To Daryl,

If the air pump is screeching when electrically disconnected, the problem is either the clutch or the idler bearing on the front. The clutch engages when power is supplied. The front idler bearing is a double row, sealed ball bearing unit made from virgin, billet unobtanium. The manufacturer won't sell them in the US and MB won't either. I have the id and od dimensions but they are useless if you can't find replacements. I even had a friend who is a nuclear grade procurement specialist try to find an equivalent in the commercial market and he couldn't. You'll know the bearing is shot if, when you remove the serp belt, the pulley flops around with fore and aft movement on the shaft. Good luck with the rebuild. If you just need a clutch, you should be ok, but anything deeper and you're probably screwed.

BTW, remember that the normal life of these pumps is about 150k miles, so think about that before putting more than $100 into any work on it.