My '86 420SEL was fully charged with freon and still didn't operate. The problem was the push-button assembly. If all other parts of the AC system check out fine, then your buttons may be the culprit. I was able to order a rebuilt part over the Internet for a mere $150 and install it in 45 minutes. If you've never taken the wood fascia off your console before, then I suggest having a pro do the work as there are many, many connectors and fiber optic lines that must be taken apart carefully. But the procedure is rather simple. Good luck.
Current: '86 420SEL, '81 380SL
Previous: '79 240D, '70 280SE, '81 300D.
Just about 600,000 miles on Mercedes products.