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I am looking for help. I have a 99 Ford Mustang v6 5speed. We have replaced the battery 3 times in the past 8 months and the battery is now dead again after replacing it less than a month ago. We believe the previous owner had tried to wire things himself or thag there is a short. I can close the doors while the car is running and they will lock and unlock themselves. There was an amp previously but we thought by taking it out maybe it would prevent the draining. Would this be the alternator that's bad?

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    Have you had the charging system tested? That would be the first thing to do. Once you have verified the battery is being properly maintained by the alternator, then look for shorts.
    – CharlieRB
    Jan 24 '17 at 16:24

My guess would be that the alternator is either putting out too much charge or too little charge. Both are bad.

What is the voltage at idle? What is the voltage at 2000 rpm?

I'd be looking for 13.6 volts (+/-). You need one of these.

free harbor freight volt ohm meter coupon


Could be lots of things at this point. You could have electronics that remain on while the vehicle is parked. Generally, electronics should switch themselves off after a while, but one unit that remains on will drain your battery in a few days.
The alternator could be faulty too, but I'd measure the parasitic draw first (see the link for more detail).

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