Yanmar with funny Tachometer

[45Sticks]

Hello:

I have a Yanmar 3JH3E circa 2000. My Tachometer seems to work (as far as I can tell) up to about 1800 RPM, then as the engine RPM increases beyond that the tach starts going down !? I have checked the alternator belt and it seems good (only 364 hrs on engine, charging works fine) and replaced the connectors on the sending unit. (I measure .75 Volts there). Have not scoped it yet. What could be wrong ?

Peter

[S/V Antares]

45Sticks, Yanmar has used a magnetic pickup on its small diesels for many years. This sensor is usually found on the top of the flywheel housing. It accually counts the teeth flywheel. about 127 teeth on most Yanmars. Some pickups are adjustable but I believe yours is fixed. They are magnetic and sometimes get bits of metal shaving and debris on them. You should get more voltage as the rpm rises. The signal is a modified square wave and you might try setting your meter to AC or frequency. This should help determine sensor or tach issue.