Hi Terminal_Crazy,

Have a close look at the optical wheel...
OWheel2.jpg

Looking at the wheel from the front it turns clockwise. If the slot represents a "1" then the opening of hi res is rising centered on the low res. A designer might use the high res to clock in the low. I believe the leading edge of the low indicates TDC for one of the cylinders. The machine then measures the duration to determine which. Once in sync it is just about keeping count. Why four and not eight? Perhaps it had to do with GMs design goals in terms of how much of a revolution takes place before sync is determined. I am sure that start up is complex looking forward to figure it out if I can.

Does anyone know if the wheel is removed from the optical path... do you measure a 1 or a 0 on high and low resolution inputs???

-Tom