I wanted to share my experience converting my factory air system to 134a refrigerant, hoping it will save you time and money. After I changed the parts, I got my system charged professionally and it works amazingly well, blowing ice cold at the vents. Here's what I did:
I started with a call to Old Air Products (oldairproducts.com) from Texas. I'm usually the guy who researches everything alone and then buys the parts online without ever interfacing with a human, but A/C is one of the only parts of a car that is (was) a mystery for me so I decided to call on the experts. I couldn't find complete information on their website, so the call was essential. They were technically competent and no pressure to sell. I told them I had a non-working factory air and wanted to convert from R12 (I assume that's what was in there) to R134a. The compressor appeared to be original and its internal condition unknown, so a remanufactured one was recommended to avoid problems. I chose an A6 over a modern replacement version (Pro6TEN) since I preferred the stock appearance. Here's the parts list and Old Air Products Part Numbers:
*Note that the compressor had a smaller pulley than original and the receiver dryer was smaller than original. See the "important notes" section below for details and talk to your supplier about options.
I encourage you to call Old Air Products (or your preferred supplier) and talk to them about what you want to accomplish.
Once the new parts arrived, I started by removing the old hoses, expansion valve, hard line, receiver/dryer, and compressor. I flushed out the condenser with AC Pro Power Clean and Flush that I bought at the auto parts store. Also flushed out the hard line from the expansion valve to the dryer (after removing it from the car). I did not flush the evaporator.
After everything was clean and blown out with compressed air, I started assembly. The STV upgrade kit comes with everything you need to change the system to cycling clutch type, so I followed the instructions and everything went well. I made sure the compressor had the recommended amount of PAG 150 oil (10 oz), and assembled all the hoses and parts. The kit contained enough new O-rings to do the conversion 3 times, haha, so don't worry if you drop one into the frame rail.
After everything was back together, I went to Autozone and borrowed a vacuum pump and a set of A/C gauges. Following the instructions, I pulled a vacuum and let it sit overnight. No leaks! Ready for charging!
I wasn't about to charge it myself so I found a reputable local shop and let them do their thing. I mentioned to them that the service manual called for 4lbs (64oz) of R12 for a completely discharged system (as mine was). I also gave him a conversion factor for R134a I found on the internet which said I'd need 53oz of 134a, because apparently less 134a is needed than R12. It turns out that the system stopped accepting refrigerant after 34 oz was delivered, and the system now functions perfectly. I can't explain the shop manuals 4lb requirement nor can I explain the conversion factor an internet "expert" suggested. 34 oz of R134a works so I'm sticking with that!
I took a few measurements to see how the A/C would perform on a hot day. With an outside temp of 88 degrees at around 2:30 with full afternoon sun, I measured 35 degrees at the driver side vent in idle and 32 at the center vent. At cruise speed and at higher rpm, vent temps are much lower - I measured 28 at the drivers vent and 24 degrees at the center! Outside surface temps of the car were over 100 but inside was comfortable.
I've driven the car on a couple of 90 degree days and have been pleasantly surprised that old school A/C in a convertible can perform so well.
A couple of important notes:
I hope this post helps you. Sorry I didn't take progress photos - I hadn't thought of blogging the experience until I was done. I added a few "after" photos below that give you an idea of what the parts look like.
Total cost was $675 for all the parts, and $175 for the refrigerant and labor to charge the system.
If you have any questions or need related info, just ask.
Photo 1: Receiver/Dryer in raw aluminum. Note that the replacement is a shorter "bottle" than original.
Photo 2: This is the remanufactured A6 compressor and the R134a fittings.
Photo 3: Another view of the fittings. I ordered 90 degree adapters because of the size of the gauge and charging system connectors. I didn't think straight fittings would allow those connectors to fit.
Also note the writing on the low pressure hose.
Photo 4: The modified STV (left) looks original from the outside, as does the expansion valve. The silver relay on the right is the compressor cycling relay. When the surface temp of the evaporator outlet gets too cold, it shuts off the compressor. Seems to activate somewhere between 15 and 20 degrees.