Nyck, cost will depend on what you have to start with. If the system is charged but just not cooling as well as you would like it to cool, I would have the system checked for leaks, repaired and charged back with R12. R134 does not work well in these cars. I know. I have been down this road several times. I tried 134, freeze 12, hydrocarbons, changed expansion valves,and nothing worked well enough here in North Carolina. Texas is generally hotter than NC so I don't think you will be happy.

The compressor went bad on my 500SEL and had to be replaced. I flushed the system completely, changed the expansion valve back to the one I took out when I put the 134 valve in. I charged it with R12 that NAPA had at the time. It was $30 per can and it took 4 cans. That was the best $120 I have ever spent. This was about 4 years ago. I haven't even had gages on it since. It works so good that on a 100 deg day, with the temperature wheel set at 22c (about 72 f) we will turn the seat heaters on to help our old bones feel better.

R12 is still available if you look for it. The correct oil for R12 is also still available.

The above is just my experience.

Trivia, R12 became the worst thing to happen to our environment about the same time that Duponts patten ran out. Dupont had the patten on on its replacement, R134.

PaulM

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84 500 SEL (307,xxx miles)