My battery went dead. The local gas station tested my electrical system. They showed me some machine that said my alternator was putting out 11 or 12 volts. They said it should be 13, and that I need a new alternator, not a new battery. Does this sound right? How much does an alternator cost (1984 300 SD) and how many hours labor to install it?

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Jacob Wissler Jr., Houston, Texas, 1984 300 SD