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Dale I

I am contemplating the installation of a tach on my 2QM20H and have a couple of questions. (I have a higher output alternator than was stock.) 1) If the tach indicates RPM's based on a pulse from the alternator, wouldn't a high-output, after-market alternator give an erroneous reading causing it to read higher/lower than the true engine rpm's? 2) Wouldn't any change in sheave diameter due to a change in mfgr. or output cause higher/lower variation in readings. 3) One responder suggested that an 'instrument' was not required to check the accuracy of your tach....only to put a piece of white tape on the flywheel and shine a florescent light on it to reveal a 90-degree pattern at 1800 rpm's and another 180-degree pattern at 3600 rpm's..... and to make adjustments for any error from that. Any one tried this? Does it work accurately?
 
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Gord May

Alternator Tach'

Diesel Alternator Type Tachometers read the Alternator Signal, which is available from the Alternator’s “AC Tap” (“AC”, “R”, or “W”). You will need to program the Tachometer according to the number of Poles in the Alternator, and the Pulley Ratio (RATIO = Crank Pulley Dia ÷ Alternator Pulley Dia.). The Tachometer manufacturer will provide a chart describing the switch settings.  You may also be able tro program the Tachometer according to the Full Scale Frequency (FSF in Hz = No. Alt’ Poles x Pulley Ratio x 33.33) Alternatively, you could use a portable Tachometer to determine the Tach’ switch setting, and Calibration Adjustment. There are also Magnetic Pickup which counts gear teath (which Yanmar uses), and Mechanical Cable Tachometers. It’s clear as mud - right? To your original query: 1& 2: If the NEW Alternator has the same number of Poles and Pulley Ratio as the old - there will be no difference in their respective readings. A change in either, will change the readings, requyiring recalibration. 3: Beats me - never tried it (tape on flywheel @ 90 & 180 degrees). Anyone else? Regards, Gord
 
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