I have had my alternators checked at O'Reilly Auto Parts since before they were even called O'Reilly Auto Parts and they 100% check your battery and alternator while it is in your car. They will ask you just to run it if it's able to run Etc. I love O'Reilly Auto Parts for this reason cuz they can determine whether your battery is bad or whether your alternator is bad. They are so efficient and they walk right out to your car usually within just a minute or so with a little device that hooks up to the terminals on your battery and it can check both the battery and alternator! Great question and there you have it.
They will more than likely ask you to remove the alternator and bring it in so they can text it on their hanging inside but a quick way to tell if it is bad or not is by starting the car and loosen and remove the positive cable (positive side only though) and if the car continues running once you remove the positive terminal then you know the alternator is doing it's job..
Yes they can
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