An update to my cooling woes.
I took it to another a/c shop.
They replaced the evaporator.The previous evaporator was only 3 yrs old.
anyway I had cool air for almost a week after that the engine temp went upto 120 C in traffic. Found the fan not working, the fuse was blown, replaced the fuse and that blew too the next day.Took it to an electrical garage. They found the fan was shorted .Replaced the fan & the resistor.The a/c again gradually got warmer and finally after two weeks it was blowing only air. With outside temp in the range of 40 to 47 C it was really bad.
Took it back to the shop which had replaced the evaporator.
They checked there was no R12. it had completely leaked out.
They pumped in air again & checked with applying soap water on the hoses searching for bubbles(This method seems to be the normal method in these places as I have found most of the A/c shops/indies use this method here).They found some tiny bubbles on the hose at the low pressure side.
They replaced the two hoses & assured me that now everything would be OK.
But it was not really cool enough. I thought the low cooling was due to the high temperatures outside & the mechanics tell me these old mercedes do not cool good.
After a week again on the way back from office the a/c suddenly started blowing hot. Found the compressor & the fan was not running.
what is the problem??? Have to take it to another shop.
Do not know whether to keep it or sell it off.

Very long post, but had to cry somewhere.

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300CE 1989 Euro Model.
ML350 2011.