Factory tach conversion

[black_out]

Hi, first post here as I'm no EE but figured this would be the place for information. I have a personal project here with a mitsubishi with an engine swap. The original engine was a distributor fuel injected v6 that received its tach signal off the distributor direct. Mitsubishi decided to redesign the tach on their coil pack powered vehicles and unfortunately this vehicle never received that coilpacks. The issue I'm having is that the tachometer with my new engine swap does not function, I have smart coils on the new engine and it just outputs a ground signal. My computer allows me to output a ground signal at 50% duty cycle based on RPM which works great in one of my other cars, however, this one seems to run off the coil spike. I have used a dakota digital tach adapter with no luck, and being a mitsubishi dakota digital didn't seem too interested in assisting me in figuring it out. I was scrolling facebook the other day and an automotive group I'm in were discussing tach operation on a distributor mitsubishi with a modern engine swap and I was linked to a thread here from one of the swap pages where they converted their mitsubishi tach to accept the new signal. I disassembled my tach which is similar but the circuit board is designed slightly different and I'm not 100% confident if 1) I can do the same to this one and 2) Which section of the circuit needs modified on mine. I currently have one spare tach unit and rather than risk destroying my one spare I wanted to come here and see if someone smart with this than I could assist me in this. I'll upload a front/back picture of the PCB of my tach and a link to the thread with the modifications.

https://flic.kr/p/2q7Q2FX

https://flic.kr/p/2q7Q2FS

I had to use the wayback machine to pull this thread, so hopefully it links properly as the forum has been shutdown for a few years now.

https://web.archive.org/web/2016091...p?t=1857&sid=6db6c70b02965435dc55eb9f5a976f54

If anyone has any insight on this that would be great. I'd love to get the factory tach working again. Thanks!