On my car, there's a slot at the 1-2 o-clock position behind the fan pulley looking from the front of the car. Slide a 3mm allen key or a *really* thick piece of wire between the ridges that form the slot and push on the key towards the front of the car while you turn the fan pulley (not just the fan since there's a clutch) until it engages one of two or three holes on the pulley.

You may have a different pulley that requires a special tool, but my recollection is that the tool is not required for W124 and W126 applications of the M103 engine.

An pneumatic tool might do the trick if you have access to one that will fit.

If you want to get creative, fit the belt around the fan pulley and have someone keep it from turning while you loosen the bolt. Keeping it from turning does not mean putting undue side loads on the fan bearing.

Regards,
Sixto
91 300SE