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Tachometer Fluctuation

PWL

By PWL
in Modern Mooney Discussion

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PWL

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We have an '84 M20-J with the IO360A3B6 dual mag drive STC.  The previous owner said that the tachometer began fluctuating by about 100 RPM on BOTH after that installation.  On either mag alone, the tach is steady.  Our A&P claims that the two mags are well timed, and after several annuals with different A&Ps asking the same question, we believe him.  Has anyone else had this problem and found a fix? 


I suspect the tach is responding to slightly differently shaped signals that are combined through 15K ohm resistors at the ignition switch.  Does anyone know the pulse shape and amplitude of the signals going from the ignition switch to the tachometer?


 


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