A lot of well-meaning but less than correct information in this thread.
Originally Posted by jectrx7
it sounds complicated cause to do it right there are a series of steps you need to take or you will be doing it year after year. that gets annoying and costs a lot more in the long run
Yes, and it is complicated beyond the scope of this thread; however, not every step is relevant to automotive AC or is reasonably necessary. Don't fix things that aren't broke, open the system unnecessarily or do things arbitrarily.
Originally Posted by therotaryrocket
The most important thing is to pull a vacuum on the A/C system for at least 30 minutes, I'd probably do it for 45 minutes that way almost all of the moisture is allowed to escape. It's recommended to replace your drier and I think you should change to R-134a compatible gaskets and o-rings, but I've done these conversions without changing either and still had great success. Also you need adapter fittings to convert your lines from r12 to r134a, I usually go to NAPA and buy them individually. GL
+1 on the quick connect adaptors used for the 134 conversion. (You can use QC's with any refrigerant.) I just don't recommend a recharge with the usual r134a (for reasons cited elsewhere) as there are better alternatives. To properly install -134 you need compatible oil, materials and barrier hoses as well. R134 is a much smaller molecule than most of the other refrigerants and it will find leaks that you never had with R12. Beyond that, it gets further complicated and 134 performance will never be as good as R12 or the ES brand I-12a hydrocarbon substitute. Additionally, there is no need to replace the dryer unless the system has been open and wet.
Oh yea and don't forget to oil up the system and you might as well add a dye to the oil so you can search for leaks if you have any in the future.
Don't overcharge the system with oil. Only replace oil that is lost, do not add extra and I wouldn't recommend adding dye at this point.
Originally Posted by jectrx7
i would not try that due to the fact that you are not only pulling moisture out of the system but small pockets of refrigerant that settle under the oil. as soon as a pressure difference is made in the lines the refrigerant will flash off and get removed by the pump. do you really want that going into the your engine? i dont. its a good theory though. also a good vacume pump pulls a lot more cfm then the engine does and electricity is a whole lot cheaper than fuel.
Oil is miscible in refrigerant, thus oil will not trap refrigerant. The refrigerant will boil off as soon as you open the system to atmospheric pressure; it may be present in the system as a gas, but it isn't gonna be trapped by oil. Moreover, in order to get a reasonably "hard" vacuum, you need a real vacuum pump. An engine, modified compressor, cheap pump from harbor freight isn't gonna get you anywhere near the 500 micron range that you should see in a sealed & dry system. The compressor shaft seal is probably the most common point of leakage in an automotive system; fortunately the Denso unit used on the FD's has a better seal than most.
Once you've pulled a hard vac on the system for at least 30 minutes, close the valves & shut off the vac pump for 10-15 minutes and see if it holds. It will rise somewhat on a high-vacuum (micro) gauge, but you will not be able to see this on a standard compound charging gauge. From experience, if the needle on your compound gauge stays at about -30" and never rises, you're probably ok. I generally turn the pump back on and vac for a few more minutes. Gettting all of the air out of the system is important for good performance, otherwise the air will get trapped in the top of the condensor and reduce capacity and raise discharge pressures. Shut it off and charge in the first can of refrigerant with the vehicle off. From here it's gonna depend on the refrigerant you use as to how the rest is charged in.