Basic System Operation
The compressor pressurizes the refrigerant then it enters the A/C condenser as a gas. The high temperature, high pressure gas is cooled as it goes through the condenser. This changes the gas to a liquid. Then the liquid goes to the evaporator through an expansion valve or metered orifice. This quickly depressurizes the liquid. This vaporizing liquid lowers its temperature rapidly. The vaporized refrigerant returns to the compressor (the compressor cannot handle any liquid).
The Three Corvette A/C System Types
The 1963-1977E Corvettes had a complex A/C system with many valves which were prone to problems. The components consist of: compressor, POA valve, expansion valve, receiver drier, evaporator, and condenser. The A/C compressor ran continuously. These systems have a Pilot Operated Absolute valve (POA valve), which bypassed the refrigerant when necessary to prevent freeze-up, allowing the A/C compressor to run at all times.
The 1973-1977E Corvette had a valves-in-receiver unit. This assembly had the OP, expansion valve, and drier in one unit. These units were prone to leakage. These systems require hose and drier replacement. The addition of an A/C condenser pusher fan is helpful. The cooling system must be in excellent shape to allow adequate cooling at low engine speed.
This system was simplified by the addition of a POA valve and the removal of the expansion valve. An orifice tube was installed in the evaporator inlet tube, which eliminated the need for an expansion valve. The A/C compressor is controlled by a pressure cycling switch. The A/C compressor runs until the pressure drops below 26 PSI, then cycles back on at 45 PSI. This also eliminated the POA valve. These cars require a new A/C hose assembly. The A/C hose assembly is not necessary on 1990 and later Corvettes. These hoses have a nylon inner barrier. The nylon barrier hoses were introduced in late 1987 but any hose over ten years old should be replaced (R-134a systems operate at higher pressures). On late 1977-1981 Corvettes, add a
14” condenser pusher fan. On 1985-1989 Corvettes with an auxiliary cooling fan, a relay should be installed to turn the fan on when the A/C is on. The 1985-1989 Corvette without an auxiliary cooling fan should have a 12” pusher fan installed in front of the A/C condenser. 1990-1993 Corvettes should have the primary and secondary fans on when the A/C is on. These cars require an additional relay. The A/C accumulator should always be replaced after the system has been opened. The A/C accumulator is the refrigerant reservoir, and houses the desiccant which removes moisture in the refrigerant. The orifice tube should be replaced any time A/C service work is performed. The typical cost is $10.00.
- Safety glasses and gloves should always be worn.
- Refrigerant systems have as high as 150 lbs. of pressure when not in use.
- Refrigerant system operating pressures can exceed 350 lbs.
- Beware of old hoses. Any cracking or checking can cause a hose to blow unexpectedly.
- Do not use compressed air to blow out any R-134a system. This combination of gases can be flammable.
- R-12 or R-134a can cause frostbite if the liquid refrigerant is sprayed on the skin or in the eyes.
- The composition of A/C refrigerant should always be verified. No exceptions.
- Beware of contraband refrigerant. Propane and butane have been found in the southeast, which is extremely dangerous.
- Blended refrigerants increase system pressure and may make the system unstable.
R-12 refrigerant is a chlorofluorocarbon (CFC) which depletes the ozone layer. Oddly, R-12 refrigerant is heavier than oxygen. The phase out date for R-12 was January 31, 1995. Since 1996 it cannot be manufactured or brought into the United States. So far, R-12 is still available, just expensive. The EPA and The Society of Auto Engineers (SAE) have requested a retrofit refrigerant for vehicles up to 10 years old. A worldwide production ban of R-12 is in effect as of January 1, 2000.
Refrigerant recycling can save valuable refrigerant. It cleans and dries the refrigerant. This is also a better way to recharge the system because refrigerant is added by weight. Refrigerant recyclers should have J1991 or J2209 certification. This certification establishes refrigerant recycling equipment, refrigerant removal, and cleaning standards.
It is also very important to verify the identity of your refrigerant. Any blend of R-12, R-22, or R-134a increases the system pressure and diminishes cooling. I have seen some R-134a conversion systems that did not have the R-134a style system service valves. Any blended refrigerants are very costly to dispose of.
For more information, check out the Mobile Air Conditioning Society (MACS) at https://www.epa.gov/ozone/title6/609/index.html.
A/C System Evaluation
- Check all A/C components for obvious leakage (oil and dirt accumulate at leak points).
- Check A/C condenser for debris.
- Check and replace radiator-to-radiator core support seals as necessary (1968-1982 Corvettes).
- Check the A/C air intake in the wiper arm area for debris (allows maximum air flow).
- Check all belts.
- Check A/C compressor. Move the drive plate by hand. It should move with some resistance. (The drive plate is on the outside of the A/C pulley assembly).
- Check A/C hoses and aluminum hard lines for kinks, cracks, and chafing.
- Check the cooling system; it should operate at peak efficiency.
When To Retrofit
- There is any major A/C component failure, for example, compressor, evaporator, or condenser.
- Any system that is missing any major components or a system that was subjected to moisture, dirt, or dust.
