When I bought my bike the tach has not worked. The prev owner said he thought that it was an issue with something being broken in the gauge. Since I can't get out to my bike at the moment and this was on my mind I thought I would ask about it.
With my gauges off I looked at the tach cable end and played with it a bit and I noticed that not only does it turn freely but it pulls out of the "sleeve" of the cable. However, the speedo cable does not have any free play what-so-ever. Perhaps a broken cable :icon_question:
It very well could be. Cables are cheap and easy to replace, I'd give it a try...
If you need a cable I have one I'd let go for cheap@!