I just replaced the air pump on my 95-S320 so I know a bit about these . Bottom line is that the electric clutch will fail at some point and the entire pump seizes up once and for all. When that happens, the pulley will no longer rotate and the burning smell and blue smoke you see coming from under the hood is the serpentine belt rubbing on the frozen pulley.

My local import parts store ordered me a rebuilt pump unit for about $325 plus the core. There is a cheaper place on-line to get a replacement (http://www.mbpumps.com/, about $200 plus core return) but I know nothing about them so you will have to do your own homework.

Switching it out is fairly easy, remove the belt and remove two bolts (top and bottom, from radiator side). The top bolt is easy to find, the bottom one is under the pulley and you have to feel around for it. Loosen the hose clamps and wiggle off the two hoses connecting the pump to the exhuast system, and replace as needed. The MBPumps site says that you should also replace the check valve. To install the new pump, just reverse the process.

Getting the serpentine belt back in position was the hardest part! If you don't have a tool suitable to release the tensioner pulley, go to your local AutoZone parts store and they will give you a free loaner tool for a $25 security deposit. It makes the belt replacement a whole lot easier.

If I had to do it again, I could probably manage the whole job in 30-45 minutes.