Thank you DSP.
I don't want to be a pain in the §$%& but I think the accuracy is only needed on the receiving side via the hall sensor to the soundcard.
If rpm drops the solenoid just needs to get more pulses that cause more diesel to be injected until it's up to the specified rpms again. No need ...

Do you think that the voltage swing of a chep hall sensor will be enough for a soundcard input?
I am still not convinced that the com port can't handle the needed 400hz adequately.
Look how this guy used up to 20khz signal to control his fan speed: http://sites.google.com/site/chrisatronics

Radian

I am building a combined power heat system with a Volkswagen TDI engine. The computer is missing
and I actually don't want it anyway since it will only run at 1500 rpm and full load to drive the 22kw
asynchronous generator.
The fuel pump is governed with a solenoid that is driven by a 200-400hz PWM ...

Thanks guys.
I only need a frequency of 200-500hz to supply a solenoid that turns a lever.
What about direct sound out. How does a PWM signal look after the soundcard?

Greets,
Radian