This past summer in late August I noticed the charging light had become
lit. I tightened the alternator belt and saw the charging light went off -- for two more weeks. A test with a multimeter on the battery indicated that
when the Yanmar was not running the voltage was approximately at
12volts, and when engine was operating.The alternator needed to be replaced. I checked the cabling and confirmed them to be sound. The original Hitachi alternator had to be replaced. I purchased a new alternator from www.spidermarine.com. The replacement alternator was rated at 60amps -- the label on the box said it was an Hitachi 12v 60amp #A12107 -- which is 5 more amps than the previous Hitachi 55amp model. I needed to replace the alternator belt with a longer belt because the form factor of the new alternator was slightly larger than the old model.
After the alternator was installed I tested the charging: 12 volts on the battery with the engine not running; 13+ volts with the engine running. The new alternator was working and the absence of a lit charging light also indicated same.
Considering the original alternator was part of the build in 1988 of Great Ketch, I think the alternator served her well.
Note: The picture above displays the old 55amp Hitachi alternator next to the new 60amp Hitachi alternator.
