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Engine RPM Gauge

Carefree Sailor

Member II
Does anyone have a suggestion as to why my engine's rpm gauge suddenly has started bouncing from zero to five or ten or so when the engine is idling? I noticed this problem yesterday while the boat was in the slip. I haven't had an opportunity to take the boat out under power yet to see if the problem still exists. I assume it does.
 

Baslin

Member III
The tachometer gets its signal from the alternator. The signal from the alternator to the tach should be 7-9 Volts AC....Sometimes depending on your battery state, the tachometer will cut in and out. You could have a loose wire either on the alternator output terminal or the terminal on your tachometer. It could be a faulty guage or possibly the alternator signal failing........ Here is a good article to help explain

http://www.amplepower.com/primer/tach/
 

redbeard1

Member II
Hello,

Just to clarify, when you write "from zero to five or ten" do you mean it bounces from zero to your normal idle speed (for example 800 rpm) and back to zero?

If so, it could be a loose wire between your tach and the alternator.

If the alternator warning light on your console is also blinking on/off when your revs are bouncing, it may mean there is a problem inside the alternator.
 

Ryan L

s/v Naoma
Does anyone have a suggestion as to why my engine's rpm gauge suddenly has started bouncing from zero to five or ten or so when the engine is idling? I noticed this problem yesterday while the boat was in the slip. I haven't had an opportunity to take the boat out under power yet to see if the problem still exists. I assume it does.

My understanding is that sometimes that happens when the batteries are fully charged so your alternator is getting regulated in a way that your tach doesn't understand (for lack of a more simple way to explain it). In other words, it may not necessarily be a "problem" (something that can be fixed) but rather just a quirk in the way your system is set up. Maybe someone with a stronger engineering background can explain better? Our tach started doing the same thing (when the batts are full) when we switched to a new regulator.
 

Carefree Sailor

Member II
Thanks for the responses. Today when I ran the engine, all was well. Yesterday I checked all the connections and they looked okay. Connections were tight and no visible corrosion. Ryan's suggestion seems the most appropriate, given the tracing of the problem that I've done. Again, thanks to all for the suggestions. I will continue to monitor the issue but hopefully it is resolved.
 

Tom Metzger

Sustaining Partner
If Ryan's suggestion is correct, and I expect it is, turning on a 12 Volt load will return the tach to normal. Bilge pump, several lights, whatever.
 
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