HELP !! Two dead 2011 iMac 21.5"

[YTM]

Hey guys (and gals).... I recently purchased two working mid-late 2011 iMacs 21.5". One was core i7 processor the other was core i5. The i7 came with a significant dent in the case at the ODD slot so I had the bright idea to swap the logic boards (complete with CPU and GPU) so the good case would have the better processor. Made the swap, reassembled both iMacs and now NEITHER one will power up. On the i7 iMac, as soon as you plug the power cord in the ODD fan starts to run and stop in a 3 second cycle (on then off then on then off) continuously. Have unplugged held power button for 5 secs tried again same result. Taken out 2032 battery to "reset" logic board....same result. The logic board #1 led light is on, but not #2....tho it does come on for half second when the power button is pressed. Did I somehow fry both mother boards?!! How??!! Help !