battery dies after a few hrs ??!! help..

[Ieatcamaros]

Sounds to me like your alternator has a diode burnt out in it. It still charges while it's running, but discharges your battery when not running. Put a voltmeter on the battery while it's running and see what the voltage is. Should be at least 13 at idle. Since we're talking about a fox mustang here. Rev it up to like 1500 or 2000 and see if the voltage comes up to 14. If that looks to be ok, try disconnecting the positive cable while the car is sitting. If that does the trick, then you need a new alternator. It is easier to just replace the whole alternator rather than trying to replace just the diodes.