J/30 Class Association Forums J/30 Forum The Boat, Sails and Equipment Tachometer Problem?

I have a Yanmar 2GM 13hp. The tach shows 200 rpm when at idle and 900 rpm at a boat speed of 5.8 mph. Is this a wiring or switch problem? I looked the back of the panel and the tach looked rusty, especially the bracket holding it on the panel. The wires seemed OK. Max rpm is about 1500. The engine sounds good and runs like it did before the tach showed low rpm. Anyone know what is wrong?

Three things to check that might be the source:

1. Bad sending unit that screws in to port side of bell housing.
2. Bad connections / wiring at sending unit or at tach.
3. Bad meter on instrument panel.

Disassemble the panel and clean up the connections there.

corrosion is a nasty beast ... on 505 it won't register at all until the motors been running about an hour ... then is fine.