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.

I have not tried this yet. Why would the ecm be damaged

This is maybe what is going on. I'm thinking your new harness is for a 3 and your ECM is a 1.
That said, I took my 1 harness, and did plug it into one side, very gently into my 3 ECM. Don't have a 3 harness other then whats on the boat and not going to try with it on the 1 to verify

I also compared my mefi 1 and 2 spare ECM's and the pin out looks the same (32 pin connectors). So if your harness is missing keys which keep a 1 from plugging in, then it would happen without knowing with my limited knowledge

From my research which is talking several marine tuners it is possible to “ go from 1 to 4 and 4 to 1.

Basically going from 3 to 1 would require changing the knock sensor and FPR pins and connector. I still have my old harness which should make things easier.

Hard to comment in great lengths but for a jist MEFI 1 does not even have the same circuits pin outs nor the J connectors per those compared to MEFI 3.

This is how the ecm can surely be can end up being damaged and I still do not see how you connected the ecm from a mefi 3 harness to mefi 1 and the engine starts and runs ???

HERE IS PRIME example how off base this is for just example.

THE IAC for mefi 1 and MEFI 2 is J2 - 28 and J2 - 13 and J2 - 14 and J2 - 29

The IAC for MEFI 3 is J1 - 28 and J1- 12 and J1-11 and J1 - 27 not even close for connections and pin outs in the ECM for jist of an example.

Do you have a service manual for this per what you are trying to do ?

Bup

You are way more knowledgeable than I am. All I did was call a few tuners and the tuner at AZ speed told me he would sell me a diagram on what pins to change to use the Mefi 1 ecm.

Another tuner Hardin Marine suggested not doing that.

All Dodge is willing to send me a mefi 1 harness if that was your 2 best choices Which way would you go?

This is what I was sent. Will it work?
https://cimg8.ibsrv.net/gimg/www.off...932ae3460.jpeg

Looks like some great info and I'm sure it can be done IF followed

Keep in mind you'll need something (tool) to unpin and swap pins in the connectors