So i recently put in a new to me alternator from the junk yard, in my 1990 240, it was visiably in good shape really clean etc, i checked out the brushes seemed to be in good shape, but heres my issue, my battery is dead and when i jump start it starts up just fine and when i disconnect the jump cables it dies immediatly unless i rev it up. The alternator is showing voltage output when i test the positive out put stud on teh alternator to ground i get about 14v. is it just because my battery is soo dead, it was sitting for a few months with out regular driving. Any suggestions!?