Old (and probably cheap) electronic diagnostic and repair - how to?

I checked my ECU manual and it states that my tach signal should be a 0-12V square wave that goes low at TDC and high at 30deg after TDC. So that means if I make a square wave in the simulator app with a duty cycle of 91 (about 11/12ths on time, similar to the 330/360 degrees on time of engine rotation), that should be similar to what i'd see from my ECU. 

I played around with simple substitutions until I found this circuit. If I replace one of the caps on the board with a 10k ohm resistor (would be really easy to do on the board), then I get a circuit that turns the 12v square wave from my ECU into the  ~5v pulses that I think the IC will need. I tried a few different frequencies with this setup and all seem like they will work. So, if I can prove that the IC still works, then this modification should allow the tach to work beautifully with my ECU. 

Rad! And much better than the first thought i had while reading the previous post. I went straight to building an adapter to turn the ECU pulses into bigger spikes <facepalm/>.
 

 Thanks for the circuit simulator link as well. You've inspired me to have another go at repairing our much-missed daylight alarm clock. NLA and replaced by garbage.