Jittery D180 Tach with video

[SammyQ2]

Continental O-200 with a Slick mag. I do have a 30K ohm resistor in series with the wire that goes from the magneto P-Lead to the EMS (pin 33?)

I thought a video would be the best explanation, so here is my D180 in flight. https://www.youtube.com/watch?v=ZhhfmKZx7Xo

It is worse around 1,600 RPM, but I am more concerned in the high RPM range.

Any suggestions on how to eliminate the jitter?

Thanks.

 

[Dynon]

It looks like it always over-reads - is that right? You may need to experiment with increasing the series resistance some to "squelch" those spikes. Too much resistance can cause the voltage pulses that you're trying to read to not be read because the voltage gets cut down too low. This would cause a low rpm reading. Too little resistance can cause phantom pulses (electrical noise) to be mistakenly be read as the pulses you're trying to read, giving you rpm indications that are too high like you're seeing. 30K is good for most people, and if anything, folks normally need to go up. People have gone up to 150k, but just how high or low you'll have to go is hard to say.

 

[SammyQ2]

Okay - that gives me something to go on.  Yes, I think it always over reads. I'll report back when I get it done.  Maybe start by doubling the resistance?

Thanks.

 

[Dynon]

Yep, that's reasonable.

 

[SammyQ2]

Problem solved!          

I got a variety pack of resistors from Radio Shack. They don't seem to be the greatest quality, but I guess an ohm is an ohm.

I boosted the resistance from 30K to 60K and it still jittered, with no discernible change. I then went whole hog and changed it to 156K (had to use two 330K in parallel) and no jitter.  The ground test seemed to be rock solid.  I didn't realize how much it had been jumping around until I got it to quit. Flight test tomorrow.

Thanks for the good advice.

Sam