Mercedes: 1986 Mercedes 190E Air Condiitoning, freon 12, coolant line

Question
I have a 1986 Mercedes 190E 2.3 with 89,000 miles on it. About 4 years ago the air conditioning still cooled the car in the summer. I tried it this summer and it would not cool. The garage says I need to convert to a new coolant that does not have freon. Can I just put an adapter on the coolant line and pump in the new coolant? Mercedes of course wants a lot of money to do this "conversion". I want to do it right, but if it is something I can do, I would rather save the money. Thanks

Answer
Jim,
The freon can leak down over time around the pump shaft etc., particularly if the the vehicle is not used on a regular basis. The lubricant circulates with the freon and helps keep the gaskets and seals pliable.
The system has a low pressures switch to keep the compressor from operating when  the freon and lubricant are low. This protects the compressor from damage.

I would find an independent A/C shop to service the system. Federal Law requires that all service be done by a qualified tech. with the proper recovery equipment. Leak are supposed to be corrected prior to recharging to keep the CFCs out of the atmosphere. Freon 12 is very expensive now as they are just cleaning and recycling out of what is being used in the US.  
The tech will vacuum out the remaining freon to see if the system will hold the vacuum. This process determines if you have any leaks,and replace it with the correct amount along with any necessary lube.
If the compressor checks OK. it will probably be less expensive to have the Freon 12 replaced than to go through a R134A conversion. Because 134A uses a different type of lubricant you would ruin the system to just ad this refrigerant to a R12 system. A conversion, requires the lubricant be washed out and the new type lubricant added in the proper proportion along with changing the fittings etc.
The R12 system is more efficient than it would be with 134A particularly in traffic.
Hope this helps. Good Luck
Rob