As many, (88-240-47), I continue musing over the bouncing tach phenomenon. Much of what I have done, following suggestions in FAQ, has been somewhat helpful. Now, however, I'm going in one last time, after noticing that bumps even pounding on dash seem to trigger it.

Conclusion, time to dedicate a ground wire from the tach to a clean ground or the battery, and dedicate a trigger wire, thus bypassing the several mechanical connections, between the tach and the coil. No?

Will this ground the entire instrument cluster? Is this a redunancy that can "over ground" the panel or the tach? Second, the negative post for the tach, looking from the back, is the upper righthand post?

I've performed this kind of dedicated grounding on my older motorcycle (the diode board) back to the battery with astonishing results to the charging system.