If the dynamo is still on the car, free the nut before taking the fan belt off.
The most common reason for removing a dynamo pulley is to fit it on a new dynamo — most replacement dynamos are supplied without one.
When refitting an old pulley, or fitting a new one, be careful not to bend or damage it. The surfaces inside the V where the belt runs must be smooth.
In many cases the pulley incorporates the dynamo cooling fan: its fins must not be bent or broken.
Usually it is best to take the dynamo out of the car before removing the pulley, though sometimes there is enough room to work with it still fitted.
Removing the pulley nut