Tach reading is pegged

[dspurgeon]

I have a 2009 Hunter 38 with a 40hp Yanmar engine. Today 1/2 hour after the start of a 5 hour motor trip the tach went from 2800 to pegged. Regardless of speed changes it would just quiver at the pegged position. At shut down it also stayed there. The charging system shows 14v during run times. What do I look for and/how to troubleshoot? All the other instruments are fine. Thanks!

 

[Don S/V ILLusion]

it's almost certainly a bad electrical connection on the panel or at the engine

 

[Bill Roosa]

The tack is just a frequency reading meter. It either gets its changing frequency from the alternator’s un-rectified stator output or a dedicated tack sensor. Either way the gage itself just measures frequency of an AC signal. So either the AC signal frequency is very high (not evidenced by it reading pegged upon engine stop) or there is something wrong with the gage itself (more likely).
I'd suggest you unplug the sensor wires and see what happens. If that causes the gage to read zero then there is something noisy causing the sensor wires to have an induced AC signal of high frequency. If it stays pegged then the only thing left to check is a good ground as there is clearly a good power or the gage would not even read.
With all that said this is most likely a broken tach and you will be needing a new one. Not much harm in opening it up as it is broken and looking for a disconnected wire/component. I'm not confident that you are going to find much in the way of user fixable though.

 

[Ed Schenck]

There are two inputs to the tach, the sensor wire from the alternator and 12 volts. I'd bet that you lost 12 volts while motoring and it stuck there.