Below is a little more information about the differences between R12 & R134a. It's from .

Refrigerants R12 vs. R134a
What is R134a ?
R134a is the most widely accepted alternative automotive refrigerant used to replace R12. There are other refrigerants: some of which may work, some of which don't work, some of which will destroy your a/c system and some of which are dangerous. The problem with these so called "drop in replacements" is that to properly handle them you may need dedicated equipment to indentify the gas and evacuate it for proper disposal. This can be very very expensive. On this website we are addressing only R134a and not other suggested "drop in" replacements. So to keep things simple and to avoid controversy and politics we have R134a. At this time you can still find R12, but, because it is no longer produced, availability is limited to current inventories and the price per pound is greater than R134a.

Should I use R134a or R12 ?
The choice is up to you. Hopefully after you have read this entire page you will have enough information to determine which type of refrigerant is best for you. If your a/c system is empty/low on refrigerant, if your system has a leak or if you need to open the system, then now is the time to decide which refrigerant you want to use. Your decision could be made based on economics, environmental concerns, or both. If you have a perfectly operating system using R12 you do not have to drop what you are doing and convert to R134a. Just leave it alone for now.

Is R134a less efficient than R12 ?
R134a is NOT less efficient than R12. Actually R134a is more efficient . Pound for pound R134a is a more efficient refrigerant than R12, however it runs at higher pressures in some aspects and therefore requires more effective condensing. Whether R134a performs as well as R12 in any given a/c system depends upon system components and the amount of R134a used. Given two identical vehicles, each with the same weighted amount of refrigerant, the vehicle with the R134a has the "capability" to remove more heat (measured in btu's) from the vehicle than the same type of vehicle using the same amount of R12. The most common influences which effect the capability of R134a to perform well are the condenser, in some cases the the superheat setting of the expansion valve or the amount of R134a. Condensers designed to release greater amounts of heat help to expel the greater amount of heat which R134a removes from the car's interior. And by "matching" the correct amount of R134a to use in a given vehicle, correcting the superheat of the expansion valve (if necessary), you can in some manner nearly balance or match the amount of heat drawn out by the evaporator and released by the condenser. These efforts to "balance" the system can not be realized if there are problems within the a/c system, such as: poor performing compressor, dirty condenser or poor air flow through the condenser, malfunctioning expansion valve, water or air in the system, improperly operating fresh air or heat input in the climate-air mixing system.
The common problem when converting from R12 to R134a is lack of knowledge or equipment. The typical scenario here is when the car owner buys an inexpensive "retro-fit" kit from the local autoparts store. These cheap kits usually have a few cans of R134a (either 12 ounce or 16 ounce), maybe an extra can of refrigerant oil, a few generic R134a charge port adapters and maybe some orings. And, unforntunately a small high side pressure gauge on a short hose. What is missing from the package? A vacuum pump, a refrigerant recover system, quality service gauge set with both low and high side gagues, and much more besides the lessons to learn what to do and what not to do. This is not to say that one of these small retro-fit kits can't be used, however to be used wisely.

But I hear bad stories about converting to R134a
Yes there are some sad stories and unfortunately we only hear one side of the story, the part told by the installer or car owner. The side of the story you will never hear is the from the "a/c system". Don't laugh. Here are some typical stories about failed attempts to convert to R134a:
"My system ran out of R12 so I put in three cans of R134a and the following month my compressor froze and locked up"
Chances are this car owner: (a) lost most of his system oil when the R12 leaked out of the system and lack of oil kills a compressor quickly, (b) "he" over-charged the system using "three cans" of R134a; he did not evacuate the system to remove air, he did not determine how many ounces to use, he did not replace the drier, he did not remove the old oil and may not have used the correct replacement oil, (c) he could not weigh the correct amount because he did not have a dedicated refrigerant weight scale, (d) he probably did not have air conditioning gauges to check the system pressures while he charged it, (e) he thought his system was full when he could not see any bubbles in the drier site glass and (f) I wonder if he removed the old R12 mineral oil from the compressor, replacing it with ester, or he put a new drier in the car. So if you are considering buying that "save dollars and buy your own R134a cans" at the local auto parts store, forget it. And if you do succeed one time or if your buddy did it to his car, you would be smarter if you put your money down at the counter at a dog race. Unless you want to change your career and become a full time AC technician you'd be better to let the experts handle the problems.
"My repair shop converted me over to R134a in the spring, in this hot August weather it's not as cold as the R12 I use to have"
If the repair shop did everything "right" the problem may be elsewhere. A few months have gone by since the car was converted. I wonder what the temperature is outside the car and what the temperature is coming from the car's interior ac outlets is? Could be there is a slow system leak and the car has lost enough refrigerant to work poorly during hot days. Is the climate control air mixing system working properly.... too much outside air mixing with the a/c air, or maybe malfunctioning heater valve.
"I've got a black 930 with a black interior. Down here in Texas when it's 110 degree's the R134a conversion just don't work"
Well we could tell him to: (a) trade the black car in for a silver colored one, (b) paint the black leather white or (c) mirror tint the windows. And we will not argue whether or not the a/c technician did the job correctly. What this fella needs to do is to remove the oversized turbo inter-cooler and put in a larger condenser. So here we draw the line as to which refrigerant to use because there is no way this 930 car owner is going to give up horsepower for cooling power. He could either install another condenser as we did in our Mr. Ice Project, or stick with R12.

