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Toyota Service
Toyota Service, Toyota Expert
Category: Toyota
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Experience:  30+ Yrs of auto experience, Award Winning Svc. Manager; Just Answer Customer's Choice Award Winner
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I took my car to Auto Zone and they said I need a new

Customer Question

I took my car to Auto Zone and they said I need a new alternator. When I took it out they tested it again and this time it passed. I replaced it just in case and the same thing happened. It failed the test when mounted on the car but passed it when tested outside the veicle (2003 Sienna).
Submitted: 2 years ago.
Category: Toyota
Expert:  Toyota Service replied 2 years ago.

Hello. Welcome to Just Answer. Please allow me to assist you. This issue is interesting; it passes on the bench but not on the car. This tells me that the alternator is faulty. In other words, the alternator will deliver a voltage output on the bench, but, when on the car, the additional loading of the unit (ECM, spark, etc), the alternator is incapable of maintaining the necessary voltage to maintain 10.5 volts DC to operate the vehicle. In this case, I suspect a bad voltage regulator inside the alternator. The problem with aftermarket alternators is rather simple. You get what you pay for. The cheaper the alternator is, cost wise, the less new parts (and usage of quality parts) is reduced. I have had, in my years in the shop, many 'aftermarket' alternators that were inoperative right out of the box form a parts store. You would be best advised to get a quality rebuilt unit. FOr the money, and the warranty that comes with it, a replacement alternator form a Toyota dealership parts department cannot be beat. You get a 12 month, UNLIMITED mileage warranty on any part the parts dept sells. This means that they are serious about the quality and the reliability of their remanufactured parts. The recommendation I have for you on this request is to return the AUtozone alternator and get a genuine Toyota rebuilt unit.

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Customer: replied 2 years ago.
Hmm, the problem is that I did just that. The alternator on the veicle has a lifetime warrenty because I replaced it less than one year ago, so I insisted that they replace they alternator altogether and I am getting the exact same results. I'm wondering if the problem has to do with wiring on the veicle or perhaps the veicle computer is the culpret.
Expert:  Toyota Service replied 2 years ago.

The computer has nothing to do with the alternator. The computer only consumes the power that the alternator delivers to run the car. The computer does not turn the alternator on or off. I would doubt highly that the wiring is bad This sounds more like the alternator, for whatever reason, is incapable of delivering enough volts and amps to charge the batter AND run the vehicle. The spec on this car is 14.4 volts DC at any RPM. A max load of at least 80 amps is needed to properly run this vehicle and charge the battery. If the test bench does not deliver 14.4 volts and 80 amps on the load test, the alternator is under specification.