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Tachometer at rest??

Zippy

New Member
For the last several years, my tachometer has been doing some odd stuff..... When all electrical systems are off (battery switch, key etc) I notice that my tachometer reads at roughly 700 - 800 RPM's. When I turn the key to start up the engine the tachometer goes to "0" RPM's and when underway it might go back to "normal" 1200 RPM's and bounce around a bit. I figured I might have a loose wire but what perplexes me is the "at rest" it showing 700 RPM's..... anyone have ideas about this and how to fix it?

Cheers,
Peter
 

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Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
Strictly as a preventative maintenance measure I used to carefully remove/clean/reattach each little wire connection on the back of our former Universal engine panel every few years. The panel looked ugly, but all the gauges did work.
Did it one wire at a time to try to avoid confusion as to where to put 'em back. Sometimes, in all of these circuits, a bit of corrosion will build up in the connections and the voltages are mostly low and resistance forms.

Check the connections on the back of the alternator, too. I put that chore off too long at one point and had one wire almost fall off while engine was running -- this led to a rebuild of the alternator. This was found by a routine check of the running engine one day and noticing some disconcerting arcing on the back of the alternator where one wire was hanging by a strand.
 

Zippy

New Member
arcing wires doesn't sound like fun.... Thanks for the suggestion about cleaning all wire connections one by one. What baffled me was without any current flowing through the tachometer the needle read 700 RPM. I wondered if there was an adjustment screw to diel it down to "0"?
 

Jerry VB

E32-3 / M-25XP
I think it is typical of boat tachometers to not have a spring return to zero so the tachometer shows a non-zero RPM when the engine is shut down. The needle remains wherever the tach happens to be when the motor is shut down. The alternator will stop generating a tach signal as it slows to a halt.

FWIW, I have a picture of my instrument panel and it is showing 300RPM. I've also noticed non-zero readings on pictures of other tachs, e.g. in "for sale" ads.
 

Christian Williams

E381 - Los Angeles
Senior Moderator
Blogs Author
Also, some tachs have a selector dial on the back for different pole settings. They're finicky, and sometimes moving from one setting and back fixes things. I just use the pole setting that gives me 800 rpms at idle, and the other rpms are then also correct.

But as Loren mentioned, a loose wire is often cause of a jumpy tach. Or at least that was my issue last year, solved with a lock washer on the alternator connection.

My old tach does zero out at rest. But some are designed to read where they are when the ignition goes off, and then become correct again when the engine is restarted. Faria-brand, specifically, I just read on another forum.
 
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nquigley

Sustaining Member
For the last several years, my tachometer has been doing some odd stuff..... When all electrical systems are off (battery switch, key etc) I notice that my tachometer reads at roughly 700 - 800 RPM's. When I turn the key to start up the engine the tachometer goes to "0" RPM's and when underway it might go back to "normal" 1200 RPM's and bounce around a bit. I figured I might have a loose wire but what perplexes me is the "at rest" it showing 700 RPM's..... anyone have ideas about this and how to fix it?

Cheers,
Peter
So glad you asked - I took the same pic this last weekend. In mine, in addition to the tach reading ~700 rpm, the temp gauge is reporting 170 degrees .. engine was cold (and ambient temp was a lot less ;-) )
 

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