zsmith3rd(at)earthlink.ne
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 Posted: Fri Aug 10, 2007 3:44 am Post subject: Spastic Tach |
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do not archive
If your tach is "electronic" and is jumping all over you may, probably will, need to install a damping resistor across the tach leads. Reason for this is the Rotax tach output is an AC impluse....one large pulse which, unfortunately, is followed by several smaller pulses. The electronic tach sees all these as "valid" indications of engine revs and therefore attempts to display a count of the total. Since some of the trailing small pulses are "smaller" than some others, the tach just goes into Stupid Mode.....at one instant you may have a nearly-correct reading, then at the next instant it will attempt to display a reading several times actual. The input threshold sensitivity of the tach "sees" some of the small pulses, does not see others, so therefore wanders all over.
The resistor damps the smaller "ringing" pulses and leaves only the one-pulse-per-rev as intended.
I'm at the office so don't have my notes.....start with a 10K resistor, work down from there.
A 'scope helps to troubleshoot this problem. The WesTach site has some tech notes which may address this problem.
Hope this is helpful.
Regards,
Zed/701/R912/etc/100 degrees F today in North Texas!
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