Why didn't I blow my alternator??

[smckean (Tosca)]

I've been analyzing the AC and DC configurations on my boat because I'm pretty sure I have some wiring errors (mostly surrounding how the batteries are configured and charged). Yesterday, I did perhaps a foolish thing. Can anyone explain it?

While underway with both the starter and house battery switches on, and my eye on voltage read-outs for both battery banks, I got the bright idea (not!) to try an experiment. I wanted to test whether the installed ACR/VSR unit combined the batteries regardless of the state of charge of either battery. Somehow I convinced myself that I would learn something if I switched off the start battery. Surprise!! As soon as I set the starter switch to "off", the engine instantly quit. It dawned on me that I may have just killed my alternator in the OB since I've heard that, on inboard engines, if you disconnect the starter battery, even for a fraction of a second, you permanently destroy your alternator. Thankfully, I turned the starter battery switch to "on", re-started the engine, and voila the alternator was fine and supplied normal charging voltage to all the appropriate places.

So my question to this august group is: was I just lucky; or do OBs have some sort of protection system that detects this situation and cuts the engine before the alternator blows; or is it false that one can blow an alternator so easily? Also, unless the 2nd option is true (which would explain it), why did my engine quit when I switched off the start battery?

P.S. Replacing an alternator on a standard engine isn't normally very hard or expensive, but I assume it could be very expensive on an OB. Anyone know?

[T.R. Bauer]

I do - yeah they are expensive! I'm glad nothing bad happened, but I think all you probably did is just cut the power off to everything - hence the engine quitting. And since there was no feedback loop from the alternator to power itself because they probably intentionally wired things in the outboard that way, it didn't have power and quit running. No idea if any of that makes any sense.....probably have to look at your outboard's wiring schematic to figure it out for sure.

[thataway]

As TR Bauer said. By cutting the engine at the same time you disconnected the alternator, you avoided damage.

Your engine stopped, because the computer on modern engines need 12 volts. The only way they get it is thru the battery. The modern 4 strokes will not run without a battery.

You proved that when you disconnect the engine from a 12 volt power source, it stops the engine. There is no "internal" feedback, as you might find with a 2 stroke, which will run without a battery.

[smckean (Tosca)]

OK, I get it now. When I turned off the start battery via the switch, I basically did the same thing as tuning off the ignition key.....so wonder the engine quit .