Background - This is a well serviced engine running great at all speeds and loads. One day, it suddenly drops in RPM to no more than 2000ish, and rough at idle. Clearly missing. Initial troubleshooting reveals that there is no spark on cyls 2 and 3. Those are common to a dual coil. Easy. Replace the coil, and same issue. OK, go to the input side and see that there is no trigger voltage coming into the coil for either point in the cycle. Ah, where does that come from? On this motor, it is fed directly from the ECM. Ugh, but ok. Replace the ECM with known good. Same issue. Put old ECM on another motor. Works fine. Well, that's not it. What feeds the ECM to tell it when to spark? The easy one is the Throttle Position Sensor. Replace that with new. No change. OK, now I'm at the hairy edge of what I know about how this ignition system works. No codes on the YDS past or now. Anyone have a similar experience with a solution? That doesn't mean it's my solution, but will give insight on where to look now. Yes I have the service manual, and yes I've been troubleshooting and fixing motors under the shade trees for a very long time. I am confident in my ability, just not my knowledge on this specific situation (yet).
TIA
TIA