I posted a few years back after purchasing an '08 R320 that had no AC at the time of purchase, and the resolution then (late 2019) was to replace the compressor. Everything worked great until this month, when the system stopped blowing cold air about 2 weeks ago, and then failed catastrophically a few days after that. On a particularly hot day, there was no cool air, then squealing, then a bang and smoke, and then all of my accessories died. The accessory belt snapped, leaving behind melted belt on all of my recently replaced idler pulleys, belt, and tensioner after the tensioner failed about a year ago. My service shop (MB specialists, but not the dealership) said that the compressor had seized up, which matched my theory of why the belt had failed. For the record, that compressor is roughly 3 years and 30k miles in service, which seems very short for the lifespan of the device. I have a few questions about this:

1. Why would the compressor seize up this time, and not spin freely at the pulley like it did when I got it? Is that just from running it hard trying to cool down, and it overheated and failed in a different way than it did before?
2. The car was parked outside at the dealership for 5 months in the peak of the summer last year awaiting the brake booster recall. Would sitting parked outside in the VA sun and heat for that long have caused something in the HVAC system to fail prematurely, like the lines or a gasket or something? What should I be looking for?
3. Assuming I choose to replace the compressor again and not just unload this thing, how can I tell if there's about to be another problem? When not blowing cold, I saw no CEL or other warnings, just spontaneous failure. I have an iCarsoft MBV2.0 reader that shows pressure in the system and some other HVAC values I believe, but I wouldn't know to check anything specific.
4. Is the compressor in this car just a poor design? I have never had a unit fail this poorly, after such a short time.

 

Probably the bearing(s) got dry and broke. I guess you installed a non-OEM in 2019? Some non-OEM parts can we very poorly built.

 

Well, the service shop only uses MB branded parts, so this would be an official MB unit so far as I know. I'll confirm this with them today.

 

When this happened the first time, was the condenser and dryer also changed? When a compressor goes bad, it can contaminate the system which could explain the reduced lifespan of the replacement.

Or it could have been a cheaper brand / rebuilt compressor with lower quality parts, or a used one?

Are you assuming that? Or do you know for sure?

A lot of them use parts of varying quality. If the customer is cheap they usually get the cheap stuff.

 

Can only speculate as to exactly why - categorically, best guess would be a problem with internal lubrication.
I'm currently having issue with (I think) compressor control valve signal - it doesn't pump as it should, and I'm about to wire in an extra diode.

I reread your thread from 3 years ago, because thinking about AC issues, and recalled my own.
I replaced mine myself, and my shop actually measured out for me how much oil to prefill the compressor with. Apparently, too much and you interfere with cooling performance, too little and Bam.
If system has small slow leak and car is at rest long periods (rather than keeping oil circulating), I can imagine is possible for oil to collect near a leak and come out disproportionate to the volume of refrigerant. But, I'm guessing.
I also read a thread today about older sedan (I think from the 90s), which had a low refrigerant sensor - I don't think R-Class has sensor to warn you of this.
Keeping a van-sized car cool is more work than for a small car, and so for us the compressor probably fails more frequently.

With these cars it's really your time or your wallet and hopefully not too much of both. I actually ride with the tow hook in bumper all the time as a statement.
Car's main virtue is - it's paid off!!!