I'm working on a 2nd method of filtering the tach input signal. My next idea is trying to use the negative pulse to trigger a mono-stable vibrator to give some control over the pulse width. I'm working on isolating the negative pulse that falls below -10V or so but I'm struggling to come up with something that works. Anyone have any suggestions for possible methods of doing this?

EDIT: Is using a summing amplifier (other voltage source being VBATT) to shift the wave up followed by blocking all the positive components to isolate that negative components we want ok? It should work, but are there any alternatives that would be better?

EDIT2: Using an op amp in an inverting summing amp? It would eliminate the need for the extra blocking step and conveniently inverting the good signal for us. Putting it into sim now.