It sounds like you need a diode installed on your blue wire which should be the #1 terminal on the back of your altenator. You need a 1N 4004 doide w/ the stripped end going towards the altenator. Radio Shack sells them in a 2 pk for .99.
It should have been in your MSD box if you bought your box new I soldered mine into the line feeding #1 terminal & used some heat shrink to cover it, then put it back into the wire loom.

I've had the 6AL for well over 10 years now and when it was originally installed, there was an internal reg Bosch alternator. So a diode? Hmmm. I'll have to get one and download the lastest 6AL instructions. No idea about the diode.
Now fitted..
Delco Remy 10DN (?) alternator with the Delco Remy external reg. Yes, back to the OE external reg set up.

Thanks.. Page 8
http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Products/Ignitions/6420_instructions(1).pdf

Bought a diode, fitted it and now the engine shuts off as it should!! :D

Best 12 cent fix ever.. ;)