Upgrading and Repairing C2 & C3 Corvette A/C systems

Corvettes are hot cars in more ways than one. Their compact design places a big V-8 in close proximity to the passengers. Keeping cool inside is often a challenge, especially for the early generations. During one summer drive to Florida in a 1970 big-block that had almost nonexistent A/C, my passenger insisted on a bag of ice to place her feet on. Getting your Corvette’s air conditioning to work effectively is a much better solution and is the focus of this article.

The first year for factory air conditioning in Corvettes was 1963. The design evolved during the C2 and C3 generations, becoming a pretty robust, reliable system for later Corvettes. Here we’ll be looking closely at restoring and updating the earlier systems, but much of the following information is applicable to all years and all cars.

The basics of air conditioning are discussed below before explaining the evolution and differences in the A/C systems in the early Corvette generations. Changes were made in refrigerants, lubricants, O-rings and hoses. Those are described, too. Then we’ll show the options for upgrading and modernizing C2 and C3 A/C systems along with how to restore the performance of original systems.

Air Conditioning Basics
The auto air conditioning system is relatively simple. A gaseous refrigerant is compressed by the compressor, then condensed into liquid in the condenser and finally evaporates back to a gas in the evaporator. Those are the three main parts and their names actually tell what they do. These key components are connected by hoses and tubes. Oil lubricates the compressor, a drier absorbs any troublesome moisture and valves or switches regulate the flow of refrigerant through the evaporator.

The refrigeration principle is also simple and is something we’re all aware of in daily life. When the molecules in a liquid change to the more frenetic gaseous (vapor) state, they absorb heat. You experience this directly whenever wearing a wet tee shirt on a breezy day. You are cooled as the water evaporates.

In an automotive A/C system, the refrigerant vapor is compressed by the compressor. This compression heats the vapor, which then flows to the condenser. The condenser looks like a smaller radiator and is located in front of the engine coolant radiator. As air is drawn by the radiator’s fan through the condenser’s fins, the hot refrigerant vapor is cooled and condenses into a liquid. The liquid refrigerant flows to the evaporator, an even smaller version of a radiator. The evaporator is located out of sight in an airbox either outside the passenger compartment (as in the case of early Corvettes) or inside the passenger compartment. Just before entering the evaporator the liquid refrigerant is forced through a valve or orifice. The pressure is lower on the evaporator side causing the liquid refrigerant to vaporize and cool the evaporator. The cold air you feel is chilled as it by passes over the fins of the evaporator.

Changes in Corvette A/C
The major components in early Corvette A/C systems remained little changed. The main difference in parts from generation to generation is in the valve or orifice used to regulate the amount of liquid refrigerant going into the evaporator. This has to be regulated because if too much refrigerant is allowed into the evaporator, it can cool so much that it becomes plugged with ice and no longer efficiently cools the cabin air passing over it.

C2 Corvettes used a POA (Pilot Operated Absolute) suction throttling valve. This changed with the C3 to a VIR (Valves-In-Receiver), a single unit housing a POA valve, thermostatic expansion valve and the receiver/dryer. These valves were an attempt to compensate for the widely varying altitudes and temperatures in which auto A/C systems operate.

However, these valves can be a problem in older systems that now have contamination, debris or corrosion internally. Most modern A/C systems have become somewhat simpler. Instead of valves, they use a small orifice tube before the evaporator. When too much refrigerant enters the evaporator, a temperature sensor switch cuts power to the compressor clutch. When the evaporator’s temperature returns to its safe zone, the compressor kicks in again. The cycling of the compressor on and off also helps fuel economy.

R-12, R-134a and Beyond
A major change to automotive air conditioning systems was in the refrigerant itself. The original R-12 chlorofluorocarbon (CFC) refrigerant had no performance problem but it was discovered that the widely used CFC’s were building up chlorine in the upper atmosphere, which eroded the ozone layer that protects the Earth from harmful UV radiation. Within several years of this discovery, nations got together to ban production of R-12. The good news is that this change worked: the ozone layer is healing.

R-134a was chosen as the new automotive refrigerant and its operational parameters were similar to the R-12 it replaced. R-134a worked well for decades but it is now planned to be phased out by 2021 in light duty vehicles because of a new environmental concern: global warming. R-134a produces a greenhouse gas that is 1,430 times as detrimental as the main culprit, carbon dioxide. Although its use in new cars is being phased out, R-134a is expected be available for quite some time.

