Erratic rev counter

Hi everyone,

I’m having trouble with the tachometer setup in my WT9, equipped with a Rotax 912ULS engine.

Previous setup:
Aviasport IM 105 tachometer – worked perfectly.

Current setup:
Upgraded to LX Navigation CS57 for the constant speed function.
I connected the lower connector of the IM 105 (pickup) to the CS57.

Initial results:
Smooth RPM indication up to about 4000 RPM, but above that the reading starts jumping erratically up to 8000–9000 RPM, even though the engine speed remains constant.

What I’ve tried so far:
Based on various forum recommendations, I tested resistors of 100 Ω, 200 Ω, and 300 Ω in series. Someone then suggested that a 60 kΩ resistor might be required.
So, I installed a variable resistor (0–100 kΩ) and set it to about 60 kΩ, all in series.

In that post there is a video, what it looks like at stable RPM below the critical "threshold"..

https://www.facebook.com/share/v/1AZFCvpweb/

Result:
The tachometer works fine at first, but once I reach around 5400 RPM, the signal starts jumping again, which means the constant speed controller cannot function properly.
When I adjust the potentiometer, the reading becomes inaccurate, so I have to fly in manual mode, where the indication is relatively stable.

While searching for more solutions, I came across a photo of the old IM 105 RPM indicator showing something like a capacitor installed between the + and – terminals.

Has anyone seen this before or can explain whether adding such a capacitor could help stabilize the signal?

Thanks in advance for any advice!




EDIT:
Sorry, wrong section — could an admin please move this thread?