Filtering External Noise

[TomGeorge]

Hi,
Try using a higher value potential divider.
Try 2k2 and 100R as well as a 0.1uF cap across the 100R resistor.
Try 22k and 1k as well as a 0.1uF cap across the 1k resisitor.

You do not know the current rating of the ignition controller tacho output?

Thanks.. Tom...

[Paul__B]

Try using a higher value potential divider.
Try 2k2 and 100R as well as a 0.1uF cap across the 100R resistor.
Try 22k and 1k as well as a 0.1uF cap across the 1k resistor.

Entirely appropriate, but only the capacitor is likely to improve the main problem.  Increasing the input impedance may well make it more sensitive to interference.