Ross,

I am much more experienced with M103 air pump operation and repair than I would like to admit, but I'm not sure if they're the same pump on a 500E. If they aren't they should be similar. The vanes in the pump are carbon, IIRC. MB will not supply any parts to the overall component. The individual vendors who supplied the parts to MB will not supply them to anyone in the US.

My first Air Pump died at around 150k miles. All that really died was the bearing on the pulley. I pulled the pump, pressed the bearing out of the pulley, and thought the hard part was over. There is an MB p/n on the bearing. MB won't sell it and it doesn't show on the dealer stock system. MB (and all of the other mfrs) sign agreements with EPA in the US that they won't sell any parts below the component level for emissions control equipment. That's why the pumps are so freaking expensive. The rebuilders reverse engineer the parts they need and that's possible because they're rebuilding hundreds or thousands of pumps. If you try to reverse engineer a single bearing on the pump, it's cheaper to buy a new pump. Others on the list have suggested machining the pump or the pulley to accept a standard size bearing. This is fine except for the day you finally eat your custom sized bearing pully and now you can't get a core charge back on the pump when you go for a rebuild. That plus the fact they usually live about 150k miles anyway and you see the point.

FWIW, I hear that on a 1991 model 300TE you can disconnect the Air Pump electrically and it doesn't cause the check engine light to come on. Maybe you can do the same on a 500E. They really don't help do anything except warm up the cats quicker so the emissions are lower quicker. Since most US tests are on warmed up cars anyway, what's the point? Since MB won't tell you how long the serp belt should be if you take the Air Pump out of the loop, you either play the game and replace them or disconnect them and run a $400 idler pulley in the loop.

Not that I'm recommending anything that the Democrats in Congress wouldn't espouse, but MBs run pretty clean anyway, don't you think?