HFO-1234yf is currently the new automotive refrigerant. While it costs more than the R-134a it replaces, its effect on global warming is much less than R-134a. Its operational parameters are nearly identical too, so only minor changes were needed to automotive air conditioning systems. HFO-1234yf is already being used in many new cars. It is slightly flammable but keep in mind the oil mist released in any refrigerant line rupture can also be flammable.

A number of other chemicals can be used as refrigerants, including propane and ammonia. Nobody wants ammonia or propane fumes leaking into a passenger compartment or releasing during a collision. Also be aware that there are other chemicals being marketed as a less expensive refrigerant replacement for R-12.

Changes in Oils, O-rings and Hoses
R-12 systems used a mineral oil for lubrication and mineral oil is often said to still be the best oil for use with R-12. However, mineral oil doesn’t play well with R-134a. Automotive manufacturers switched to a PAG oil with the change to R-134a. However, a number of people prefer not to use PAG oil for repair or replacement work due to concerns about it not mixing well with any residual mineral oil in the system and PAG’s harshness on paint and skin. For those reasons, a POE (ester) oil is often used. POE oils are compatible with both R-12 and R-134a and mix well with mineral oils.

Changing to R-134a, PAG oil and higher temperatures also spurred changes in the materials used in O-rings and hoses. The traditional black NBR (Buna) O-rings are commonly replaced by HNBR (Hydrogenated NBR) rubber, frequently colored green. Two cautions: not all green O-rings are HNBR and not all HNBR O-rings are green. Tip: be sure to save the old O-rings to compare their diameter and thickness with the replacements. Also, O-rings packaged by an air conditioning specialist are the safest bet for A/C systems. Really finicky repairers will also make sure the O-rings are relatively new because all rubber products age and O-rings have a shelf life, at least for military and aerospace applications.

When R-134a first came on the scene, it was believed that this new refrigerant would slowly seep through older rubber hoses. This appears to be an unfounded fear but in any case replacement hoses are likely to have a barrier layer to better prevent refrigerant seepage. When you are not replacing the A/C hoses, be sure to closely inspect any portion of the rubber that is near an exhaust manifold. If any cracking or other signs of rubber deterioration are evident, replace the hose.

Changing R-12 systems to R-134a
Let’s start with a disclaimer. There’s been surprisingly little independent, verified testing comparing the performance of the various automotive A/C components. Almost all reports are anecdotal, often with too few specifics to make a solid comparison. Therefore we will have to resort to qualifications such as “It is commonly said …”. But one indisputable fact is that R-134a is more easily available and costs much less than R-12. R-134a is even available at Wal-Mart for as low as $6 for a 12-ounce can. R-12 currently costs about $30 online for the same size can.

A system designed to use R-12 is likely to cool better using R-12, all things being equal. That said, many older R-12 systems have been successfully converted to use R-134a. The first thing needed in the conversion is to flush the system to remove any of the mineral oil used with R-12. R-134a-compatible O-rings and compatible oil should be used and it is desirable but perhaps not essential to change the rubber hoses. Optimum cooling is achieved when the POA valve is recalibrated for R-134a. Other upgrades that may improve A/C cooling are a parallel flow condenser, a modern compressor and electric radiator fans.

R-134a systems also need to use a different desiccant, such as XH-7 or XH-9. The original desiccant (XH-5) used in R-12 systems is broken down by R-134a and its oil. If you find a N.O.S. desiccant bag, don’t use it in a R134a system. The XH-7desiccant works with both refrigerants. Zip Products offers this in their VIR Maintenance Kit. Zip also offers a R-134a Conversion Kit that includes new Schrader valves, flush solvent, ester oil and a retrofit guide to help with the conversion. Plus, they have an inexpensive kit of O-rings for Corvette A/C systems that are compatible with all oils and refrigerants.

A/C Upgrade Options
Corvette owners have a number of choices for modernizing older A/C systems. Here are some, starting with the easiest and least expensive. Zip Products offers a VIR Eliminator Kit to replace the complexity of the VIR assembly with the simplicity of an orifice tube. The kit includes an accumulator/drier, orifice tube and thermostatic switch. A more extensive kit is available that also includes a new evaporator and all hoses.

For those who want to modernize their A/C system but don’t want to go to the extent of disassembling the dash or replacing the evaporator, more efficient components are available. Parallel flow condensers are said to offer much better cooling and are often recommended, particularly for R-134a conversions. Smaller lightweight compressors such as aluminum Sanden units have a good reputation and are designed to handle the cycling on and off of orifice tube systems. Zip Products offers over 200 A/C parts for C3 Corvettes alone. Local automotive A/C repair shops can custom-fabricate hoses if needed.

