Tach reads high suddenly

[Preston Nash]

My tachometer is suddenly reading around 350-400 RPM higher than actual.  I did have the alternator rebuilt recently, but we did a short trip after the alternator repair and everything worked normally throughout the trip.  After having the coach in storage for about a month, the tach is suddenly off.  At 700 RPM idle, it reads about 1050.  At 1500 RPM on the highway it is reading around 2000 RPM.  The Silverleaf verifies that the actual RPM is where I think it should be. The alternator is putting out normal voltage.   

After reviewing previous posts about the tach, I have gathered there can be issues with the ground wire.  Since the problem is so proportional and constant, I am assuming it isn't the ground.  What could possibly be causing this?

[Fred Brooks]

     Preston,
I believe the gauges are VDO with a Magnum label. They are normally considered to be a quality product. I can't guide you to a resolution other than check all the connections on the back. You may try jumping a temporary ground from another metal ground source. My coach sporadically does the same thing. It jumps about 400 rpm higher than the Silverleaf indicates. Sometimes when I tap on it with my knuckle, it drops down, sometimes no joy. I normally drive by my Silverleaf as that indicates what the ECM is seeing. Many forum folks have made the comment that the factory gauges are for "entertainment purposes only". Sorry, Fred

[Preston Nash]

I took another drive today and it was reading closer to normal, although still in error.  My main concern was whether this may be indicating a problem with my recent alternator rebuild.  It sounds like it is probably isolated to the gauge itself.  I'll check those connections.

Thank you for your insight.  I greatly appreciate it.

Preston

[Fred Brooks]

  The tachometer is dependent on the alternator rpm. Not real clear how that works, however the rebuilder must have completed the job correctly or it would be inop. You may look for a loose connection on the alternator itself or try a temporary ground to the alternator case. Fred

[Eric Maclean]

Fred and Preston
the tach is driven by an AC frequency picked up off a stator tap in the alternator because the connection is made internally in the alternator at one of the three alternator stator legs before the rectifier bridge the signal will be an alternating current and the frequency will be dependent on the alternator pulley size and engine rpm.
It is important that the alternator be grounded properly and that the connections at the tach also be good to get proper tach readings.
I believe the tach in your vintage of coach is built by Teleflex this can be proven by read the face of the tach at the bottom in very small letters it should say Teleflex.
As Fred said check the connections at the back of the tach for tightness ( they have a habit of getting loose)
And provide a good ground from the electrical bay ground buss to check if you can rectify the problem.

Hope this helps
Eric