Had an interesting experience the other day. I was driving on the highway doing about 70, when my tach went from around 2200 or so then jumped to about 4000, and then drifted back to 2200-ish. Went along a bit more, and it happened several times until I got home.

I didn't hear/feel the engine speed increase, so I think it's only isolated to the gauge. Although at 75mph, wind and road noise tend to drown out a lot of external sound, so I can't say with exactly 100% certainty on that, but I don't believe there was any notable increase in engine speed.

Any ideas on what the cause might be? Circuit board maybe? Or possibly an indication of a problem elsewhere?

 

You might want to ask the advice of Greg .

https://www.thirdgen.org/forums/engi...achometer.html