yeah, it was the bearing I was most keen on changing. The parts suppliers call it a clutch but I can well imagine it's simply because so many other AC compressors have that part as the clutch. Usually there is a wire running under the pulley to activate a solenoid to engage or disengage the clutch but I can see from pictures of replacement compressors that this one does not have it...

 

Turns out some versions of the CL500 have a magnetic compressor pulley clutch and others don't. Mine does not have the magnetic clutch. I had to order twice though as the first one was a magnetic type despite being listed as compatible for my car. The second one was also listed as a magnetic clutch but wasn't. I had to use the photos to figure it out for myself which part I was buying!

Anyway, that has solved the issue which has been with me for such a long time.

I made the mistake of going down the ABC rabbit hole when it wasn't anything to do with it. The Mercedes dealer diagnosed the problem as a noisy ABC pump and would have charged the customer thousands to replace it. I paid something like £350 for a replacement pump, fitted it myself and at least I now know mine is good for a long while.

The noise was coming from the same area as the pump because the AC compressor is directly below the ABC pump.

When I got the pulley off the compressor (a bit of a fight due to corrosion) it was immediately obvious the bearing was shot.

 

Does anyone have a Drawing of all three systems with MB Part numbers?