J/30 Class Association Forums J/30 Forum The Boat, Sails and Equipment Yanmar 2QM15 1970 - Gages

If everything stops working I suspect it is the return line connection common to all the gauges. Take a meter/continuity tester and ring out all the lines from the panel back to the engine. I suspect you'll find a broken wire or bad connection.

For the tachometer, there is a sensor mounted on the bell housing by the flywheel that generates pulses for the tachometer. Most likely there is a loose connection there. Pull the spade type contact connector off, clean the terminals and if needed gently squeeze the female spade terminal to close the contacts so they seat better.