Ignition simulator

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.

I was going to put a high voltage amplifier into the design and drive the simulator with a digital to analog signal but more thought made me change to a simple high voltage driver along with a 12 volt driver and just mix the two signals with diodes.

My driver is capable to running up to 600 volts but I found 80 volts is driving the stock idle ECU. I power my high voltage driver with a DC to DC converter that is adjustable from 80 to 390 volts.

I'm going to install it all in a box and it will replace the adjustable signal generator I now use testing my units. But now I can test OEM units as well. I have a switch to select 675 RPM or 875 RPM (stock idle RPM + and - 100). I will probably bump the spike voltage to 100 volts or more.

I finished putting my ignition simulator in a box. Now I can test OEM idle ECUs and OEM RPM relays.