Someone asked me in e-mail how I'd do a conversion or recharge. Maybe, for the 2 cents worth and for those who are considering a $30 drop in kit, it might be illustrative to describe a conversion.
First thing is I check the type of refrigerant in the system. Mainly is it "contaminated" ? I have a tester to do this, it tells me the type of refrigerant and the purity. If it is contaminated I stop. In CA disposing of contaminated refrigerant is expensive, so I do not even go further. Many shops are similar. Remember this when you want to put in the whizbang "SUPER R-12 replacement". Often too then contain propane or butane. BANG!
Ok assuming it passes, I next check it for acid. Why ? It often shows a failing system in progress. If it fails it usually means replacement of major components, but I still proceed. I evacuate the system and recover the refrigerant.
Next I disassemble the hoses to/from the compressor, remove the expansion valve and take out the old "receiver/dryer or accumulator (depending on the type of car and what they call it). I now check for signs of black oil. If the acid was high the chances of blackened oil are high. If I find it, I start by assuming I have a system in failure. Often the expansion valve will already be gunked up with the "black crud".
Depending on the severity, I may try and flush the condensor. If I can flush it clean after a couple tries that is a good sign, if not, I assume the system is a total loss and replace the condensor, compressor and receiver/dryer (which gets replaced in 100% of the cases anyway). If I can flush it clean, I then flush everything I can get to. I often start with alcohol and finish with commercial flush.
Rubber hoses get tossed, and replace with barrier hoses. All o-rings get replaced with barrier ones, they are green in color. I replace the receiver dryer and any other inline filter. Any hint of compressor failure previously and I add an inline filter/dryer to the system.
I reassemble everything, add oil along the way (often to the receiver dryer and as a matter of course to a replacement compressor) up to specs.
I replace the schrader valves with new ones. Any rubber sealed part gets treated with NY-LOG - a terrific sealant. I add new connectors at this point too.
I hook up my vacuum pump and pull an initial vacuum. I stop and watch it to make sure it holds vacuum for 15 mins or so with no loss. If it does I restart the pump and run it for an hour. I feel the hoses and parts to see if any moisture has frozen (it will under vacuum). If I feel cold I stop and let it sit a while and then restart the process. I measure the vacuum and look to pull down to 50 microns or less (a damn good vacuum, near perfect). I have an electronic vacuum gauge that tells me these things.
Finally I connect my manifold set (used to hook up the vacuum pump earlier and monitor the state of vacuum - and at this point hooked into the automobile system high AND low sides) to a can of refrigerant and add liquid refrigerant to the "HIGH SIDE" with the can upside down until the flow stops. Once it has I disconnect it. I hook up a can to the low side port, and place the can in a coffee can of hot water. I can usually now start the engine and there is enough refrigerant for the car to run the AC. The balance of the necessary charge goes in as gas - can upright and into the low side with the car AC operational. NEVER feed a can into the low side upside down. The compressor cannot compress a liquid and a slug of liquid hitting the compressor can damage it. Once I have put in the required amount of freon (from the manufacturer's specs) I'm done.
I check the vent temps, and they are usually around freezing.
So for a usual job, that's it. Compare that to the instructions of your $30 death in a can kit.
Hope that helps.
Chris
