Did I damage my alternator

[stinger222]

1986 Merc V8, 2 batteries with a Perko switch, I accidentially switched the Perko to "off" while engine running. Fired the boat back up and the alt is charging battery one fine, 14 volts. Did I for sure damage the diodes or regulator ? can these things survive?

 

[azlakes]

Re: Did I damage my alternator

... i believe thats more the kiss of death to an O/B. experts will chime in to be certain. on an auto V8 of that year what you did just prooved the Alternator was ok when briefly pulled from positive batt terminal and the motor remained running.

 

[Boatist]

Re: Did I damage my alternator

You are lucky that you did not damage it. With most alternators if you switch thru off with the motor running you will blow the output diodes or the regulator. If your getting 14 volts then you are ok but I would not try it again.

Some battery switches have a alternator field disconnect curcuit which will prevent damage to the alternator even if switched thru off with the motor running. However they have to be wired up to work. On older alternator where they had an external regulator the field disconnect circuit will disconnect the field winding in the alternator before it breaks the battery connection and save the alternator diodes and regulator. It basicly turn off the charging circuit before the battey disconnects.

 

[stinger222]

Re: Did I damage my alternator

... i believe thats more the kiss of death to an O/B. experts will chime in to be certain. on an auto V8 of that year what you did just prooved the Alternator was ok when briefly pulled from positive batt terminal and the motor remained running.

the motor did not remain running, when you turn the BATT switch to off it kills the motor. Cranked it back up and the alt showed a charge. Hopefully I dodged a bullet.

 

[Boatist]

Re: Did I damage my alternator

Turning thru off will kill alternators on both outboards and I/O's.
Understand what happends is the motor is turning and the field winding has current making a magnetic field. When you remove the load of the battery the output voltage can shoot up to 1000 volts. The diodes in the alternator are rated for a reverse voltage of 50 to 100 volts so the high reverse voltage will cause it to fail.

 

[TerryMSU]

Re: Did I damage my alternator

Boatist is mostly correct. What he described is known as "load dump". When you remove the main load (usually the battery) the alternator will jump to about 70 to 80 volts for about 1/10 to 1/2 of a second. In an automotive environment, load dumps are clamped to about 30 to 40 volts. I don't know whether the newer boats have a clamp for load dump or not. If your diodes are rated for about 50 volts (standard rating), you might get away with it, but only because the diodes have some safety margin to cover manufacturing tolerances. You may have gotten away with it, but only becasuse you were lucky.

TerryMSU