Can I leave my stock ECU
in place to control non-EMS related gadgetry, while letting the MegaSquirt control the engine?
Yes you can-- This is what is known as a 'Parallel Install' and on some newer cars where the OEM
is putting more and more responsibility on the ECU
(more than just an Engine Management role) you may need to go this route. Some of the other responsibilities that OEM's are putting on the stock ECU
are gauge control, cruise control, auto tranny lockup, etc.
You’d basically leave the stock ECU
in place, and leave the stock sensor inputs connected to it. Then you’d run a new CLT and IAT
sensor for the MegaSquirt, and then share the TPS
signals between the stock ECU
and the MegaSquirt. You’ll disconnect the spark output(s) from the stock computer and wire the spark input to the MegaSquirt (pin24). You should be able to share that signal in most cases. Next wire the MS spark output (pin36 usually, depends on model and mods)to the stock igniter or directly to the coil if modded for direct coil control. Then disconnect the fuel injectors from the stock ECU
and wire them to the MegaSquirt. This will basically give your factory ECU
control of everything but the engine, and give the MegaSquirt control of the engine.
Note that's just a quick rundown, and not necessarily in the order you might go about things. For example, you might prefer to start with fuel control, go ahead and do all of the above EXCEPT the spark output change (you do need the spark input shared with the MS at this point). This way you've got full fuel control, but the stock ECU
is still controlling ignition. You can dial in your fuel map and get the car running good, and then move spark over as well with this transition being much smaller than taking it all on at once. Not to mention you'll know the MS ECU
and the tuning software MUCH better.
Note some may call this 'running the MS as a piggyback' though it's not. A piggyback ECU
simply manipulates the stock sensor signals to 'trick' the stock ECU
into performing differently. There is no trickery going on with an install as described here, this is a full standalone EMS implementation, we're just leaving the stock ECU
around to control the NON-EMS stuff. Call it a 'Parallel Install' as the two computers are working in parallel each playing their part, not tricking each other.