[LPG2CV]

The more simple approach, would be to take a single signal from the camshaft, or distributor, or as psig has suggested on other topics, even from a cam lobe.

[Nitro]

I thought about the distributor too. But I don't feel safe messing with the ECU electronics and wiring. The extra hall sensor was to avoid this.
But if its safe to pick the signal without messing with the ECU, I want to try it out.
This engine doesnt have cam sensor. The distributor is the only source for this signal.

[JHolland]

The count is what leads the system and the count will say where it stops. Unless you turn off the arduino. Most of ECUs keeps powered after the engine stops. So the system also will be.

Try it.

[Nitro]

I need to code first. No success yet