the hobbs meter will not tell rpm but it will tell you the hours used as for the rpms you either rely on the flywheel sensor or your alt ...i personally don't like the alt method as the belt can get loose and lie to you about rpms the hobbs meter is just an electric clock that only runs when your ignition is on ...oh and the hour meter from yanmar runs off the power being on on your panel but if the lcd is bad then get a hobbs meter