Tach RPM unstable

[DocRock27]

This evening, I was going to test fly my plane after installing ADSB, uavionix Echo. I have a EMS D10 monitor, 0360 Lyc with P mags. after start up, i noticed the RPM was unstable, jumping around 0, 1200, 2600 with low RPMs; when I would run up the RPM over 1000-1300, the RPMs would settle down and work fine... however, when I would reduce power, less than 900 rpm. The tach would drop to 0 and start jumping around.
I mention the install of the ADSB, because the only thing I did different to the EMS was use the same ground bolt for the Echo/GPS and the master ground to the EMS ; all other indicators are fine.
Any suggestions?

 

[jakej]

I’d power down the ADSB unit & see if the problem persists, if it does then it should be moved further away from the other devices like the D10.

 

[Dynon]

It's possible the change to the ground is introducing some noise from the other device. Definitely try Jake's suggestion first.