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Old 01-06-2009, 08:29 PM   #1
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Will a Bad Alternator drain my Battery?

I took my car out for a spin yesterday,when i got back i had it sitting for about 3 hours and i went to start it and my battery was dead.So I put another battery in there started it and checked it with a volt meter it was showing about 13.5.So i shut it off disconnected my negitive post off the battery and put my volt meter in line and sure enough i got a drain being its reading 12.7.so i started disconnecting things ECC,pulling fuses ect,,,and when i unpluged my alternator my volts droped to zero so it appears to be draining it..(Seems strange to me i never had that happen.)...I hooked every thing back up and went to pull it in the garage and my fanbelt started squealing big time!
What do you all think is my has alt gone bad ?,,,,thanks jim
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Old 01-06-2009, 08:40 PM   #2
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Yes, bad diodes in a Alt can drain your battery, I had it happen to tow of them . with the battery charged and the car/engine cold, Place your hand on the alt, it should NOT be warm.
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Old 01-06-2009, 08:41 PM   #3
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yes I had the same problem last spring..it can be caused by the brushes getting crooked in their slots and it causes more of the brushes material to pile up inside the regulator housing part to create a dead short..I had a spare alt that wasn`t charging so I just swapped the regulator over to it and ...voila...it works flawlessly,hope this helps
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Old 01-06-2009, 08:42 PM   #4
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Sounds like a bad alt but take it off and take it to an advance auto or auto zone to have it bench tested before u buy one. Also check the pigtails that plug into the alt because they sometimes go bad.
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Old 01-06-2009, 11:23 PM   #5
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Il pull it out and get it checked out,,,thanks everyone
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Old 01-07-2009, 01:09 PM   #6
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before dumping $$$ for a new alt - pull the voltage regulator off the back of it... If the thing has many miles on it, or has never been replaced - the brushes inside the regulator are shot, which is what has caused the alt to fail.

You can get a new voltage regulator w/ brushes for about $30 +/- depending on the store & brand. It's a 30 min fix at the max...

If the alternator has not seized up, 90% of the time, it's just the voltage regulator is bad...
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Old 01-07-2009, 06:13 PM   #7
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Quote:
Originally Posted by Cobra Jet NJ View Post
before dumping $$$ for a new alt - pull the voltage regulator off the back of it... If the thing has many miles on it, or has never been replaced - the brushes inside the regulator are shot, which is what has caused the alt to fail.

You can get a new voltage regulator w/ brushes for about $30 +/- depending on the store & brand. It's a 30 min fix at the max...

If the alternator has not seized up, 90% of the time, it's just the voltage regulator is bad...
it was still charging but theres a drain on the system after its shut off which in my problem the armature was ground down so bad that a new reg and brushes wouldn`t have lasted more that a few weeks at most... so yes take the reg off and inspect if the armature is still fine then theres a bad diode which means a rebuilt alt anyways
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