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Old 09-01-2020, 18:16   #1
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Yanmar 3GM30F Tachometer Issue

I have a tachometer which seems to be reading several hundred rpm below the actual rpm. Iíve confirmed this with a portable laser tachometer on the main flywheel.
Does anyone have any fixes for this. Is the problem more likely in the sender or the gauge and is there a simple way of determining which is faulty.
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Old 09-01-2020, 18:24   #2
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Re: Yanmar 3GM30F Tachometer Issue

really this is very common, especially with alt driven tachs.

Most tachs have an adjustment pot inside. Some you have to open the crimped cover (use a small screwdiver) to get at it, then calibrate with your digital tacho... Not hard to do, but a bit fiddly to get the crimp back on nicely!
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Old 09-01-2020, 18:49   #3
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Re: Yanmar 3GM30F Tachometer Issue

the tach on my 3hm35 receives signals from a sensor on the flywheel by the starter. I often have to clean those two wire connections or the tach fluctuates and reads low.
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Old 09-01-2020, 18:55   #4
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Re: Yanmar 3GM30F Tachometer Issue

Is there an simple way to identify whether the tachometer sensor is flywheel driven or alternator driven.
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Old 09-01-2020, 19:14   #5
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Re: Yanmar 3GM30F Tachometer Issue

I'm away from the boat for two weeks or I would send you a photo. the flywheel sensor is located on top of the flywheel housing above the starter. this diagram isnt as helpful as I was hoping but it gives you an idea of location and appearance. the sensor (optional part) goes where bolt #4 is located. it has two wires connected to it. hope that helps.
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Old 09-01-2020, 20:36   #6
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Re: Yanmar 3GM30F Tachometer Issue

Thanks Iíll have a look when I get back to the boat
I appreciate your help
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Old 09-01-2020, 21:07   #7
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Re: Yanmar 3GM30F Tachometer Issue

If I'm not mistaken, the tach on a 3GM is driven by a sending unit on the top back end of the engine, not the alternator.
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Old 10-01-2020, 10:32   #8
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Re: Yanmar 3GM30F Tachometer Issue

That engine has a sensor on the flywheel.

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Old 10-01-2020, 10:38   #9
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Re: Yanmar 3GM30F Tachometer Issue

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Originally Posted by CAVOK View Post
Is there an simple way to identify whether the tachometer sensor is flywheel driven or alternator driven.



For Yanmar models 2GM, 3GM (with a 97 tooth flywheel) the range selector must be set to the 90-120T tooth setting and will be reasonably close to correct
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Old 10-01-2020, 10:44   #10
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Re: Yanmar 3GM30F Tachometer Issue

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Originally Posted by CAVOK View Post
I have a tachometer which seems to be reading several hundred rpm below the actual rpm. I’ve confirmed this with a portable laser tachometer on the main flywheel.
Does anyone have any fixes for this. Is the problem more likely in the sender or the gauge and is there a simple way of determining which is faulty.
Try removing and clean the leads of the tach sending/sensor unit...
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Old 10-01-2020, 17:02   #11
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Re: Yanmar 3GM30F Tachometer Issue

I had a similar problem when I upgraded my alternator. The tachometer is non-adjustable so I used a portable tachometer to get the actual RPM and made up a “deviation card”.
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Old 11-01-2020, 07:16   #12
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Re: Yanmar 3GM30F Tachometer Issue

Your taco should have p=97 just below the X100 if it is an original unit.

I would check the resistance of the pickup, if it is not between 1.5 and 1.7k, then check the wires and possibly replace.

If it has moved or you install a new one don't forget to set the clearance, from memory 1.3mm
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Old 11-01-2020, 20:14   #13
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Re: Yanmar 3GM30F Tachometer Issue

Iím not sure how I would check the sensor clearance. Is it a difficult process?
Iím not sure if the current tachometer is original or a replacement. Itís a VDO unit.
Thanks for all the replies. There are quite a few leads to try.
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Old 27-01-2020, 08:33   #14
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Re: Yanmar 3GM30F Tachometer Issue

Sorry for being late but this may help. There is a copy from the Yanmar service manual 3GM30F
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Old 27-01-2020, 17:33   #15
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Re: Yanmar 3GM30F Tachometer Issue

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Originally Posted by CAVOK View Post
I have a tachometer which seems to be reading several hundred rpm below the actual rpm. Iíve confirmed this with a portable laser tachometer on the main flywheel.
Does anyone have any fixes for this. Is the problem more likely in the sender or the gauge and is there a simple way of determining which is faulty.
When you install a new flywheel sensor itís important to get the sensor to flywheel distance correct

Normally the installation manual give you these clearance specs

Check that your sensor has not moved and that itís flywheel clearance is correct

On some installations the flywheel sensor sends its signal to a control board that allows fine tuning
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