Updating Air Conditioning from R12 to R134a
With the engine compartment wiring and vacuum lines now installed I decided it was time to give the air conditioning (A/C) system a bit of attention. However, many of the original A/C components were in pretty tough shape and well beyond repair. In addition, the factory system was set up to use R12 coolant, otherwise known as Freon. So, since I had a fair bit of work to do on it anyway, I decided to convert the A/C system from Freon to R134a, a more modern and environmentally friendly refrigerant. It’s worth noting that, if your existing R12 system is in good working order, leave it alone. A swap to R131a is only necessary if your system needs a major overhaul or has been open to the atmosphere for quite some time.
To start this process I needed to educate myself as to the various components of the A/C system, their role in the cooling process, and whether they would work properly with R134a. Keep in mind that the information presented in this update is specific to a 1971 Corvette – you may find some variations if your Corvette is a different model year. General Motors changed the A/C system numerous times throughout the C3 generation.
So, let’s begin with a diagram of the air condition system you’d find in a model year 1968-1972 Corvette.
The cold air that blows through your vents is generated by an A/C system consisting of several key components which are shown in the diagram above. The process starts at the compressor where power from the engine is used to compress the refrigerant to a small fraction of its original volume. Immediately after being compressed the refrigerant passes through the muffler, a large silver cylinder located immediately behind the compressor, which quiets the operation of the system.
The process of compressing the refrigerant increases its temperature significantly so, before it can be used to generate cold air, it must first pass through the condenser which is mounted in front of the radiator. The condenser looks and operates just like a radiator. As the hot, compressed refrigerant moves through the condenser its gives off its heat which causes the refrigerant to transform from a high temperature, high pressure gas to a liquid.
Once through the condenser the refrigerant is fed through the receiver/dryer, which acts as a reservoir of liquid refrigerant and also traps any moisture that may be in the system. This is an important component since any moisture in the refrigerant would freeze in the evaporator and cause the system to stop functioning properly.
From the receiver/dryer the refrigerant travels through the Expansion Valve which regulates the flow of refrigerant into the evaporator and thus, the cooling. After passing through the Expansion Valve the liquid refrigerant turns into a low pressure gas in the Evaporator and, in the process, absorbs heat which drastically lowers the temperature of the Evaporator. The blower motor then passes air over the evaporator which generates the cold air that comes through the cars vents.
The passage of refrigerant through the evaporator is actually a delicate balancing act between the Expansion Valve and the Suction Throttling Valve, also referred to as a POA valve. Together these two valves maintain 29.5 psi of pressure in the evaporator for maximum cooling efficiency while keeping the temperature of the refrigerant above freezing. The latter is maintained by a temperature sensor in the POA valve which shuts the compressor off if the temperature gets too low.
The POA Valve and the Expansion Valve are also connected by a thin copper line called an Equalizer Line. In addition, the expansion valve has a temperature probe connected to the Evaporator outlet. As the Evaporator outlet temperature drops, the valve will close, thus decreasing cooling. This prevents Evaporator freeze-up.
Lastly, there is an oil return line from the bottom of the evaporator connected to the Suction Throttling (POA) valve that prevents compressor oil from getting trapped in the evaporator.
Finally, the low pressure gas enters the compressor again and the cycle repeats
Now that we have a basic understanding of how the A/C system functions, and we’ve seen the diagram, let’s look at a few photos of the engine compartment to see what these components really look like. Note that not all of the components are installed in the photos below – namely the compressor and condenser. If anyone has one they want to donate I’ll gladly take them off your hands and give you a shout out in my next update! 🙂
This first photo is of the passenger side firewall / inner fender area showing the following components:
- Yellow arrow: Compressor Bracket and Muffler (compressor not shown)
- Red arrow: Expansion Valve
- Blue arrow: Suction Throttling Valve / POA Valve
- Green arrow: Evaporator Outlet Tube and Temperature Probe (under insulation)
This next photo is taken near the front of the passenger side fender just ahead of the radiator frame. A few more components can be seen here including:
- Yellow arrow: Inlet to condenser (condenser not installed in this photo)
- Pink arrow: Receiver / dryer
Additional photos of these components in their final condition are replaced a bit later in the update so keep reading!
With the basics of how the air conditioning system works it’s important to understand which stock components are compatible with R134a refrigerant, and which need to be replaced. It turns out that most of the original components are forward compatible with the exception of the receiver/dryer, o-rings and the mineral oil lubricant used with R12. This means many of the original components can be reused but they must be individually drained and flushed cleaned of any mineral oil lubricant prior to the conversion.
In the case of my project several additional components were replaced. The original system had been open to the atmosphere for quite a long time and they just looked really rough. Ultimately the following components were purchased to support my A/C conversion from R12 to R134a:
- Rebuilt compressor and condenser (these were missing from my car)
- Receiver/Dryer (with R134a compatible desiccant)
- Expansion Valve
- A/C insulation tape (used to insulate the expansion valve temperature probe)
- POA Valve
- Low / high side adapter fittings
- Assorted O-Rings manufactured for R134a A/C systems. R134a refrigerant O-rings are of a different composition than those for R12 systems. R-12 O-rings are black in color. R-134a O-rings are blue or green in color.
- Ester oil charge for lubrication (amount varies by model year). You can also use PAG (Poly Alkylene Glycol) or Castrol Retro Oil for lubricant.
- R134a refrigerant (amount varies by model year).
Note that most of these parts can be bought much cheaper through a local auto parts store (e.g. Napa or AutoZone) rather than through a Corvette parts house. They may not be exact replicas from an appearance standpoint but, if originality isn’t a big deal for you, there’s an opportunity to save 30-50% on these parts.
