Spark when key is turned to the off position?

AJ/FormS said

All Chrysler ECUs that I have tested do the one spark thing,at voltage collapse to the coil . I imagine all standard coils do that if the negative side is rapidly interrupted. I mean It's just like the points are being opened right? The ECU is just an amp for the pick-up, and a current breaking system; a glorified, maintenance free, set of points, that don't suffer from premature point-bounce.

Click to expand...

Describing the ECU as an amp is not exact. Yes it has a transistor that pulls the coil (-) to ground, but the interface to the reluctor sensor does not resemble an amplifier. The interface rejects the negative swing of signal, then triggers at a point a few tenths of a volt above the zero cross transition. The pickup signal amplitude increases proportionally with engine RPM. The cranking signal with too much reluctor to pickup gap, signal ampitude is too low, and may will result in no start. It may have been running well at idie before shutoff, the signal at idle 3 to 4 times greater than cranking signal.
The amplitude threashold after zero cross, mentioned earlier is necessary for noise immunity, without the ignition would falsely trigger between reluctor teeth, and by EMI.
The spark _Sam gets when key is turned off, happens because the ignition switch interrupts the coil current, however the rotor is pointing to a random place, so spark may not make it to a plug.