What are the basic steps to converting to R134a ?
It's not that difficult but the procedures of evacuation, charging and testing should be done by a licensed a/c technician that is experienced with R134a and your particular car. Here are the basics for converting a system that does not have a charge of R12, it's empty :
1. Unplug the a/c's clutch wire
2. Loosen the a/c belt
3. Disconnect the two hose lines to the compressor, protect the ends from getting dirty.
4. Remove the compressor.
5. Remove the old mineral oil from the compressor by turning it upside down, side to side, and turn the front hub on the clutch assembly.
6. Recharge the compressor with a/c ester oil and a "tracer dye". Typically 5 to 6 ounces of a/c oil is adequate but this may vary depending upon the car.
7. Put the compressor back on the car.
8. Reattach the hose lines using new R134a compatible o-rings, lightly lubricate with compressor oil.
9. Reattach and tighten the a/c belt.
10. Plug in the a/c clutch wire.
11. Attach the R134a charge port adaptors, there are two types.. one of which you need to remove the old R12 valve cores.
12. Replace the drier with a drier that has a desiccant that is R134a compatible, use new R134a o-rings.
13. If your car does not have a high pressure cut-out switch in the AC system then you'll need to have one installed (call us if you need help).
At this point it is time to make an appointment with the a/c shop. The shop will:
14) Do a pre-inspection and then Evacuate (vacuum down) the system, the longer the evacuation the better but there is a point of diminishing return, 2-3 hours is generally very good.
15) The system will be checked for "gross" leaks while under vacuum.
16) Generally the system will be "initially" charged with approximately 80% to 85% of the original amount of R12, using R134a. For example, if your system originally had 36 ounces of R12 then you multiply that by .80 and you have 29 ounce of R134a. The exact amount will vary with each car.
17) After the "initial" charge the system is then "tweeked" : you determine the optimum amount of refrigerant to match your system by monitoring the a/c outlet vent temperature and the high side pressure as you add or remove refrigerant. When the ambient air temps get above 80F the gauge readings can run a bit higher on the high side and lower on the low side when using R134a. You do not want to exceed the system's high side recommendations. The safe high side pressures are determined with a "PT" (pressures and temperatures) chart. This chart has a column of "ambient" (outside) air temperatures on one side and a corresponding range of high side pressures on the other side. The higher the ambient temperature the higher the high side pressure. By noting the ambient temperature you can look up what the preferred high side pressure should be.
18) Since you will be using less R134a than R12 you do not want to rely on the drier's site glass to determine if the system is full. With R12 you use to check to see if the site glass was "full", if it had bubbles it meant you had a low charge. With R134a you do not rely on the site glass.
19) After the optimum referigant charge is determinend the car should be test driven to check for performance of the system.
20) After the system has been tested out it should be checked for leaks. When you add fluorescent "tracer dye" to with the refrigerant oil it helps to track down leaks that push oil out of the system. By using a "black light" the dye will glow in areas where oil leaks are visible. The next piece of equipment to track down leaks is a "leak sniffer". This is an electronic tool that sucks in air through a tube and across a sensor. When the sensor detects the refrigerant leak the sniffers lights glow and the sniffer makes a noise. What is usually "sniffed" are all the connections in the system: hoses and compressor, condenser, drier, expansion valve and evaporator fittings. Generally you follow the refrigerant flow moving from connection to connetion, component to component, and you sniff the a/c vents to check for evaporator leaks as well.
21) To "bless" the system the technician should attach a R134a sticker somewhere in the engine compartment to note the car is using R134a. The sticker typically has a place to note the amount of refrigerant used, the type of oil and amount of oil, the technicians sign off or name and the date of the conversion. When ever possible you should ask the technician to note the same information on your repair bill and keep a copy in your glove box for future reference.

