Tach on 1985 36C

[TomT13]

I have a 1985 Hatteras 36C with 3208TA Cat´s. Hatteras installed AC gauges at the helm, I only have the flybridge helm. This boat is equipped with a Glendenning synchronizer so there is a mechanical cable coming out of the tach housing on the front of both engines that lead to the Glendenning. The CAT book shows a magnetic pickup at the flywheel that should run by electrical wire up to the tachometer at the helm but I haven´t verified that as of yet. My starboard tach all of a sudden stopped reading correctly. It reads approximately 1/2 of the true reading. I know the engines are running at the same speed because the boat is tracking correctly and you can hear the engines are in sync. It appears the Glendenning is working correctly but the gauge is not. I am about to diagnose if the sensor is bad or the gauge. If the gauge is bad I would like to replace with a digital type tach. Does anyone have any ideas as to how or if this can be done? Any help would be appreciated.

Thanks

Tom