I found a couple posts on this but couldn't find an answer. I currently have an efi 502 with mefi 1. I have a mefi 3 harness and a tuned mefi 3 ECM. Can the mefi 3 harness and ECM just be swapped or will I have to change anything? Thanks.

I keep reading posts that describe the differences between them, but can't find anything that says wether they are plug and play.

You have an MPI harness it is different than the EFI , as long as the mefi 3 harness is from a MPI motor and not a 500 EFI I believe you can just swap it out, as long as the connector's are the same, and the engine harness will connect to it.

Thanks for the reply. I believe it is from a mpi motor, but not sure. I bought it from a member on here and have cleared pm's. If there any differences that would be noticeable between the two?

The ecu I know is from an efi 502. If the harness plus into the ecu do you think it would work with everything else?

The mefi 3 harness does plug into the mefi 3 ECM from an efi/mpi 502.

easy to tell the difference, with the 502 MPI harness the ecm connectors set in the middle right side of the intake manifold, on the 500 EFI the connectors mount on a metal plate bolted to the back of the head on the same side, good luck

it will go,you may have to reroute some of the wires that connect to the throttle body

The motor will be coming out soon so if I have to put the new harness on I'll just wait.

Here is my new question. The mefi 3 ECU plugs into my harness that is currently on the boat. Does this mean it will work?

I know the M1 & the M3 connectors are the same but I don't know if the pin out is the same, there's one way to find out

they are not pinned the same,,not even close the same.I have the cross reference chart over to make an mefi1 to mefi3.

I did not get it on today. I spoke with Eddie (really nice guy) at young performance and at least on connector needs to be spliced in from the mefi 1 harness. A few of them will likely need to be extended too. Seems like a job for when the motor is out.