Posted by Peter Kennedy on 11/22/2019 to Alternators
The nut is on tight and if you try to undo it with a regular wrench the pulley turns instead. Some people try to stop the pulley turning by jamming a screwdriver into the interior of the alternator but that invariably damages it. The tool being used here is a power impact wrench, not a drill. It spins the nut off so fast the pulley doesn't have time to start turning.