Crazy Tach

I have a tach at my upper helm that is progressively getting worse each new season. This year I turned on the ignition to the port engine and the tach jumped to 3000rpm. The lower helm station tach for the same port engine is normal. After starting the engines that faulty tach drops to 2800rpm while the lower helm reports 800rpm. Eventually the tach drops 200rpm to 2600 while cruising along at 1800. Last year and the year before it would eventually drop down to near accurate readings. The tach is mounted in a Cummins Marine instrument panel. I'm guessing I can just replace it but wondering if there is a way to recalibrate it. If not where can I find a replacement?

Thanks

Paul

Check and clean your connections. I had a tach act like that and it was a bad connection.

At the back of the tach or at the engine?

Never mind - stupid question.

Love it! Same thing (similar, at least) with one tach. Had yard mechanic come over to advise a few days later. Fired up and of course it worked perfectly. Lower helm tach works off the same sender, I think, for both. I'll check all connections. Thanks kenk.

Had that and the connections on the engine sensor was not clean enough, has to be really clean and tight.

If the lower helm tach is correct and the upper is way off it seems as though the problem is at the tach in this instance.
Is the sensor reading from the crank or the output shaft at the trans? Or does that vary by application?

I had the same problem. It was caused by some light corrosion at the upper station.