As far as I can tell, the tach senses the coil firing, there's no "detector" at the coil. When I changed to the 4.3 (6 cyl) from the 2.6 (4 cyl), I swapped in a Mitsu V6 Gen 1 tach, and simply ran the wire to the coil.

If your tach is "stuck", it may have an internal issue rather than a signaling issue.

I'm not the expert, but that's my understanding of it.

Don `87 Mitsu 2dr, Rubicon survivor, GModified.