A) Though you can purchase 12 or 16 ounce cans of R134a from local retailers it is very difficult to measure out the correct amount into your system. By using a qualified a/c shop with the proper equipment you odd's are in your favor of having the job done correctly the first time. You may have heard of a friend that did the job himself with borrowed equipment and 12 or 16 ounce cans of regrigerant. Statisically your friend was lucky. If you you don't have the free time to "play" then you are better off to pay a technician.
B) If you are replacing other system components, such as condensers, driers or hoses, you may need to add more a/c refrigerant oil to the system. Before we mentioned that you can add approximately 5-6 ounces of ester oil to the compressor. Generally this works well for the whole system. We say "generally" because the vehicle manufacturer may have a formula already worked out for the R134a conversion process. They might say for instance, add 3-4 oz of oil to the compressor, 1 oz of oil for when you replace a drier, 2 oz oil for the evaporator and so on. We chose to use 5-6 ounces of oil to be on the safe side: 3-4 ounces for the compressor, 1 oz of oil for the drier and another ounce of oil for "system losses". You do not have to split up the 5-6 oz of oil and add it to each component, just simply put it all in the compressor as it will move throughout the whole system once the system is operational.
C) If your compressor requires replacement or its seals and o-rings need to be replaced due to leakage, it would be wise at this time to either obtain a compressor that has o-rings and seals that are compatible with R134a or have your unit remanufactured with compatible components. You do not however "need" to replace all the o-rings or seals for a simple conversion, it is simply a logical thing to do when and if the time comes to work or replace certain components because of age issues.
D) You do not need to replace every o-ring in the system, but if you happen to open a connection that has an o-ring you should replace the old o-ring with one that is compatible with R134a and lubricate the o-ring with ac oil.
E) We mention using "ester" type refrigerant oil. Some compressor manufacturer's or OEM's suggest PAG type oil for R134a. But some types of PAG do not mix well with the old residual R12 mineral oil left in the system components whereas "ester" does mix very well.
F) You do not need to replace the expansion valve in most cases. The original expansion valves work very well with R134a. But, if your old compressor has lock-up or if your current expansion valve is not functional it would wise to replace the expansion valve with a valve designed for R134a.
G) The original rubber a/c hose lines in your car will leak out R12 through the hose's lining. R134a has a smaller molecule than R12 so R134a will leak out sooner than R12. So if you car has a large amount of rubber hose, such as a 40 feet with the 911, then you will be re-charging the system often and that is expensive no matter which refrigerant you use. To prevent refrigerant loss you should consider replacing your old hose lines with "barrier" type hose.

So should I use R12 or R134a? If your car's system has a sufficient condenser design and air flow to expel the greater amount of heat extracted by R134a, then by all means you should convert over when the time comes. But, if your car lacks enough condenser and you have many 100 + degree days then you may want to stick either increase air flow, add another condenser as we mentioned in The Mr. Ice Project or stick with R12. For those of you who are considering what the cost is for R12 you can think of it this way: "Don't go out to dinner this weekend. Stay home and grill your own steaks and stir your own martini. The money you will save by will pay for the R12".

What about this other refrigerant I saw advertised?
Besides R134a there are some other refrigerants that are "approved" to replace R12 and there is a long list of refrigerants that are "not approved". The approval is very legal one, governed by the U.S. EPA. Failure to abide by the EPA regulations can result in expensive fines for either you or your service technician.

Approved list link is

The problem with using an approved alternative refrigerants other than R134a is that when the system is serviced it requires certain types of fittings, labels and dedicated equipment. For instance you can evacuate FRIGC FR-12 into a refrigerant recovery bottle that has R134a in it or visa versa. This would become an expensive investment for a repair or service station to have unique dedicated equipment for everytype of alternative refrigerant.

Non approved list link is:

We consider the the various types alternative refrigerants that are illegal or not approved to be in two sectors: non-flammables and flammables. The non-flamables can have R22 type gas which is not compatible with most rubber hoses or they have gases that contribute to global warming. The flamables have hydrocarbons.