Well, the symptoms have changed a little from my first post about this subject. Not sure where this leaves us, but here we go...
First, here's the pic again for reference:
The part highlighted in red, which I call the "clicker", used to flip from plain white to a black line every second or so whenever the key is on. It's now stuck on black line (though you still hear the "click"), and the hour meter is also stuck.
Here is what happens:
The "clicker" makes the clicking sound, but does not rotate. You can see it try to rotate (moves a tiny bit), but it is stuck.
At the same time, you can see the hour meter, which is now at 1519.9, try to rotate to 1520.0, but it is also stuck. Again, you see it try to move (so there is power), but it doesn't actually rotate. It tries to move in unison with the clicker.
Sometimes I still get the tachometer needle "dip" with this behavior, other times not. I checked all the connections on the back of the tach, and everything seems OK. I tried checking connections at the alternator end of things, but am still figuring out the wiring since my alternator is not original and there's a separate voltage regulator in the mix (also not original), so the wiring diagram in my manual doesn't match my boat exactly.
So, is this still some sort of electrical/ground problem? Or is the hour meter just bound up (mechanically)?
Any ideas on next steps to figure this out?