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post #1 of 12 Old 05-24-2017 Thread Starter
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Tachometer fault?

The tachometer for my 3YM20 rests at about 200 - 300 rpm with everything turned off. As soon as I turn the ignition on the needle deflects below zero and then when the engine starts it appears to give a realistic - if questionably accurate - reading. Is this an adjustment issue? Or would I be best to source a new tach?

The control panel is an older Yanmar - with a key start. What other questions - if any - should I be asking?

Any insights would be appreciated.
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post #2 of 12 Old 05-27-2017
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Re: Tachometer fault?

As long as it indexes on zero when the power is applied I think it should be reasonably accurate as it is not capable of measuring anything with the power off. You would assume that the zero mark would be the normal at rest position but apparently not with your tach. If you know anyone else with a Yanmar tach you could look at?
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post #3 of 12 Old 05-27-2017 Thread Starter
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Re: Tachometer fault?

Thanks for the feedback RMB. I think I'll live with it for a bit and see what happens. If I don't over rev and flog the engine I think I'll be ok
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post #4 of 12 Old 06-02-2017
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Re: Tachometer fault?

If you can get ahold of a hand held tach you should check the accuracy of your tachometer. I always wondered why I could never get to up max rpm ( not that I would run there but I was not even close) When my tachometer was reading 2000 rpm it was actually 3000 rpm.
A quick adjustment with the adjusting screw at the back of the tachometer and all is good now. ( I did check it with my handheld and one that belonged to a friend just to confirm the handhelds were right.
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post #5 of 12 Old 06-02-2017
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Re: Tachometer fault?

Electric tachs do [B]not[B] drop to zero when the power is turned off, they sit approximately at whatever RPM's they were registering when the power (key) went off. This is normal, relax! As said above, with the power applied (key on) it should zero out until the alternator is putting out, then read something near what you expect in RPM's.
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Re: Tachometer fault?

Thanks dbruce85. I'll have to see if I can round up a handheld and check it out.
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Re: Tachometer fault?

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Originally Posted by capta View Post
Electric tachs do [B]not[B] drop to zero when the power is turned off, they sit approximately at whatever RPM's they were registering when the power (key) went off. This is normal, relax! As said above, with the power applied (key on) it should zero out until the alternator is putting out, then read something near what you expect in RPM's.
Ok thanks capta. I'll have to watch more closely and make sure I understand what the tach is showing just as I shutdown.
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post #8 of 12 Old 06-04-2017
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Re: Tachometer fault?

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Originally Posted by capta View Post
Electric tachs do [B]not[B] drop to zero when the power is turned off, they sit approximately at whatever RPM's they were registering when the power (key) went off. This is normal, relax! As said above, with the power applied (key on) it should zero out until the alternator is putting out, then read something near what you expect in RPM's.
Kiling the engine and turning the ignition (key) off are two different operations right? I have a manual pull kill and even if its electric its a separate button. Im trying to see a scenario where tach power is killed with the engine shut off, Gas engine maybe.
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Re: Tachometer fault?

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Kiling the engine and turning the ignition (key) off are two different operations right? I have a manual pull kill and even if its electric its a separate button.
Yes, that is how it is on most small craft. We push a button to shut down the engine (releases fuel solenoid) then turn the key off. If the key is turned off first, the stop button will not work.

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post #10 of 12 Old 06-04-2017
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Re: Tachometer fault?

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Originally Posted by albrazzi View Post
Kiling the engine and turning the ignition (key) off are two different operations right? I have a manual pull kill and even if its electric its a separate button.
Yes, that is how it is on most small craft I've operated. We push a button to shut down the engine (releases fuel solenoid) or pull a cable (shuts down fuel flow at injection pump), then turn the key off. If the key is turned off first, the electric stop button will not work.

"Any idiot can make a boat go; it takes a sailor to stop one." Spike Africa aboard the schooner Wanderer in Sausalito, Ca. 1964.
“Believe me, my young friend, there is nothing - absolutely nothing - half so much worth doing as simply messing about in boats.” ― Kenneth Grahame, The Wind in the Willows

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Last edited by capta; 06-04-2017 at 11:29 AM.
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