I am putting MS on my daily driver in prep for a turbo. If I wire the MS to the stock ECU harness, leaving the stock plug, can I just switch back and forth between the two ECUs until I get the MS tuned, or will that mess things up?

If I understand correctly, it should be fine so long as I only have one ECU plugged in at a time, and I'm using all the stock sensors. (All this is until I get the turbo on, of course. Once I do that there is no turning back)

I only have the one car, and if I need to go somewhere before my tuning is complete, it would be nice to be able to do that. If not, I'll have to borrow somebody else's car.

-Corey

 

If you're keeping the injectors stock for now, and leaving the factory sensors in place, you'll be fine. You'll probably be using a non-standard (GM) IAT sensor and a wideband O2, so wire those direct to the MS connector. Also, don't forget to configure all the pullups in the MS for standalone operation- these would be the temp sensor resistors and the CMP/CKP input pullups.

Truthfully, if you bench-test the MS properly beforehand and start with the DIY base maps, you should be able to have it driveable in a day- a weekend at most. But there's no harm in leaving the factory connectors in place. Mine are zip-tied up out of the way above my MS (note that I cut, rather than spliced most of the wires, but same basic idea):



The other option, which would make the in-car portion of the wiring even simpler and cost only a small amount extra, would be to build yourself a plug-n-play harness as described by Braineack in This Thread.

 

if you want make the PNP addapter harness with switches to swap between ecus for fuel and spark. I didnt end up putting switches in cause it takes me <5 minutes to swap between ecu's