Hi! So, I've been riding my '05 GS500f for a few years now and haven't noticed this happen before with a good battery (more on that later).
While riding, usually engine speed around 4-6k RPM, my tach will waver between 3-7k, occasionally dropping to 0. Bike never loses power, however. This seems like a connection issue, nothing mechanical within the cluster.
I can compare this to when I had a dying battery (multiple cells were dead, had to bump start to avoid being stranded.. Not a fun night). The tach would jump all over the place.
I've verified this is very likely not caused by another bad battery, as I pulled the battery and the tender showed 95%.
I see that this tach is likely electrical driven, but I can't find anything on it. Wiring diagrams I see don't show the tach signal cables, and I couldn't find anything on any other forums.
Any ideas? I'm thinking a bad ground somewhere, but I know nothing about the colors, or even where the signal is fed from (cam sensor (doubtful), alternator/regulator, or pulse generator?)
When I first got my GS (second hand) the tach would jump between 0 and ~1200rpm when sitting at idle, but was fine after that. I figured it had to be an electrical issue so I pulled the tach out of the housing and saw that its ground wire (the black one) that connects to the back of the tacho proper was all corroded. I cleaned it up with some sandpaper and screwed it back on and ever since it's been working perfectly.
Might be worth checking out.