The “proper” way to do a conversion is NOT the only way.
THE reason that you have to remove all traces of R-12 in a system is because the introduction of R-134 turns the R-12 traces to glue. This is in fact, a good thing, as hoses now become sealed up, instead of leaking even more like was previously believed.
It is a myth that all of the O-rings need to be replaced. Also, the oil is the one of the biggest issues, so:
1. Do NOT EVER get a new “R-134” compressor just because you had an R-12 compressor. It’s the same compressor as an R12 compressor, but it has a different oil charge from the factory. It’s a simple job to flush the compressor with alcohol.
2. The same applies to the condensor and all of the other parts, except the receiver/dryer – all of the system can be flushed with alcohol (denatured). It is NOT necessary to replace major parts of the system because you want to convert it. (The residual alcohol boils out when you pull a vacuum on the system – it has a high vapor pressure.)
3. Put in the required amount of Ester oil, usually about 6 oz. This is synthetic, and is THE perfect replacement for the old crappy mineral oil (R-12) and PAG (R-134).
4. Replace the Receiver/Dryer. It’s cheap, and it affects performance.
5. If you put in R-134, fill with a charge equivalent to 80% of the R-12 charge, which is 28-32 oz. for ’91-’93 MR2’s.
6. Your vacuum pump may not pull to 30 Inches of Mercury (not PSI). Any reasonable vacuum below 20 in. Hg. will do fine. The real trick here is to boil out any residual moisture. The greater the vacuum, the more effective the procedure.
7. Connect both manifold hoses when you get started. There’s less chance of getting air into the system if you pull a vacuum through the whole system, all hoses included. (Open both manifold valves.)
8. A properly charged and functional MR2 A/C system will in fact cool pretty well with R-134. I have proof of this with a few MK2’s.
9. If you really want a fluorescent dye added, get one that’s Ester oil based, as it will work in almost any system.
10. Ditch the crappy R-134!!! It’s molecules are smaller (so we are told) than R-12 molecules, so it leaks out faster. (This is one reason for the O-ring story – the newer O-rings are supposed to seal “better”. Maybe…)
ES-12a, manufactured by EnviroSafe in Pekin, IL, is THE stuff to get. It is:
1. Non-ozone depleting
2. Non-toxic (well, not severely brain damaging like R-134)
3. Flammable (but not like propane is)
4. It’s only a mail order product for most of us
5. You use less than R-12 or R-134
6. It’s a petroleum product, so it’s molecules are larger than R-12 and R-134 molecules, so it has less chance to leak.
7. It is compatible with almost every A/C oil out there – no need to change oil.
8. For 12 bucks, you can add a can of ProSeal to fix most small leaks.
8. COLD. No, I mean COLD. I HIGHLY recommend the Industrial ES-12a, as it gets colder than the regular version. MR2’s will cycle off about 42 degrees F (measured at the center duct). My Lexus SC300 goes down to 31 degrees. The colder it gets, the more efficient it is, and the less it needs to work to get your car cold. My MR2 has will pull the duct temp up to 50 degrees below the inlet temp. R-12 and R-134 will not ever do that.
I’ve put this stuff in so many cars, I’ve gone through two cases of cans and a 30 Lb. tank.
AND, don’t forget to check your heater water valve to make sure it’s closing all the way. (The heater core lines should be cold to the touch.)