Yanmar 3ym20 VDO Tachometer Settings

[jussip]

Hi,

I have a Yanmar 3ym20 motor in my Beneteau First 31.7. The tachometer went crazy, showing first full RPM and after the next start zero RPMs. Also digital hour meter started blinking.

The tachometer is a VDO made unit, in a Yanmar B Panel.

I first thought it must be a wiring issue, or alternator issue but then proved both assumptions were wrong. Finally I tried to change tachometer settings and that solved the problem: now I get some near correct readings out of the tachometer and hour meter also works.

Now the problem is how to figure out correct pulses/round settings without any meter to show the RPM?
Does anyone know how many pulses per revolution the standard Yanmar alternator produces?
Or even better, what is the correct setting for this yanmar tachometer?

I don't have a clue how the tachometer lost its settings. Does anyone have similar problems?

BR,

--Jussi

[Sunspot Baby]

You can use a strobe to determine true RPM and then adjust the tach to match.

George

[GordMay]

See “Calibrating the Tachometer”

➥ http://usa.vdo.com/NR/rdonlyres/09F0...eHourMeter.pdf
or
➥ http://www.egauges.com/pdf/vdo/0-515-012-037.pdf

If you have additional questions contact VDO:
Repair & Service for OEM Instrument Clusters and Systems
Merri MCIntyre
Phone: 540-665-2446
Fax: 540-662-2515
Email: mmcintyre@vdo.com

[jussip]

Thanks for the documents, however I already found them earlier.

I don't have a strobe or anything else to measure the real RPM.

I'd just neet to know how many pulses per revolution the alternator produces, I'd then be able to calculate correct pulses/revolution factor for the tachometer using following formula:

pulses/motor rev = alternator pulses/rev * alternator gearing

This is not a real big issue, I already have it close because I know the idle speed was somewhere near 800 and I have set the tacho for that...

BR,

--Jussi

[GordMay]

Hitachi alternators typically have 8 or 12 poles.
You can count them by removing the pulley & fan. You could ask a Yanmar dealer.

Pulses per Revolution = (NUMBER OF POLES ÷ 2) x PULLEY RATIO

The standard alternator for a 3YM20 is a 60A or optional 80A Hitachi.