Fluctuating Tacho Reading

[bugsy63]

I've just completed an annual inspection on my RV9a. I powered the avionics up today and without the engine running i witnessed large fluctuations from the RPM indicator. It sat at around 370 RPM but jumped to approximately 3000 on occasions.

My thoughts were perhaps a poor ground to the P MAG which I changes during the annual, but got no unusual RPM reading during the previous ground runs and got normal drops on both mags.

Thanks in anticipation of your opinions

Regards

Michael Brooks

 

[kurtfly]

If your prop was setting at the zero degree point it will sometimes jump between 0 and 359 degrees. when it does this it sends out a tach pulse. it is just an artifact of the position sensor resolution/ accuracy. You can replicate this when setting your timing and the LED is Yellow. Not green or red, but in between. Nothing to worry about. if you just nudge the prop a bit it would have stopped. not a condition you can get with the engine running.

 

[bugsy63]

That is an absolute possibility. I was able to get a zero reading temporarily by isolating mag power through the PMag internal alternator test function, but experienced the same spurious rpm reading when I recycled power. I confirmed the integrity of the PMag grounds

I took the aircraft flying today and had no issues at all and the tacho is reading zero after engine shutdown

Thank you for your suggestions

Regards

Michael Brooks