Mitsubishi Repair: Transmission problem, transmission control unit, auto tranny

Question
I have a Mirage 2000 DE with automatic transmission.  The old tranny was replace with a used auto tranny because it slip, don't even move, tranny overheats fast and de "N" light flashes (dash- board). This  new tranny (second hand) and another one that we try throw first gear. If the tranny selector lever is in "L" do not throw first gear and the car moves, and if the lever is move to "D" at some speed (15 MPH)upshifts and downshifts normal. If you stop and the tranny downshift to first gear it throw out fist gear.  This two transmission that we try do not slip and the "N" light do not flashes, looks OK.  The "N" light should flashes when the tranny have a problem or when tranny overheats or both, but it do not.  Mirage comes with two identical auto tranny, one with external oil filter and another one with internal oil filter, but they are exactly the same.  Everything is the same, wires, connector, etc. The original tranny had internal oil filter (hard to find) and the two tranny we test have external oil filter. Could I have a problem with the transmission control unit that may be not exactly the same?, are those tranny from different manufacturer?

Answer
Erick,
My guess would be that your problem would be with the transmission control unit.  You may want to remove the TCU and inspect it for damage.  Installing a working TCU from a Mirage that uses the same type of transmission as the one you currently have in your car may help.
Good luck!