Join Date: Mar 2010
Location: SF - South Bay
Thanked 22 Times in 22 Posts
Rep Power: 7
As a start, I would begin to trace wires and develop a rough wiring chart for the boat. A camera, sketchbook and patience helps. I am doing the same thing myself, and in the process have found 3 basic wiring problems - 2 of which were unrelated to the problem I was diagnosing!
Also, if you don't have it already, Yanmar should have a wiring diagram available which will help.
Once you have a rough idea of where the wires go and what they are supposed to do, your ammeter/voltmeter becomes your best friend; just work methodically. But I am betting that during your charting phase you'll find a loose connection.
Not sure of your experience level, but Nigel Calder's book is a good place to start for advice.