A complete modern A/C system kit is also offered by Zip. It features the Vintage Air unit that moves the evaporator into the passenger compartment completely behind the dash. The engine compartment will have a lot more space because the big evaporator box is no longer needed. The appearance of the interior remains the same. The factory air vents and thumbwheel controls are retained, but the controls are now electronic. The kit also includes a parallel flow condenser with all hoses and a modern, smaller lightweight Sanden compressor with brackets.

This amounts to a complete new modern air conditioning system for your Corvette and cools very well using R-134a. Kits are available for both cars that came with A/C and those that didn’t. The main difference is a larger firewall block-off plate for cars that came equipped with A/C. Therefore if your factory A/C was removed, make sure to order the kit for A/C equipped cars.

New Life for Factory A/C
Authentic restorations are required to stick with the factory A/C system. This is also a viable choice for Corvettes when their evaporator, condenser and compressor are in good condition. Whether sticking with the factory system or upgrading, if the system has not been working for a while or when any components are replaced, the receiver/drier or desiccant (in a VIR assembly) should be changed. Zip Products offers a VIR rebuild kit that includes a R-134a compatible desiccant, pickup filter screen and O-rings. After removing a splash guard, these parts can be installed from under the car

System components should be flushed with an A/C solvent to remove the old mineral oil if changing to R-134a, and or to remove contaminants if the compressor failed or the system was left open for a long time. However, flushing may not be necessary when just changing hoses for preventive maintenance or replacing other parts in a working system.

A/C flushing solvents dry completely and leave no residue or chemicals behind. 32 ounces is sufficient for flushing the condenser or evaporator. A small flushing tank with a hose and nozzle makes the process relatively simple. Tip: securely attach a hose on the exit side of the condenser or evaporator to direct the flushed debris, oil and solvent into a container. You don’t want any oil or solvent, especially PAG oil, on fiberglass panels or painted surfaces. After flushing, blow air through the part to ensure all the solvent has evaporated.

For optimum cooling when switching to R-134a, it is recommended that POA valves and VIR assemblies be adjusted for this different refrigerant. Therefore be sure to specify R-12 or R-134a when ordering rebuilt units. VIR assemblies can be disassembled to inspect the valves for condition and cleanliness.

The long cylindrical A6 compressors used on 1963-’76 Corvettes are available new or remanufactured from Zip. If the compressor is slinging oil from the front (often seen on the underside of the hood) or leaking refrigerant but is otherwise OK, consider replacing the shaft seal. Although this job requires long nose snap ring pliers and a special clutch removal/installation tool, replacement is relatively easy. Some people say that the currently available ceramic/carbon replacement seal is not as good as the original seal. A modern replacement double lip seal is available from Century Auto Air, along with a reseal kit and clutch tool for the A6 compressor. Disassembly and resealing this compressor is straightforward as shown in the accompanying sidebar.

Final Steps to Cold A/C
During reassembly of components, make sure that each new O-ring is the same thickness and diameter as the original. Lubricate the O-ring with mineral oil, and tighten fittings just sufficiently to keep them from loosening.

Whenever A/C parts are changed, it is highly recommended to draw a high vacuum for 30 minutes or longer. This can be done by any A/C repair shop or you can rent or borrow a vacuum pump. Vacuuming the system removes any moisture from the system. Moisture is public enemy number one inside an air conditioning system. It corrodes metal parts, makes valves stick and degrades the oil or refrigerant. And if the system does not hold vacuum after the pump is turned off, that’s an important indication of a leak that must be fixed prior to adding the refrigerant.

When adding refrigerant, use a good gauge set to measure the high and low pressure. Adding too much refrigerant can diminish cooling and detrimentally increase pressures. R-134a is said to be particularly sensitive to overfill, and compared to R-12, fewer ounces may be needed. R-12 is less sensitive. For example, the GM manual calls for 48 ounces of R-12 for the 1973 Corvette system we worked on but duct temperature cooled to under 40 degrees with only 36 ounces.

Finally, belts and pulleys are often overlooked when repairing or replacing the air conditioning system. When the compressor seal leaks, a mist of oil can get on the pulleys and belts. Thoroughly clean all the pulleys and install a new high-quality belt.
Your Corvette can be very hot on the outside while keeping you very cool on the inside. There are many avenues to accomplish this, ranging from an entirely new modern air conditioning system to upgrading individual components to revitalizing the original system. There’s no excuse now, Vette has shown you how to be cool.

