What brand are the tachs? I replaced the VDO tachs with LCD hour meters in our boat last winter. Ones as old as yours probably have DIP switches to set them for the RPM part. The new ones were programmable and the directions were very difficult to understand. I just sent them to Lauderdale Speedo and the set the engine hours and programmed them for the pulse count of our engines. It cost about $65.
If you are one there is no explanation necessary.
If you arenít one, there is no explanation possible.