It sounds like your A/C compressor is locked-up/ seized

This will require a trip to your favorite MB tech to repair, and the expense of around $1000.00+. You will need to replace the compressor, receiver drier, and expansion valve, and flush the system to remove metal contamination. Mercedes recommends replacing the "manifold/muffler" hose assembly in the case of a compressor failure, which can be an expensive part. I always recommend using OE remaned compressors.

Then you will have to decide which refrigerant you will go back with, 134a or r12? I won't get into a debate on which is the way to go on this issue, but my opinion is for 134a retrofit for the difference in expense and future availability of product, JMO:p

Hope you are still breathing, sorry to have brought you to a bad conclusion on your problem, but at least you know.

Until next time!