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My alternator is undercharging(12-13 volts), what are the possible causes?

Tiny
RON DEKOK
  • MEMBER
  • 1995 FORD MUSTANG
  • 84,000 MILES
Tested alt @ regulator with voltmeter, output was 12.5 volts @ idle and 13.5 @ 2000rpm. Bought new alternator, installed, initial test 14.3 volts @ idle. Drove home and voltage dropped, and will not go above 13.5 while driving. Turned the a/c on and voltage dropped, alternator output did not increase as it should. Where should we go next?
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Thursday, September 1st, 2011 AT 6:14 PM

1 Reply

Tiny
CARADIODOC
  • MECHANIC
  • 33,873 POSTS
First check for a loose drive belt. Next, have a load test performed. It sounds like the generator has a bad diode. That will limit its output to one third of its rated capacity. Since that's barely enough to keep up with an electric fuel pump and the ignition system, the battery will slowly drain and you'll see the voltage gauge on the dash drop over time.
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Thursday, September 1st, 2011 AT 6:55 PM

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