Dynon Avionics EMS units have two inputs ("Left" and "Right") for RPM / Tachometer. The EMS unit only displays ONE RPM / Tachometer. The one that is displayed is "the first input that was receiving pulses". If, at some point, such as a Mag check", no pulses are received on that input, then the EMS unit switches inputs.
What you're describing - "... after doing the mag check it starts oscillating again", means that the EMS unit has switched to the other input.
Background:
The EMS-D120 and our other EMS units simply count pulses. With the Pulses per Revolution setting, the unit does a simple calculation: x number of pulses / pulses per revolution = displayed RPM.
While adding a resistor in series USUALLY "cleans up" the RPM signal so the EMS unit can reliably count pulses, some RPM signals are simply noisy, outputting extraneous pulses, and the EMS unit cannot distinguish between the noise pulses and the RPM signal pulses.
Potential solution 1:
Since, in your situation, you seem to have one "clean" RPM signal, and one "noisy" RPM signal, the "clean signal" is the first one seen, and the "dirty" signal remains during / after the mag check, perhaps if you swap the two RPM signals, then the signal that the EMS unit sees during and after the mag check will be the "clean" RPM signal.
Potential solution 2:
Some customers that aren't happy with the RPM display resulting from the normal RPM signals have resorted to installing a "tachometer generator" on the engine. This is a small unit that is mechanically coupled to the engine that outputs a "clean" RPM signal.
Ok...we have tried with 150k-ohm resistances, these improved the signal...however it was still a bit jumpy on the left magneto, ocasional blips up to 300rpm. We tried a 180k-ohm resistance on the left magneto...and it didn't improve, if anything it looked worse..So we went back to 150k-ohm....any other ideas? Thanks for your time.
