Yanmar 3GM30F: Tachometer is accurate!

I’ve heard quite a few experts say that the Yanmar 3GM30F tachometer
reads high, by as much as 100 rpm.

Until now, I have accepted that as gospel, and even repeated it to
others. No more!

But today I used my handheld optical tachometer in a calibration
exercise. And found that the boat tach is accurate to within 3% (ie
the error never exceeds 70 rpm) and for engine speeds in the range of
usual operation (2500 - 2900 rpm), the accuracy is quite good (within
1%). At the very top end of the range (>3300 rpm), the tach does read
high, but only by 50 - 70 rpm (1.86% at 37 hundred rpm on the tach).
The low end has inaccuracies of around 3%, but the errors aren’t
consistent (sometimes high, sometimes low).

If you’re interested, I’ve uploaded my findings as
Yanmar3GM30Ftach.pdf (about 84 K) and placed it at
http://finance.groups.yahoo.com/group/bcc/files/BCC/Yanmar3GM30Ftach.pdf (you likely need a Yahoo identity to access the file).



The Yanmar tachometer pickup reads off the flywheel ring gear, therefore generally considered more accurate than types driven off the alternator; Alternator driven tachometers tend to be less accurate when proper belt tension is not maintained, pulley size or higher output alternators are installed. In either case, the we also need to take into consideration the percent error built into the instrument used to verify accuracy. Most digital optical tachometers, depending on the quality of the instrument, will have a built in margin of error, which can degrade the accuracy of the reading obtained.?Lazer infra-red thermometers are a prime example of potential false readings, as reflective surfaces can cause significant variation in readings.?

Marty Chin
BCC Shamrock
Bay Marine Diesel.