How to change a pulley on an alternator

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.

Note that the nut should go on with a lock washer.  If you don't have a lock washer then you need to use Blue Locktite to stop the nut vibrating loose.