Tach reads 500 RPM when not running - SailNet Community

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  #1  
Old 08-29-2007
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Tach reads 500 RPM when not running

Anybody have any thoughts on why the Tach on my Universal M4-30 might exhibit this condition. The boat was repowered a few years ago from an Atomic 4, and the tach works, except for the incorrect zero. I'm thinking the gauge might be stuck, rubbing, etc. From what I read, the tach is probably electronic, not manual, but the gauge is a D'Arsonval-type gauge (I would guess) run off the alternator. Thanks in advance.
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Old 08-30-2007
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That's normal when power is not applied to that brand of Tach.
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Old 08-30-2007
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The same thing happens in my granddads freightliner truck. Its normal
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Old 08-31-2007
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That's interesting!

I would have imagined there would have to be some voltage on the meter to make it deflect when the motor is off. Anybody have any idea why this phenomenon would occur?
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Old 08-31-2007
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Our Perkins/Irwin runs continously at 600 rpm. Until we fire her up.

The Wester generator runs at 900. When not running as well.


Actually, my '97 F350 gauges do the same. Go figure..........
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Old 08-31-2007
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Definitely NOT normal for an analog gauge in general, but there are some types (like gas gauges) that simply "lock" at the point they were last reading when the power to them was shut. It sounds like that is how your tach is built. So it is "locked" at the tach reading what it was when the power was cut. (Somewhere between idle and stall.)
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But to be fair, on all three of my goodies, once the key moves to the first "click", (energized) they all drop down to zero.

BTW, the two gauges in the boat are 23 y/o Stewart Warners, which I've had on many different machines. I like them, especially the oil gauges that use the plastic tubing.

The Ford truck gauges are ?..........
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Old 09-03-2007
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Some needles use counterweights that are not balanced properly, others use a return spring that is so light to read low rpm, that it doesn't get the needle back down to 0 unless you have the volts applied to pull it back with the magnetic force.

All in all, it is purely harmless, and quite common.
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Old 09-03-2007
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Quote:
Anybody have any thoughts on why the Tach on my Universal M4-30 might exhibit this condition.
Your boat is getting ready to sink - sell it and buy a new one immediately
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