How can I call if my tachometers are accurate in providing accuracy between two engines? My RPMs do not read the same when the RPM synchronizer dial is showing balanced revs. Also, the gauge seems to work intermittently. Is a hand-held optical digital tachometers of any use? If so what brands work on a 2000 Carver 374 with twin Cummins 6BT 220 HP?
Likely the tachometers are alternator-driven. A replacement alternator may not have the correct # of poles and/or pulley. The tachometers may have an adjustment or selector switch on the back, if not, you will have to replace the alternator(s). The VDO instrument most likely is installed with its own sensors and if so should be accurate when it works. Intermittent operation is likely a connections issue.
On Amazon.ca you can find an optical tachometer for $40, shipped. It’s all you need.