XJ900 Tachometer "sticking"

I took the 900 out yesterday to make a quick run to the grocery store before the snow started flying. When I was a couple blocks from home, I noticed the tach wasn't working - it was stuck on zero.

First thought was dangit, must be a loose wire or something. But when I got out to a major road and rev'd it out a bit, the tach went up to about 3200...and stayed there. A little further down the road and it had dropped to around 2400, but it certainly wasn't following what the engine was doing.

I went in the store, got what I needed, then got back on the road. Now the tach was working fine. Go figure.

It was a cool day - probably somewhere in the 40's, and I parked in the sun while I was in the store, so it probably got some solar heat into the instruments. I assume maybe it was cold and sticky when I started out and once it warmed up it got back to normal.

Any suggestions on what to look at? I assume it is electronic in some way, probably triggered off the ignition. Has anybody had one of these apart? Are they serviceable inside?

 

Usually rust particles in the clock..

 

Guess that will be one of the winter projects to pull it apart and look inside.

 

@co.dirtbiker I think you can do a search on here and find some links of the different types of RPM gauges that were in the 900s. I know there were two different types. I had the one that was not easy to fix. I could have broken down the little ckt board and checked each component. Don't have the right tools to do that, and I am sure I would heat sink mess up the components.

Here is the link for my thread that I was working on it. I ended up going to ebay and getting a good working one.
https://xjbikes.com/forums/threads/xj900-1488.133012/#post-689480

 

@Timbox Thanks for the thread link. That looks like it might be helpful.