In my experience, the heat usually works ok. It's the AC that is the problem. I'm not sure why an OM617 would be any different than any other older car with AC. On mine, my mechanic replaced the heater-dryer units and filled with R12 and the AC worked. The second year I had a line rebuilt by a AC shop once it started leaking.

Now, as far as AC performance, could the cabin get colder? Yes. But, I'm generally just fine in it driving around in triple-digit temperatures. Someday my AC compressor will fail (or I'll get done with everything else on my list), and then I'll put a Sanden in.

Dkr.