1. A complete modern A/C system kit for 1963-’76 Corvettes is offered by Zip Products. It features Vintage Air’s unit that moves the evaporator into the passenger compartment completely behind the dash and uses electronic controls. The kit also includes a parallel flow condenser with all hoses and a modern lightweight Sanden compressor with brackets.

2. For those who want to modernize their A/C but want to retain their existing compressor and condenser, Zip has an Evaporator Conversion Kit. This replaces the complexity of the VIR assembly with a simpler modern orifice tube system that regulates cooling by cycling the compressor on and off.

3. Another option Zip Products offers to change the VIR assembly over to the modern orifice tube system is their VIR Replacement Kit. This enables the existing compressor, condenser and evaporator to be used but will require some hose fabrication by a local A/C shop.

4. To switch the original factory system over from the increasingly expensive R-12, Zip has a R-134a Conversion Kit that includes new Schrader valves, flush solvent, ester oil and a retrofit guide. An inexpensive O-ring kit tailored to Corvette air conditioning systems is also available.

5. Solvent flushing an older A/C system is highly recommended whether modernizing or restoring. The accumulations on this VIR filter screen show contaminants that can cause a system to stop cooling or even damage newly replaced parts.

6. This flushing tank with a hose and nozzle is filled with solvent and then pressurized with an air hose to make the process relatively simple. A/C flushing solvents dry completely and leave no residue or chemicals behind.

7. To flush the evaporator, press the blow gun nozzle to the upper tube. Securely attach a hose on the lower tube to direct the flushed debris, oil and solvent into a container. A quart of solvent is usually sufficient to flush an evaporator or condenser.

8. The discharge hose is secured to prevent any oil or solvent, especially PAG oil, from getting on fiberglass panels or painted surfaces. A container for the discharge solvent helps to show when the component is flushed clean of contaminants and old oil.

9. The condenser is flushed in the same manner, from top to bottom. The return line is disconnected at the bottom of the condenser to attach the discharge hose (not shown).

10. The desiccant absorbs moisture and is a critical component of an A/C system. Whenever the system is opened, the desiccant bag or receiver/drier should be replaced. Zip Products offers a VIR Maintenance Kit that includes the necessary O-rings, pickup filter screen and a desiccant that is compatible with both R-12 and R-134a.

11. Closely inspect and clean all the A/C components before reassembly. This VIR assembly had deposits on its internal surfaces. These deposits may have been from the past use of A/C sealer or from degraded oil.

12. The POA valve capsule and the expansion valve capsule are located under the VIR’s inlet connector shell and require special tools for removal. For optimum performance, POA valves should be recalibrated when changing to R-134a.

13. Keep the desiccant bag in its sealed packaging until the VIR assembly is thoroughly cleaned, inspected and ready for assembly. Lubricate these O-rings and the other A/C O-rings with mineral oil.

14. The VIR assembly is barely visible underneath the coolant recovery tank. The desiccant bag can be replaced from below, but remove the tank for easier access from above to change hoses or to remove the entire VIR assembly.

15. After connecting the hoses or the evaporator’s tubes to the VIR assembly you will understand why someone cut an access door into the inner fender. At least this provided you with a better view of the VIR connections.

16. Tip: Fine brass brushes from Dremel work well to remove corrosion or contamination buildup on O-ring sealing surfaces.

17. A/C hoses are often compromised, particularly in areas close to the exhaust manifold. Fortunately, new replacements are available from Zip Products. Inspect and order these parts carefully because some look very similar but are not identical. Compare the length, fittings and alignments of replacements to the old hoses; the large A/C hoses do not bend or twist easily.

18. A/C hoses are often compromised, particularly in areas close to the exhaust manifold. Fortunately, new replacements are available from Zip Products. Inspect and order these parts carefully because some look very similar but are not identical. Compare the length, fittings and alignments of replacements to the old hoses; the large A/C hoses do not bend or twist easily.

19. Use a straightedge to make sure that the compressor pulley is in alignment with the water pump and crankshaft pulleys. Also install a new, high-quality belt, especially if the compressor had been leaking oil.

20. Attaching the low pressure hose of the gauge set to the VIR assembly can be challenging. This 90-degree adapter was modified to facilitate connecting the hose in these cramped quarters.

