The A9P is for an automatic, while the A9L is for a manual. I found this in the archives:
There is a hardware difference between an auto processor & a manual processor. The difference is because of the way the NDS [neutral drive switch, manual trans
] & the NSS [neutral safety switch, auto trans
] are wired. They are both wired to the same pin#30 but the auto trans
wiring grounds the pin to the starter solenoid & the manual grounds to signal return, during crank mode there is a voltage spike sent to pin-30 so the auto trans
processor has a diode on pin 30 where as the manual processor doesn't, most of the time this won't cause a problem unless you get into a prolonged crank condition. Long cranking can burn the "signal return" (pin #46) trace in a manual trans
I've been warning people about converting an auto to a manual and using the manual EEC with the auto wiring harness; those that want to convert their automatics to manuals, and swap the ECM, need to clip the wire going to pin 30 or else the ECM could fry the trace.
Here is the link to the thread: http://www.allfordmustangs.com/forum...s-a9l-a9p.html