New alternator or rebuild meeded

1985 Thompson 4.3. Installed the new GPS and Downrigger’s and took it out to test everything. Got the lines down and within 15 minutes the alt was under 12 volts. Had a voltmeter and checked the voltage at the battery and it was 12.37. Shut the motor off and it was 12.37 so I was only measuring stored battery voltage. Started it back up, pulled the pos terminal and she stalled. The local marina wants $300 for a new alternator. Am I to assume based on this information the alternator is bad? The Mercruiser part number is 98555 and the Vendor part number is AR150-AA. Does anyone know where to get one online or where to have it rebuilt in the Watertown NY area? I’m hoping Bondo might know someone local.

Start by checking the belt tension on the alternator.

You have a 55 amp Mando alternator. These are made in Korea. Really nice alternator but the output is not that great. You may want to upgrade to a higher output particularly if you are charging multiple battery banks. Any 2 inch foot alternator will fit (like a Delco 12si). You could just put the same thing back on tho. There are a few used ones on eBay for about $30, which will probably get you back on the water for the rest of the season. You might ask at your local marinas sometimes they have good used stuff laying around.

o2batsea said:

You have a 55 amp Mando alternator. These are made in Korea. Really nice alternator but the output is not that great. You may want to upgrade to a higher output particularly if you are charging multiple battery banks. Any 2 inch foot alternator will fit (like a Delco 12si). You could just put the same thing back on tho. There are a few used ones on eBay for about $30, which will probably get you back on the water for the rest of the season. You might ask at your local marinas sometimes they have good used stuff laying around.

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Would I get the Delco 12si at an auto parts store?

FWIW, don't ever pull a battery cable off with the engine running...NONE of the electrical items like the results.

As far as a rebuilt goes, any alternator shop should be able to do it 'same day' if they have access to the parts...and they are pretty common...

I got in touch with a local man who rebuilds alternators and starters. He will be 87 tomorrow. The voltage regulator was bad so he cleaned it up and it’s charging good. I do have another question. There’s a spacer that’s about a quarter in thick. I’m assuming that goes between the bracket and the threaded screw hole for the short screw? I didn’t notice anything when I removed it but then saw it laying there. Thank you.