21. Tip: Warming the refrigerant speeds installation. Bill Coddington modified this bucket to hold the small refrigerant cans. Submerging a 12-ounce can of R-12 in a few pints of very hot water enables it to be emptied in just a couple of minutes.

22. After installing only three 12-ounce cans of R-12, the system was already performing well. The low side pressure was what it should be for R-12 at about 30 psi, indicating that the POA valve was functioning properly. The high side pressure increases significantly with the outside air temperature, exceeding 300 psi when it’s above 90 degrees.

23. Measuring temperature and humidity in front of the condenser is important in evaluating an air conditioning system’s performance. A duct temperature of 50 degrees is impressive in 95 degree weather but not so much in 75 degree weather.

24. This is the kind of duct temperature that will cool your body and warm your heart. Ambient temperature was in the low 80s, rpm was 1,800, and the car was parked. Later when it was slightly cooler out, the duct temperature reached 39 degrees when the car was driven at about 1,800 rpm. This Vette is now ready for summer cruising.

A6 Compressor Reseal & Rebuild
The large, long cylindrical-shape A6 compressor was used on the first air conditioned Corvettes in 1963 and continued in use through 1976. It’s a very robust and reliable compressor. Zip Products offers completely new, made in the USA A6 compressors and remanufactured units, too. For those that want to retain their existing compressor, replacing the front seal or resealing the entire unit is a straightforward job.

1. A special tool is required for removing and reinstalling the compressor clutch. The tool is relatively inexpensive, but a compressor clutch tool set is also offered for loan by several auto parts store chains. This tool set from O’Reilly fit the A6’s threads; the tool from AutoZone did not fit.

2. The original style ceramic shaft seal is visible behind the snap ring. Some people question whether the currently available ceramic seal replacement works as well as the original ceramic seal. A newer style double lip rubber seal is also available for the A6 compressor.

3. The original style ceramic shaft seal is visible behind the snap ring. Some people question whether the currently available ceramic seal replacement works as well as the original ceramic seal. A newer style double lip rubber seal is also available for the A6 compressor.

4. Examine the snap ring closely. One side has a beveled edge. During reassembly this beveled edge needs to face out, toward you. The compressor clutch coil also has a beveled snap ring. Tip: save the old snap rings and O-rings to compare to the replacement parts.

5. The clutch coil needs to be removed to further disassemble the compressor. Again, long nose snap ring pliers are required. On this particular compressor, removal of the snap ring was the hardest part of the job.

6. Remove the four nuts from the rear of the compressor. If necessary, gently tap on the plate (rear cylinder head) to separate and remove it.

7. Grasp the small steel oil drain tube to pull it out. Also remove its O-ring. Century Auto Air offers the double lip front seal, a seal kit with O-rings and a clutch tool for A6 compressors.

8. The cartridge assembly with its pistons can now be slid out of its case.

9. The crud buildup on the seal surface of the shaft needs to be removed or the new seal will leak. It would be difficult to clean this area of the shaft if the compressor was not disassembled.

10. The seal surface of the shaft has been restored to good condition after scraping the crud off with a plastic tool and polishing with 2,000-grit paper.

11. Rotate the shaft to move the pistons and inspect the cylinder walls. These are in good shape, the crosshatching is still visible.

12. After inspecting and cleaning the compressor parts, thoroughly lubricate them with the proper oil. Tip: reassembly is much easier if the case is held with its rear end up by clamping the adjustment bracket to a bench.

13. Reach up through the case to grasp and support the front cylinder head while lowering it into the case. This way, the weight of the cartridge assembly keeps the front O-ring in place between it and the cylinder head.

14. Next, install the reed valves, valve plate, oil pump gear and the large rear O-ring. Make sure that the two alignment pins are not bent or damaged.

15. Install the rear cylinder head and make sure it engages properly with the pump gears and alignment pins. Stop tightening the four case nuts if the head does not go down smoothly and easily.

16. This black tool screws on to the shaft and is essential to protect the double lip shaft seal during installation. After the seal is installed, use a large, deep socket or piece of PVC pipe to make sure it is pressed fully in place.

17. Install the seal snap ring with its bevel side toward you. Gently tap it down if necessary and make sure it is fully seated in its groove.

18. Clean all surfaces of the compressor clutch before reinstalling them and setting the proper clearance. Oil can get on these parts from a leaky seal.

19. The last step is adding fresh oil. If necessary, turn the shaft slowly to help draw the oil in. The compressor is ready for installation and years of trouble-free use.



Vintage Air

(800) 727-7094



Zip Products, Inc.

(804) 746-2290


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