Ignition simulator

[Bitsyncmaster]

I've decided to make an ignition simulator for my playing with my electronics and testing stock components. It turns out the RPM relay and idle ECU have an AC coupled ignition input and they use the high voltage spike generated by the ignition coil to detect RPM. My units use a DC coupled opto coupler with a threshold around 7 volts to detect RPM.

My car uses a GM ignition ECU, no ballast resistors and a 1.5 ohm ignition coil so I took an oscilloscope "shot" of my ignition and also put a stock ignition ECU on to test (still no resistors and non-stock coil).

The GM ignition ECU generated a double 450 volt pulse about 16 us. long. Don't know why it got a double pulse.

The OEM ignition ECU generated a 150 volt pulse about 100 us. long.

My guess is the OEM ECU clamps the voltage for some reason. Normally ignition ECUs use a 400 to 600 volt driver transistor so why would it clamp at 150 volts?

I hope to get some parts any day now to build my simulator. It will be interesting to see what the stock idle ECU threshold is.