| I had a similar situation on race car tachos. For some tachos the output signal is too big, and it causes the tacho to pulse. The cure was to put a resistor in series with the tacho signal feed, and hey presto all was well. I seem to remember using a 1k resistor, but it can do no harm to experiment. if the tacho still pulses, try a higher resistance, if it stops reaading at all try a lower one!
Unfortunately for Desmobob I suspect there is no easy cure if the signal is too low, and the tacho is not reading at all - unless you are up for building a small signal amplifier! | 
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