Welcome to summer! In my part of the world the mercury has risen well above 90 and humidity to match. And lucky me, it stays this way till October! I swear, Alabama is the only place where you can get a sunburn, heatstroke, and dehydrated at a football game! And since I am what I call "hypo-thermatically sensitive", strong A/C is a MUST!
I have been pleasantly surprised with my BMW climate control system. It keeps things nice and cool in the hottest of temperatures. But last year about this time I was sitting at a red light and all of a sudden warm air starting pouring out of the vents. For me, this was worse than overheating. It was a giant unknown and I started having flash backs of my old Jetta. The air died and they said it would cost $1500 to fix it (so I traded it in). However, since I didn't have $1500 and I was not gonna sweat through an Alabama summer, I dove in to fix it. Here is my experience and some tips.
First thing I did was buy a quality pressure gauge. You need to know what your low side pressure is at a minimum to start with the diagnosis of the problem. On the E46 BMW the low side port is right next to your positive battery connector under the hood. I have found them to be in similar locations on the E36, E39, and E34.'s that I have worked on (note any pre 1993 vehicle like the E30 or E34 might still have R-12 in it which requires a conversion to R-134a style fittings). Now to read the pressure merely attach the pressure gauge with your A/C at full blast and see what you get. It is hard to say what "normal" is because pressures will rise and fall with the ambient temp. You could get a 25psi eading in 75 degree weather and the same car could read 35psi in 90 degree weather. But as a rule you should be bewteen 25 and 40psi and most gauges will have a blue, green and red section to help you see what is normal. On my car, I got really high pressures on the low side, close to 60psi. I did some research and it seemed my suction side of my compressor was not, uh......sucking. So I decided to replace the compressor. Normally I would have dug deeper, but my car had over 120k miles on it at this point so I knew it was time for a new one. I called BMW and they wanted over $900 for the compressor, stealership ripp off at its finest. I checked online and a new OEM one was still $450! I didn't have $450 so I sourced one from a used BMW with 90k miles on it for $150. SCORE! (note, if you want to find a used compressor try www.vinesauto.com ask for Martin and tell him Shad told you to call).
If you decide to do your own A/C work just know that it is illegal to vent R-134a into the atmosphere. If you do it I doubt the EPA is going to come to your house with guns drawn, but since I don't know the legal liability of blogging this stuff, I will just say it is illegal and don't do it. Since you may not have an E46 like me I will save the detailed install process of the compressor, but just know I did it on a Thursday night, after work and it only took me 1 hour having never done it before. You also should install a new dryer to be "correct", however I didn't want to spend the extra $50 and since the compressor was used I figured, what the heck.
After the mechanical parts where replaced and connected I decided to proceed with a DIY re-fill of R-134a. Again, to be 100% correct in the repair you should pull a vacuum on your system for at least 30 minutes to make sure you have no leaks and get all the air out of the system. I don't have a vacuum pump so I bought an air vacuum pump from Harbor Freight (http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=96677) for $13. It is basically a Venturi pump that uses forced air from your air compressor over a diaghram to create a vacuum. I let it run for about 20 minutes. (note: I have done many of these repairs without doing this and all were successful). Now I was ready to fill the system with R-134a. If you take only one point away from this article let it be this: ONLY PUT PURE R-134A into your A/C system! Do not buy the stuff with leak sealer in it, any synthetic stuff, or any stuff with seal conditioner or oil (you buy that seperate) in it. All that stuff can clog your system and cause a failure, yet millions of people buy that "recharge" crap they sell at AutoZone and ruin an otherwise properly working system. Don't do that! Then simply add the R-134a to the low pressure side attachement and add 2 cans to start. Then check pressure and if the vent temp is cold. If it is not quite where you want it, add another can or can and a half. Sometimes the can will empty really fast, sometimes it won't. If it seems the can isn't emptying into the system, you can put the base in some hot water to help move it along. On most BMW's I have worked on, you can actually set the can near the exhaust manifold and accomplish the same thing, but do NOT put it directly on the manifold or it will burst, just near it to get some of the heat from it. If you overfill the system you may not get any cold air at all, the systems are equipped with a high pressure valve that will kill the compressor if pressures get too high. It is best to fill the system in the hottest part of the day when the ambient temp is hottest. This allows the R-134a to expand as much as possible and will help you not to overfill. On mine I did the re-fill at night and I had perfect pressure and 39 degree air coming out of the vents. The next day it went hot again because the pressure had risen in the heat of the day. I had to evacuate a little R-134a out to make it start working again. After that, I have had no problems and still have ice cold air over a year later. Total cost to do my budget A/C redo was $300. That included all R-134a, oil, and the tool from Harbor Freight.
Much better than $1500 in my opinion!