The large pulley in the center moves in and out (about a quarter inch) while the engine is running and makes a clicking noise. About the same time this started happening, the battery failed to hold a charge and the engine overheated. I'm assuming the belt is not functioning properly because of this loose pulley, thus causing the other problems.


Is this a routine repair - or does it require an "authorized" dealer?
Any advice appreciated!

 

That's your water pump going bad. Get it fixed immediately! That would have nothing to do with your battery failing to hold a charge though. Get a new water pump installed and then see what's next to do.

 

EDIT: If as Bob says above it is the black steel pulley that the belt runs over with the non-grooved side, then ignore all I said below .....

STOP NOW !!!
DO NOT PASS GO !!!
That is the crankshaft pulley ( or harmonic balancer )
If it comes off, all sorts of bad things can happen.
You will need to find a good MB repair shop that can work on a SLK320 and get it hauled to them,
or fix it yourself. ( This would require a new bolt, possibly a new woodruff key and a HUGE torque wrench,
not to mention the special tool for locking the flywheel )

 

Thanks guys!