I replaced an a/c compressor in a 05 equinox because the owner had no cold air. You can see why because the compressor had no oil and chunks of metal missing which could not compress the refrigerant, which oddly it still had some.
Opened up the old one and it had no oil. Checked the bypass valve (cleaned it before taking pics) and had the black death. Luckily I flushed the system of any metal and replaced the filter dryer.
My diagnosis is that the o-rings have a few splits in them where the oil leaked out and out the case (the outside of the compressor has a greenish mold look to it like the S-10 compressor which also had an oil leak.) which metal on metal on high temps and pressure is not good and the compressor scroll ate itself (yummy :D)
Interesting stuff. I want to teardown the S10 Harrison V-7 compressor, but I need a special tool that I will probably only use once.