Can you mix R-12 Freon and R-134a? Yes.

Can you add R-134a to R-12? Absolutely.
Can you mix R-12 and R-134a? The answer may surprise you. No doubt this is a very controversial subject, and only you can decide what is right for you and your car. The simple answer is yes, you can mix them. For the most part, you can add a R-12 to R-134a retrofit valve (for about $8), buy a hose with a gauge ($19), and a can of standard R-134a, and simply top-off your system. Of course this is when someone will barge into this conversation screaming, yelling, and declaring that you can't do it, so let me explain.

First we need to accept there's a lot false information on the Internet. This subject is no different. I'll be honest, I don't have a good reason to tell you exactly why the EPA recommended complete retrofits, instead of mixing the two refrigerants, other than the scientists who made the recommendation decided the "average person" might not understand  the complexity of the consequences in such a decision, or simply the powers-that-be anticipated R-12 would disappear from the market quicker than it has. It should be noted that in some stationary equipment, "mixing", is legal and acceptable according to the EPA. The most likely answer is that the EPA simply did not want the new R-134a supply to be tainted by the old o-zone depleting R-12. Remember, most people take their car to the mechanic where he or she recovers the old Freon, but have no way to store or separate the two refrigerants.
Here's the new refrigerant port adapter installed.

So here's the crash course in why your mechanic, and many on the Internet will declare you can't mix R-12 and R-134a. It's not the refrigerants that can't be mixed, it's their respective oils. R-12 uses mineral oil, while R-134a uses PAG oil and the two don't go together. It should be noted Ester oil can be used in both systems, but this fact is irrelevant to our purposes because if you have to purge the system, none of this matters anyways.

However, knowing that it's the oil and not the refrigerants that's the problem (as long as there's enough oil currently in the system to lubricate the compressor), then everything will be fine up to the point that there's not enough oil (which could be years from now) and yes, we can mix R12 and R134a. In other words, if your compressor is working just fine now, but just not cooling all that great, then adding R-134a will work perfect. On the other hand, if your system is completely dry, you should repair your system and likely do the retrofit.
Filling the service port with 134 goodness.

The reasoning for this is that R-134a does not carry the mineral oil. However, having some R-12 in the system will allow the mixture to continue to carry the necessary lubrication until the oil is completely depleted. For instance, even in a slowly leaking air conditioning system where the owner adds a can of refrigerant a year, it may take five or more years before the system leaks enough oil that the compressor fails.

For instance a 50% (above average) rate of refrigerant depletion per year:

Year 1: 50% R-12 and oil remains, system is topped off with 134a.
Year 2: 25% R-12 and oil remains, system is topped off
Year 3: 12.5% R-12 and oil remains, system is topped off
Year 4: 7%R-12 and oil remains, system is topped off
Year 5: Compressor fails because of oil below 3%.

Prior to the upgrade, this gauge never could even get into the blue. 
At this point, the compressor would cost about the price of a retro-fit kit, but you've now had five years. In five years you might crash your car or sell it. Even if you don't, there's an added benefit of an R-12 and R-134a mix, there's actually a performance increase. The two refrigerants together actually exhibit a property together, that they don't separately (sort of the way the colors of blue and yellow make green, but neither of the original colors have green in them). The end result is a capacity increase that actual cools about 5F better than either of the refrigerants do alone.

Okay, so if you've read this far, you probably  want to know how to add refrigerant. I recently did this exact procedure on my 1993 Ford Taurus (3.8 V6). Prior to the conversion, my car only cooled to about 44 F. It could only achieve this temperature under the most ideal conditions and after many minutes of driving. After the conversion, my vehicle reaches 27-28 degrees instantly, exceeding every factory and industry benchmark for cooling performance in the automotive industry. To say that I'm impressed, is an understatement.

What you buy:

  1.  1 R-12 to R-134a retrofit valve. Make sure this is one with a center plastic pin on the female side. This pin presses down on the old R-12 Schrader valve and allows you to retrofit without evacuating the old Freon (remember we don't want to lose the old oil or freon). Some versions of this adapter require the removal of the Schrader valve which would defeat our purpose. You do not need the larger conversion kits, only this single adapter. I used A/C PRO brand, part number VA-1L. I found mine on Ebay, new, for about $8 (seller: kressautochris).
  2. 1 R-134a hose with gauge. These are about $19 at your local parts store and will work on any car with 134 fittings. 
  3. 1 can of plain Jane R-134 (some include oil, we want the one with out for the reasons mentioned previously). $12.
You will also need.
  • Some parts cleaner to clean the old mating surfaces of the R-12 valve.
  • 17mm wrench to tighten the new R-134 adapter.
What you will do:
  1. It's pretty simple, locate the low-side service port and clean with parts cleaner (allow to dry).
  2. Screw on adapter, have the wrench ready to tighten down quickly (to prevent Freon loss).
  3. Attach R-134a can to hose, and pierce can with needle valve.
  4. Connect the host lifting the coupling and firmly press down onto the adapter, then release coupling.
  5. Start the vehicle,  turn the A/C on high, setting intake of air to pull from outside (not-MAX).
  6. Unscrew R-134a valve and allow the system to fill until desired pressure is reached or the can becomes warm to the touch (about 15-25 minutes).
One thing I did notice is that the gauge initially read very high pressure readings during the initial fill (when it was mostly R-12) as the compressor would engage. This caused the compressor to shut off which was what I was experiencing prior to this top-off. After several minutes the readings stabilized, and the compressor started working appropriately without high pressure readings.

While this isn't for everyone, if your driving around an older car but only have mediocre a/c because you refuse to pay for R-12 or a conversion, then this may be the answer you've been looking for.

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