Once you understand the system, and have the necessary parts on hand, the steps for completing the conversion are generally as follows:
- Remove any remaining R12 refrigerant in the system by taking your vehicle to a licensed air-conditioning professional. Releasing R12 directly into the environment is dangerous, harms the environment and is punishable by law.
- Inspect and make necessary system repairs to all components to remain. Check the condition of expansion valve, POA valve, condenser, evaporator, etc. If they’re found to be bad or in questionable condition, replace them now. Oil and dirt accumulate at leak points so pay special attention to any suspect areas where grease and grime have accumulated.
- Completely flush out all components to be reused with MEK or ethyl alcohol. The mineral oil used to lubricate the original Freon-based R12 system is not compatible with R134a and will cause a reaction if mixed. This may damage your system.
- Put half the required new lubricant in the compressor and half in the receiver/dryer. The amount of lubricant you’ll need will be as originally specified for your vehicle. However, excessive lubricating oil in the system can result in warmer outlet air temperatures so don’t overdo it. The quantity of oil needed for various model years is as follows:
- Install the new receiver/dryer.
- Install the new receiver/dryer.
- Replace all o-rings in the system with properly lubricated R134a compatible pieces.
- Use a tire valve tool to remove the schrader valve in the muffler and hose assembly. This type of valve is not compatible with R134A systems so they need to be replaced with adapters. Place the high side and low side retrofit fittings over the old fittings and use a wrench to tighten them to about 20 ft-lbs of torque.
- Verify all connections are properly made and tightened and then have the system vacuumed and checked for leaks. If you pull a vacuum on the system yourself be sure to use the proper equipment in accordance with the manufacturers instructions. Pulling a vacuum of 29.9 inches is recommended and will vaporize any moisture in the system while also purging any air from the A/C system. The vacuum should be held for one hour once the vacuum has been achieved. After evacuation, shut all of the valves tightly and let the system sit for a while. The vacuum should hold. If it doesn’t it means you have a leak somewhere that will need to be found and repaired before you can charge the system.
- Fill the system with R134a refrigerant. Because the type of refrigerant is being changed you’ll use about 8 ounces less R134a refrigerant than R12. The actual quantity of refrigerant used will be determined by taking pressure readings throughout the charging process. The quantity of refrigerant needed for various model years is as follows:
2 pounds 12 ounces
2 pounds 8 ounces
2 pounds 4 ounces
1 pounds 12 ounces
During the charging process take periodic pressure readings to help you determine when the system is full. The readings at 90 degrees ambient temperature, 100% relative humidity, and with the motor idling at 1,000 RPM should be 350 psi on the high side and 55 psi on the low side.
- Once charged complete a thorough check of the system for leaks. Refrigerant is heavier than air so pay extra attention to the bottom of each A/C component. Also check for evaporator leaks at the drain hole in the bottom of the heater case.
- Place a retrofit label, showing that you have retrofitted the vehicle for R134a, in an obvious visible spot in your engine compartment.
So there’s the process. Now a bit about the work undertaken to convert my C3 to R134a refrigerant.
The original system was in pretty rough condition with missing components, paint overspray, grease and grime everywhere. To properly clean all of the components, and to verify everything was functioning properly, I removed the heater / air-conditioning box from the firewall. In addition, it also allowed me to verify the evaporator was intact and free from any obvious leaks.
While cleaning up the parts of the A/C system that were to remain the replacement components I ordered arrived at my doorstep. These included a new POV valve, insulating tape, receiver/dryer, new high-side and low-side retrofit fittings, o-rings and a new expansion valve.
After reinstalling the A/C box the new replacement parts were also installed. The replacements were all direct fit parts so it was just a matter of installing new gaskets and tightening the various fittings. It was really a piece of cake. The hardest part was keeping the greased o-rings clean and in place while assembling and tightening the hoses. A few photos of the finished system are provided below. I still need to install the condenser and compressor, but those will be put in once the body is installed back on the chassis and the radiator is in place. Below is a photo taken from within the engine bay looking forward through the passenger side of the radiator support. Note the receiver/dryer located in the lower left of the photo.
Next is a photo of the heater / air conditioning box located on the passenger side of the firewall.
Here’s a close-up of the new expansion valve and POV valve. Note that I simply cut the two green wires to the POV sensor for now. After the body is lowered onto the chassis these wires will be spliced into the factory wiring harness.
The expansion valve temperature probe, located against the evaporator outlet tube, is secured in place by wrapping it and the outlet tube with self-adhesive insulation tape made specifically for A/C systems such as this. The insulating tape can be seen in the photo below.
And that’s about it for converting the A/C system. The system still needs to be checked for leaks and then filled with new R134a refrigerant, but that will need to wait until the compressor and condenser are installed. For now the lines While working on the A/C system I also took a bit of time to clean-up and re-install the heater control valve. The valve’s function is really pretty simple – when vacuum is applied the valve opens and allows hot coolant to enter the heater core. Without vacuum the valve closes which essentially turns off the heat. Once cleaned the operation of the valve was checked and then installed in its proper location on the firewall.
And lastly, before wrapping up this update, here are a few before and after photos of the A/C system. Once in a while I do this to keep my motivation up- it allows me to see how much progress I’ve made. After taking the time to dig through my photos I figured why not share them here as well. So, here they are! Heater valve and lower A/C hoses:
Passenger side fender and A/C components:
Well, that’s all for now. I’m currently working on preparing the chassis and body for the final body drop. It will be nice to finally have them both together once and for all. Plus it will free up a bay in the garage so my wife can park her car inside again! If you have any questions about this update please feel free to post them on the Corvette Restoration Forum! You can also Like this project on Facebook to receive the latest news regarding project developments, updates, and progress