Bendix Rpm pick up

I have a UMA rpm pick up in a Bendix Mag wired to a Garmin engine module (24) . When I first wired the system 2 years ago it would not pick up rpm , after checking all connections and wiring it started working ( never found any thing wrong ) Today at almost 150 hrs of use , on start up rpm read zero . Checked both mags at idle and both were functioning . I decided to fly "as is " on the taxie out the tach came to life . On the return flight home from breakfast it did the same thing .
Going to start my Anual this week , this is on my squawk list . Any thoughts ??

Sounds like a broken wire or bad connection somewhere. These can be a BEAR to find. I will try to get out to FIN and give a hand but probably just slow you down.

RPM

Kinda odd , I hate intermittent problems , always seem to work when I decide to work on them !