The PCM drives the EST line low, or in other words its an open collector output.
I'd rather stay away from tuning requirements, and I think I can - connecting the EST line to ground through a 470 ohm resistor seems to prevent the DTC. I just need to figure out this voltage comparator circuit so the microcontroller can see the signal.
This won't increase any inherent RPM limitations, but it should give stronger spark than the opti above 4500 RPM.
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