One further think.

If the coil wire is grounded, the tach should peg full scale, if I have the little 'lectrons right this time....

On recollection, I have had the short initial bounce of the tach on startup when I had a disconnected tach wire. Look for a wire break. I have found that the insulation on some mitsu wiring will get very brittle over time if overheated, and crack. The wire them flexes at the crack and eventually fatigues and breaks one strand at a time until all are broken. This is the common failure of the LC filter on the v6 tach.

Not responsible for advice not taken...