This is from experience with a 300 SD W126. The R134 has no noticeable difference. It works well in 90+ plus temperatures. Before I replaced the evaporator I was running R134 and it worked great. To the OP, if you search the forums you can find a number of people who live in the deep South and are very happy with R134. I do not doubt that R12 is slightly better and will admit that it seems R134 can be hit or miss, but the idea that R134 cannot be successful in a 126 is factually incorrect.

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1984 300 SD 384,000 Miles