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First of all, if your compressor is good, I assume you already have r-12 in the system and I would recommend keeping r-12 in there. I say this because many people (even compressor rebuild people) have said that your old compressor won't last long with r-134a because it operates at higher pressure than the r-12. Furthermore, 134a oil doesn't really mix with r-12 lubricant. The compressor performance diminishes fast with the higher pressure of 134a, and the less pressure it can pump the less cooling ability - see why so many people claim their car doesn't cool as well after they convert?! I converted when I put in a new compressor, so I didn't worry about this.
If you are going the new/rebuilt compressor route, the following will give you results comperable to r-12:
1) Use an air compressor and flush all the lines, condensor and evaporator with mineral spirits. Follow up with lots of compressed air to blow out residual mineral spirits. This is a PITA and takes a while, but a thorough flushing will give you great cooling.
2) Install your new parts. First of all, you don't need to replace your old lines unless they leak (the "replace the lines" thing is another holdover from old, 1970's cars where smaller 134a molecules would leak through the walls of the hoses. You won't have problems with modern lines.), secondly ALWAYS replace your reciever/drier AND expansion valve when you have the system open. R-134a gas requires a r-134a specific expansion valve. The last thing you do when buttoning up your system is install your drier. It comes sealed and you only want this thing open to the outside air for as short a time as possible - as long as it takes to get it threaded to the lines. Also want to replace all the o-rings while you are at it. Lubricate them with fresh lubricant before you install them and DO NOT crank down on the fittings. The connections seal from the o-rings, not from ham-fisted mechanics cranking down on them as hard as possible. Too much pressure will actually damage the o-ring and cause a leak or strip threads. Use anti-seize on all threads.
3) Before you button everything up, distribute oil throughout the system.. You should check, but I think the whole system takes around 8oz. So put like 4oz in the compressor, 2oz in the condensor and 1oz in the evaporator and 1oz in the other hose (turkey baster with measurement marks on it works great). Check on the exact amount of oil. For a new system with a new/rebuilt compressor, definitely use PAG oil. The other crap is for use when you don't flush the system and try to mix 134a with the old r-12 oil - this is another source of complaints of inadequate cooling after conversion because mixing two incompatible substances like this actually puts a film on the inside of your a/c system and reduces its cooling ability.
3) Using a high powered, professional vacuum pump (rent one if you have to), put a vacuum on the system for around 45 minutes. This is necessary because lower pressure causes moisture to evaporate at a lower temperature and you need to remove all the moisture from your system as it will reduce the systems ability to cool and can damage the compressor. Consequently, vacuuming with a professional pump is essential, and the lower the outside air temp is, the longer you should leave in on the pump. After this you can put the new refrigerant in. YOU REALLY NEED GAUGES TO DO THIS THE RIGHT WAY. Check http://www.townsendimports.com for the appropriate pressures, but when I did this last summer, they told me to only start with two cans of 134 and only add if it doesn't seem to be cooling enough or the pressures aren't high enough. Remember that proper pressure is a factor of ambient air temperature, so gauge pressures will vary depending on if you are doing this in Arizona or Maine. Anyway, first without the car running, add a can to the high side. You will probably have to put the can in hot water to get all the refrigerant out (not boiling water, not a propane torch, not some kind of heating element, just hot water)) Then start the car, hit the a/c button and then add from the LOW SIDE ONLY. Trying to add from the high side with the compressor running will result in an exploded can and a nice trip to the hospital with God knows what kind of injuries. Add the two cans and you should be done. Check your gauge pressures to make sure you didn't put in too much (remember, high pressures will not only wreck your old compressor, but it will take out a new one too, so don't try to save yourself $45 on the cost of gauges when it will cost you a new compressor.)
That should be about it. I did a lot of research on this last summer, and I went this [complex] route because it is the only way you can be sure you will get really good, r-12 like cooling. Everyone complains about r-134a because they cut corners when they do the conversion, and as you can see from my explanation, there are a lot of corners that can be cut. You can be sure some garage won't so such a thorough job like this and if they do it will cost you a fortune. I am quite sure my system works as well as it did when r-12 was in there. While I don't live in Florida, it worked great in D.C. traffic last summer and it is pretty damn hot and humid there.
If you do decide to go the easy way and just pump in one of those kits from the parts store without doing any of the above, please don't go around keeping the myth alive that 134a doesn't cool as well as r-12.
Good luck,
Greg
'85 SPG 214k
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