The other possibility is that somehow the alternator is working extra hard due to some other problem...Years ago I had a car with 5 amplifiers (yes 5) and I regularly went thru alternators because they had to be working all the time because the 5 amps drew so much current the alternator was having to charge all the time. If the alternator is turned on all the time (charging) the brushes will wear out much sooner. If there is some other problem with the car such as bad grounds and or poor positive connections the alternator could be switched on more than it should be. If the car has bad grounds the alternator has to work overtime to overcome this poor complete circuit. It is not uncommon to see older cars go thru alternators more so than when the car is new. These were the kindsof things I saw in 25 years of experience in my car audio shop.

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83 300CDTurbo 307,000
2005 Honda Element 266,000
56 Nash Ambassador Country Club Special 34,000(under restoration presently)

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