Have you gone to the RMI website
and downloaded the manual
I built and used an RMI microMonitor over 20-years ago and used it 10 to 15-years before upgrading to the Dynon EMS-D10. It was the lowest priced option to fit in the vertical space that the micorMonitor required.
IIRC, there is a voltage output from the microMonitor that is required by some transducers. It is possible that there is a broken wire or open connection that may be creating the issue you are having. It has been years since I worked on one and they say that the mind is the 2nd thing to go. I cannot remember the first.
Hope the link to manuals helps you find the problem.