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. Itís 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 Ė 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, letís begin with a diagram of the air condition system
youíd 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 weíve seen the diagram, letís 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 Ė namely the compressor
and condenser. If anyone has one they want to donate Iíll 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
- 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
itís 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
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 isnít a big deal for you, thereís 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 theyíre
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 youíll need will be as originally
specified for your vehicle. However, excessive lubricating oil in the system
can result in warmer outlet air temperatures so donít 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 doesnít 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 youíll 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
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 thereís 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.
Hereís 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 thatís 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 valveís function is really pretty simple Ė 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 Iíve 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, thatís all for now. Iím 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
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