tonyarkles, I have no idea, however the output waveform is exactly what I expected. It's a 12V square wave, which is normally 12V, but goes low for a few ms on each ignition pulse. There were actually two 10k resistors, one from 12V to collector, and one between the collector and the tach. I was looking at the voltage at the tach, and it was still what I expected, so I don't think there's a current issue.
Junkers, I don't have any screen shots of waveforms, but like I said. It looks like 12V, then goes to nearly ground, for a few ms, then comes back up, and then gives a very damped very high amplitude ringing as the pulse comes back to 12V.
Rerouter, Is it looking for it to go below ground or above battery voltage, the high voltage looked much more distinctive than the below ground voltage, but maybe you're right. Could you draw a schematic, I'm really not following what you are describing about a push pull. Are you thinking of something like this (driven by a 5 or 12V CMOS buffer):
![](http://i61.tinypic.com/2ih8obb.jpg)
Or maybe something more like:
![](http://i57.tinypic.com/2ah9wns.jpg)
In either case, I'd think R=10k is probably good, but you'd probably want a time constant measured in microseconds, maybe 10us, which would put it at about 1nF, not 22uF. 22uF would could give you a time constant of 220ms, which means the cap wouldn't be fully charged before its discharged, nor would it fully discharge if it were allowed to charge.