Waverunnering

So I am trying to determine if my alternator is on it's way out. Now every other boat I have had it needs a little throttle to excite the alternator and it will charge at an idle. My current boat is my first with EFI. I am getting almost no charge at an idle whether you try to excite the alternator or not. At about 3000 RPM I am getting a full 13.2V to 13.9V as soon as the RPM's come down my voltage comes down to just above the battery voltage. What is everyone's thoughts on this? Should this motor be able to run just on the alternator if I turn the battery switch off? Engine is a Merc 502 Mag EFI

stevesxm

your numbers make me wonder if your voltmeter is wrong. full charge should be 14.2 or there abouts... if your instrument or wiring are bad or the ground is bad you might see the low voltage offset you are reading which would make your at idle voltage correct. and running the motor w/out a bat in the system is about the worst thing you can do to an alternator... and on an efi motor where it is looking for a stable 5v ref signal for the sensors, i'm not sure what result you would get.

the bottom line is that it takes about 12 nano seconds to get the alt off and napa or about a million other people will test it for free... why guess ?

Ted G

Don't try that! If you run the alternator with no battery attached it will fry. It sounds like you may have lost a field already anyway. There are 3 fields in an alternator, each one produces 1/3 the current rating. In your case that is about 33 amps per field. But more important, each one also provides a smaller amount at idle speed, if you lose a 1/3 or 2/3 at idle you will get no charging. My 502's usually kept the batteries at 13.5 or so all the time, even after several hours of idling. You should load test the alternator, either in the boat or at an auto parts place, most likely it is down at least 1 field.

wjb21ndtown

I've never had good luck with having an alternator tested. IMO the testing mechanisms suck. They won't tell you when an alternator is about to go bad. They only tell you that it is currently operational. I've had numerous junk alternators test as operational, and it drives me nuts.

That said, if there aren't any special issues with the efi setups I would just go ahead and swap it out. A $169.00 alternator is a lot cheaper than a $500 tow... and you still have the repair to do after the tow...

I'm not sure, but I think killing the Batt. switches will shut off the motors. I've never tried it, but it seems that it is wired that way.

FIXX

check for a loose belt..

Waverunnering

This is being tested with a load tester hooked to the battery that also checks the charging system. everything is tight so I am most likely going to just buy an alternator and if it doesn't change anything my parts guys is great and will take it right back.

speedreeder

If it is a Mando alt, It will have two pointed looking lugs on the back side. These are plastic covers over the studs. take a 12 volt source wire and hold it on the post farthest from the battery terminal on your atl.(after removing the plastic cover), This will excite the field and sometimes that is all it takes to get it to charge again. Hold that wire on for about 2-3 seconds. If you can watch your volt gauge that would be good. Good Luck

kdratch

I'm with Ted. Don't EVER run and engine with no battery in the system!!!! You could easily see 30-40 volts in the system with no battery! Not good for electronics.