Accessory belt tensioner/pulley
The outer belt is not turning it's the accessory belt to ac and alternator the car
starts and idles rough how do I know if it's a bad tensioner or just the pulley on
Remove belt then spin the alternator, tensioner and other related pulleys that go around belt if one make noise or does not spin easily that is the culprit. The only accessory you can not check by hand is the A.C compressor so if all other accessories turn out ok suspect the compressor.
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