Yanmar tacho test

This one is for the technos.... I have had the pleasure of getting to know the wiring loom on my Yanmar 3YM30 and one thing I can't seem to clear up is the Tacho on the panel is not working. Continuity proven between the 'P' terminal of Hitachi 80 amp alternator and the loom connection at rear of tacho itself. I understand it requires load to excite the alternator to produce power and have been through most tests I can think of and have had the multimeter confirm voltages with and with out load ( 14.61 VDC with no load and haven't got notes in front of me atmo but it was lower and def not by much ... maybe 14.4 VDC ? ) I've measured the voltage between the 'P' terminal and neg at idle and wide open throttle - and yes, on the AC range of the multimeter and came up with 4 - 6volts AC
The needle does 'move' off zero about 2mm when engine starts and moves up maybe 8 mm near wide open throttle.
I am hoping the brains trust can shed some light on what I have missed or not checked, and I thought it best to ask before I spend on just replacing it with an authentic Yanmar replacement $$$$ (or another 80mm generic Tacho at 1/3 the price) and cover all aspects
I'd like to keep the panel as original as I can, and haven't gone down the path of pulling the tacho itself apart - don't really want to do that in case I inadvertently destroy a perfectly good Tacho but am willing to do it for authenticity sake.

Most of the replacement 80mm generic tacho's Ive seen available are for 4,6 and 8 cylinder engines and nothing I have seen is specific for 3 Cylinder? Not sure if there is a specific reason ( calibration maybe?) ) or just the way they are marketed.
I could live with an 'adjusted' reading on the Tacho if necessary and that's my only option.

Any tips, solutions or similar issues are appreciated crew.

Seapig.

Have you looked at the fly wheel sensor adjustment ?

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Trek said..
Did it ever work? In other words has something failed but the installation is correct or could it be miswired?

Thats a question I asked myself as well Trek, although I have nothing at this stage to suggest that it did or did not. The wiring is continuity proven from back of Alternator through loom to Tacho so this could be a valid call as well.

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seapig said..

woko said..
Have you looked at the fly wheel sensor adjustment ?

Unofrtunatley my only option at this point is the feed from the Tacho sensor wire but if need I will look at installing a fly wheel sensor, it would appear easier and more cost effective to see check all obvious things first though. Like I mentioned before I don't mind spending the $$ to use a replacement /aftermarket job but I would like to keep the panel as original as I can

I am not an expert on any of this.

Am I correct in thinking you are seeking to drive the tacho on the original Yanmar engine panel using a feed from an alternator ("the 'P' terminal of Hitachi 80 amp alternator"), rather than from a flywheel sensor?

Looking at one of the manuals that cover the 3YM30, it looks like they came with both flywheel sensor driven tachos and alternator pulse driven tachos at different times.

"12.6 Instrument panel
The new type instrument panels are applied for 3YM30/3YM20/2YM15 series engines. The features are compactness, waterproof and independence from pulse by ring gear teeth number. The engine speed, indicated with the instrument panel is activated by the pulse from flywheel ring gear. The engine speed with new panel is activated by alternator B terminal pulse."

That's from a 2005 edition of the Yanmar Service Manual M9961-03E101. So if you have the "new panel", then it should expect/accept a alternator signal, in which case your problem may be that the number of stator pulses per revolution of the engine may be different for the Hitachi alternator than the original alternator. If that's the case, I have no idea if you can recalibrate the original tacho.

As I understand it, the P terminal is the stator pulse, which can drive a tachometer built for such a signal (variable frequency). I doubt a flywheel sensor would put out the same signal as a P terminal on the alternator. The diagram posted by Woko suggests the flywheel sensor puts out a higher voltage at higher RPM ("1.2vac at idle increases with RPM"), so the tachometer would be, in a sense, a voltmeter, albeit with a scale in RPM rather than volts.

The output from a P terminal is a variable frequency signal rather than a variable voltage signal. The calibration of an electronic tacho for such a signal is discussed at www.electricalrebuilders.org/eren/archive/ere_2008/ere_01_08_analyzing_the_alternator_tach_signal.pdf

If the tacho on the engine panel is expecting a variable voltage signal from a flywheel sensor, then you will need to either install a flywheel sensor and give it the signal it expects, or replace the tacho with one that expects a frequency signal from the alternator - preferably one that can be calibrated for the P signal (number of stator pulses per RPM or interval between pulses for a given RPM) from your Hitachi alternator.

Apologies if I am missing something or have misunderstood.

Thanks to all who have replied, I was called back to Patrol unexpectedly and have not been able to get back to the engine ( which is still strapped on the pallet ) I will keep all updated. As far as I could tell ( and remember ) there is no sensor on the fly wheel that came with the engine.
Stand by for more .......