Tachometer Glitch

I recently got a 2006 America; love it, but hard to have a bike with no tach. All the info I can find says the Speedmaster tach should work, so I put one in. Worked fine, til I turned the bike off--indicator needle stayed at final reading. Next time, tach seemed to work but started from the last rpm number; on turn off, stayed at the final number again--final rpm plus first stuck.
Seller sent a replacement; exact same result.
Any ideas of the problem? Something in the bike, or just bad luck?

 

Thanks; unfortunately, no buttons, whistles or bells on the Triumph tach. The manual I found tells me what color wires connect to the tach, but that's about it; zero about the instrument itself. The hunt goes on, tho.

 

Dude, Kansas doesn't even have hills. I've passed tapping to rapping and striking affirmatively; not a jiggle. I need a shop visit anyway, I'll see what experts have to say. Even not working, it looks better than the filler emblem, so that's something.