Once you've got the alternator off of the engine – which I understand some people find difficult – the chances are you'll have found out that some bolts and nuts are not meant to move! If you are struggling with alternator removal take a look at the pictures below. Perhaps they will help.
Start by removing the nut from the pulley. It is a good idea to leave the adjustable mounting bracket on the side of the alternator in place for this, as you can use it to stop the fan from rotating whilst undoing the nut (22mm socket).
If this doesn't work then I recommend trying to stop the pulley from turning with a strap wrench before resorting to clamping it in a vice. Whatever you do don't damage the pulley – if you have a knackered pulley then you are better off replacing it. A pulley needs to be straight and true otherwise you'll have trouble with V belt tension, vibration, bearings, V belt durability etc etc etc...
Under the nut on the pulley you'll find a split washer. Remove this, a thick spacer, the pulley, a thin spacer,
...and the fan from the alternator.
Underneath all of this stuff you'll see that there are two sets of four screws.
The inner set hold a bracket that helps to locate an inner bearing assembly on the rotor, and the outer set (four long screws) hold the two aluminum casings in place on either side of the stator.
Unlike in the OM617 starter motor (
An OM617 starter motor is for LIFE not just for Christmas!) the armature in this alternator is a stator – it doesn't turn. Whilst at first glance this alternator seems quite similar to an electric motor / dynamo it is subtly mechanically different as it does not have a commutator. Instead it has a slip ring assembly.
When the rotor is turned an alternating current (AC) is induced. To obtain direct current (DC) the AC is converted to DC through a set of diodes that are positioned in the plastic assembly at the back of the unit – at the opposite end of the pulley.
See
http://en.wikipedia.org/wiki/Alternator#Automotive_alternators for more information regarding the workings of alternators.