Eburklo, Check all connections from the engine to the gauge and I suggest starting at the gauge and working backwards. If someone removed your bridge during transport then a connection could be bad. Look at your propulsion manual and determine where the signal is generated from (Computer, Crank sensor or what ever a Volvo D12 uses) and determine the wire color/number for tracing. The RPM gauge and Sync. gauge are tied hand to hand. I have a 2001 406 that has a Faria sync and sometimes it just does not work for some reason. I can tell you the sync gauge in my boat recieves the signal from the tachometer and both tachs show correctly but the sync gauge is wrong. Here is a link to an example:
http://faria-instruments.com/site_manuals/IS0079D.pdf Check with the gauge manufacture and determine what signal is needed for the digital tach to indicate speed. Is it a variable DC voltage or is it a change in frequency. If it is a variable DC voltage then connect a VOM and see if it changes value when the digital does not. The malfunctioning gauge should not affect your performance as it is only an inidicator unless the problem exist prior to the gauge in the electronic control. Good luck.