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.
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.