Step One: Obtain a factory wiring diagram specific to your model AND serial #
Now, your mechanic must be replacing the "diode trio", which normally supplies the alternator's internal power requirements, and (apparently) is used to sense an "engine running" condition to keep the fuel pump running ONCE THE ENGINE STARTS. If the engine will not fire at all, there is something else wrong, as another circuit energizes the fuel pump while cranking. If the engine starts and stops as soon as the key is released, you need to look for a short (grounded wire) in this circuit that comes from the alternator to the fuel pump, or more likely, a fuel pump relay.
Hope that's helpful
Sep 08, 2007
Boat Alternator Solutions by: Anonymous
Diodes don't go bad as such. They blow by going open or short circuit and this is usually caused by overheating or a change in the load which forces them to conduct too much current.
Its very difficult to fault find the system over Internet, but repeated replacement of diodes would suggest that your electrical circuits need to be checked through piece by piece to ensure there are no partial shorts to earth causing the alternator to be forced to deliver too much current.
Get the alternator checked first by removing it from the boat and taking it to an auto electrician. This removes this from the equation.
If there is nothing wrong with that, start looking at your boat circuits for a fault
