RPM gauge not accurate

[mlh2013]

On my 1999 diplomat my rpm gauge is reading 2500 or better when it should be reading 1800. I may go all day with it way off like this. Then next day it might work just fine, any suggestions?

[Robi.1014]

Well, if the tach is failing inside, a replacement is in order. First, however, let me suggest that you (while driving) press your finger, hard, on the tach rim starting at the noon position, then 3, then 6, then 9 pm locations.

My tach would occasionally fall to zero RPMs. When I applied pressure (for me: the 6 pm location), the tach would start working again. I then found that stuffing some tape under that position of the rim made the "fix" permanent.

[Mekanic]

How are you determining it is off 700RPM?
Are you sure your in high gear?
Are tires the OEM size?

[walt2137]

On Monaco coaches they are well known for inacurate gauges brought on by faulty ground system and other. A lot of Monaco owners call the gauge pakage a entertainment center, what i did on our 99 Dip and the curent Dynasty is use the VMSpc program that i run on a laptop ( see thread on this site on it) with the gauges set up like i want and problem solved on all the inacurate gauges.

[mlh2013]

I know it runs 1800 around 65 mph and on the last couple of trips it started acting up. It would start climbing without any acceleration and then come right back to it's norm. While idling it will start to climb without any acceleration from the engine, and then sometimes it works just fine for hundreds of miles. I have decided it has a mind of it's own.

[mahon1993]

Two things: The tach is driven off of the alternator. There is a separate terminal on it just for the tach. Check the terminal for corrosion. You could also have a failing alternator. Second, I had weird dash problems including "waving" speed-odometer. I solved the problem by running a separate 12ga ground wire from the dash cluster to the MH frame.

[mlh2013]

I had Cummins powersouth put a new alternator on last year. Maybe you are right I will check the wires on it.

[onechaddude]

Originally Posted by mahon1993

Two things: The tach is driven off of the alternator. There is a separate terminal on it just for the tach. Check the terminal for corrosion. You could also have a failing alternator. Second, I had weird dash problems including "waving" speed-odometer. I solved the problem by running a separate 12ga ground wire from the dash cluster to the MH frame.

I had exact same problem, speedo would fluctuate. Also, occasionally all gauges would drop to zero and then be back normal, kinda like they would reset. I added new growing and no more problems.
Chad

[walt2137]

The tac was driven off the altanator on the 8.3 but it is driven off of the engine computer on the newer engines.