I've got Toyota COPs wired through a gutted stock ignitor, as suggested in this sticky thread (https://www.miataturbo.net/useful-sa...writeup-12704/), and my tach is behaving spastically. It jumps between reading high and reading low, depending on load. under load, it reads low, unloaded it reads high. Adding the 1Kohm resister to the diagnostic port changes the tach reading up or down, but doesn't change the fact that it jumps between two distinct states depending on load.

I am running Megasquirt, but this issue was happening on the stock ECU as well. So the problem isn't megasquirt related, but I'm open to megasuirt solutions...