Digital RPM in 90s/2000s saratoga, how does it work?

[easyski]

Does anyone know how the digital RPM gauge seen in the picture gets it's signal? I would assume some sort of sensor on the magneto but am trying to trouble shoot mine. It will work occasionally. Typically works on start up (whether warm or cold outside) but quickly fails (just shows "- - -") as if it's not getting a signal vs reading the wrong RPM and then will occasionally and intermittently show up again in flight to then again fail.

I have an analog RPM gauge that works fine but would like to get this as it is much easier to "fine tune"

 

[Unsafervguy]

 

[easyski]

 

[sierrakilo99]