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Q: 3 alternators replaced!

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My mechanic has no idea what's wrong. Went for a new alternator, it ran but won't charge battery. Then dash lights, running lights and brakes and do tag lights all went out. Replaced relay and alternator and again same issue. Battery won't charge ,alternators are going bad. Replaced again today to only have this happen again. Please please help me.

My car has 312000 miles.
My car has an automatic transmission.

This may suggest a failing voltage regulator. The voltage regulator is a unit that regulates the charging of the battery by the alternator. When the voltage regulator is not working properly, this may result in the alternator allowing too much power to be delivered to the battery resulting in damaging wires and prematurely sometimes destroying the battery. A common sign of this is usually the acid inside the battery boiling causing the battery to swell. In other cases it may result in the alternator not supplying enough power to the battery, resulting in undercharging the battery or not charging it at all. I would recommend having an expert from YourMechanic come to your location to diagnose your vehicle’s charging system.

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