The factory computer in a non turbo eclipse controlls all engine functions, and that is pretty much about it. The cruise is seperate, a/c has input to the ecu but is not needed to for the a/c to work (it just lets the ecu know it is on and that is where the extra drag on the engine is coming from so it can kick up the idle and prevent stalling on deceleration). The guages are all seperate, the tach gets its signal from the stock ignitor. So properly done the stock ecu can come out, and an aftermarket can be plugged directly in. This is why I would recommend the Professional EFI System. They have precalibrated maps, and automapping of the fuel and timing tables so any modifications made can easily be re-tuned.

Jason.