2020/10/30 - Tach Pullup Resistor
So, it turns out there's a few different ways to run a tachometer. I elected to use the low voltage signal the ECU puts out, via bulkhead connector. I hooked it up, started the engine, and nothing happened. Turns out, there's not enough Oomph in my low volt signal to get the tach to read it.
I had a similar issue with the RPM window switch (see 2020/11/01 - A/C Protection for deets on what that is).
The internet already had a solution for me - use a pullup resistor.
To help makes sense of why this works (I scratched my head for weeks not knowing this), I'll preface this with the fact that the ECU is apparently opening/closing a GROUND connection/pin to send the RPM signal.
Fig 1. Shamelessly taken from Google Images, closely describes what I did
Basically, the tach now sees that "bouncing" +12V signal much better, and reacts as expected.
I'll note that I used a 10k ohm, 1/2W resistor. I wanted very little current to be used. I was also able to hook up both the tach AND the RPM window switch (again, see 2020/11/01 - A/C Protection for deets on that) in parallel with (so far) no issues and accurate readings.