I have a Hanger 9 tach. On overcast days it doesn't work well.
The measurement jumps all over the place. With one small prop (a 9x6 on an OS46FX) it refused to provide any reading. I have also had problems in the past when using wooden props (versus APC nylon props).

Today I illuminated the prop with a flash light from behind, then
measured the RPM also from behind. All readings were very steady, even the small 9x6 prop. Actually, the tach has never worked better that it did today with the illuminated prop.

I used a rather large flash light, but I suspect that even a tiny flash light (e.g. the single AA battery type) would be sufficient.

I have read at least one similar account of using a flash light with a tachometer. Search the lists you may find it.


It makes sense... you won't get a strong signal without some reflected light.