Not sure if there is a problem with the alternator. Have turned the key on and off and hand cranked the diesel
on our last boat many times with the key off without any problems.
There is no ignition switch on a diesel
. The 'switch' activates the electrical system
, flashes the field on the alternator so it will can produce electricity with enough rpm's and, if wired and has a position for it, engages the starter. Without the switch on and no electricity to the alternator when the engine
is started, it will just sit there and spin without producing current
. Once the field is flashed, the alternator is self energized and will continue to produce electricity whether the key is on or off. The question is whether the engine
switch could disconnect the alternator from any electrical
load which could blow the diodes the same way as turning the battery
switch to OFF with the engine running above idle. I almost always turn the key off before pulling the engine shut off cable because I don't like the sound of the oil
pressure and temp warning horns with no problems on our Yanmar 3GM30f. Crank started our old Volvo
MD2 for over ten years, most of the time with the key off, without a problem.