I have 100% MEFI 3 harness. If I found a MEFI 3 ecm it needs to be for a 502 correct? Also all my other components dist. Injectors etc will still work properly?

The MEFI 3 would need to be programmed for your motor. No expert but most any 3 can be reprogrammed but would ask Smitty (articfriends)

Could also ask Bob at obd diagnostics

I spoke AZ speed he is going to send me the pin out so all I have to do is switch the pins so I can use my ECU ( well for 25.00) This is great news. My ECU already has a tune in it.

The problems I foresee, IF Im following correctly ( aside from the fuel pump relay being backwards).
!. You have Mefi1 ecu and new mefi3 efi harness, correct?
2. You fix the relay problem by rewiring the relay
3. You make ecu work by re-pinning the two J connectors FROM mefi 3 to mefi 1
What about the Knock module then, the mefi 3 harness doesnt even have it wired into it, maybe you can add the wires into the J connector, IDK for sure. We have rooted all of it out before to convert a mefi 1 harnes to mefi 3 but never backwards and last time we did THAT was a couple years ago so its not fresh in my memory.

I do have my old harness to which I was just wire the knock sensor wires into the new harness. Seems better solution I hope

Instead of all the repining, how about I send you my mefi 1 harness.
If it works lets just say happy birth day, if not send it back

Edit: and lets hope nothing happened to the ECM during this time

I am waiting for the tuner to send me a diagram and if it’s just a few pins I’d rather go that way
“ for now”. I would hate to take everything back apart again that’s a lot of work. Thank you for offer I may end up taken you up on that.

Just thought I would chime in and say, thats a nice offer!

Yes it is still good people out there. If I had the ability to help someone I always do it always come back to you in some way

How did you get a MEFI 3 harness to plug into a MEFI 1 ecm ? and if that was the case there was a likely hood the ecm possible could been damaged internally.