Keep in mind that retrofitting will change the external appearance of your A/C components.
When retrofitting an empty A/C system, it should be leak tested first, but only after the system has been properly evacuated. (R-134a refrigerant can be used for leak testing in an empty system only).
Partially charged systems should have refrigerant verified and then leak tested. Leak testing should be done with an electronic tester for R-12 or R-134a. Soapy water or a refrigerant with a leak detecting dye can be used for hard to find minor leaks.
R-134a refrigerant O-rings are of a different composition than their R-12 counterparts. R-12 O-rings are a natural black color. R-134a O-rings are blue or green in color. Lubricate all O-rings with refrigerant oil before installing them All A/C connection O-rings must be changed to R-134a compatible ones.
R-134a systems require different system couplers (adapters and O-ring retrofit kits are readily available from NAPA Auto Parts stores). All R134a retrofit conversions must be labeled. These are also available from NAPA. Be sure to fill out the retrofit label completely with information such as the type and quantity of the oil. The amount of refrigerant charge should also be noted. Typically, R-134a systems require 8 to 12 ounces less refrigerant than R-12 systems do.
Contaminated A/C Systems
Flushing is necessary when the A/C system is not operational and left open to the elements, also if there is a compressor or desiccant failure. Some of the older Corvettes with the POA system had a receiver drier and desiccant that would become saturated with moisture and disintegrate. This would disperse a fine grit throughout the system. On late 1977-1986 Corvettes, remove the main hose assembly from the backside of the compressor. Take a white paper towel and wipe the inside of the hose assembly to check for metallic particles. This will require a system flush also. Inexpensive flush kits are available at NAPA.
A/C Compressor Oil
The quantity of oil to be used follows:
Originally, PAG (Poly Alkylene Glycol) oil was the only lubricating oil recommended for R-134a systems. Castrol Retro Oil is compatible with PAG and mineral oil.
Be careful- excessive lubricating oil in A/C systems can result in warmer outlet air temperatures.
When retrofitting I prefer component removal and draining of the lubricating oil. It is difficult to determine the oil level in the components. Compressors should be removed, drained, and then refilled with lubricant. A-6 compressors have a fill plug on the lower side of the compressor. Remove the 3/8” drain plug. Refill with six (6) ounces of refrigerant oil while holding the compressor level. The compressor will not sit flat due to the oil sump. A-6 compressors are axial-drive six cylinder compressors. R-4 compressors are four cylinder radial compressors with no drain or refill provision. After removing the compressor, lay it on the back side where the hose assembly bolts to. It usually takes an hour to allow the oil to drain. I usually rotate the compressor at its drive plate every ten minutes or so to remove as much oil as possible. The easiest way to turn the drive is with a 9/16” twelve-point socket and ratchet.
When refilling R-4 compressors, add retro oil at the suction side of the hose fitting on the back of the compressor. The side with the large hole is the suction side. The small hole is the discharge side. After adding oil, rotate the compressor drive plate by hand to disperse the oil in the compressor.
Use the same procedure for R-4 compressors and Nippondenso compressors.
TYPICAL R-134a A/C REFRIGERANT PRESSURE READINGS
Readings at 90 degree ambient temperature, 100% relative humidity, and 1,000 RPM.
High side- 350 PSI
Low side- 55 PSI
As the A/C system pressure rises so does the refrigerant temperature, which directly relates to the A/C outlet temperature inside the car. As the engine temperature increases, so does the high side A/C system pressure.
After recharging the system, always check for leaks. Refrigerant is heavier than air; be sure to check for leaks at the bottom of each A/C component. Check for evaporator leaks at the drain hole in the bottom of the evaporator case.
REPLACEMENT COMPONENT OIL QUANTITY
When replacing any major component, refrigerant oil should be added to the new components.
|1963-1977L||Receiver drier/ VIR assembly||1 ounce|
Evacuating the system for one hour is recommended. Drawing a vacuum at 29.9 inches vaporizes any moisture in the system. It also purges any air from the A/C system. Evacuation timing should start after 29.9 inches of vacuum has been achieved. After evacuation, shut all of the valves tightly and let the system sit for approximately 30 minutes. The vacuum should hold; if not, recheck all of the fittings for tightness.
SYSTEM CHARGE PROCEDURES
|1963-1977E||3 pounds 4 ounces|
|1984-1988||2 pounds 12 ounces|
|1989-1993||2 pounds 4 ounces|
R-134a requires eight (8) ounces less than the typical R-12 refrigerant charge.
The figures above represent R-12 factory fill levels.
If you are recharging the system with D.I.Y. equipment, beware of charging liquid into the suction port of the compressor. Liquids will damage any compressor. If there is no alternate charging port, be sure to charge as a gas. Submerging the refrigerant can in a container of warm water while charging the system will speed up the charging process.
R-134a refrigerant is to be charged as a gas only!
TYPICAL R-12 A/C REFRIGERANT PRESSURE READINGS
Readings at 90 degree ambient temperature, 100% relative humidity, and 1,000 RPM.
High side- 300 PSI
Low side- 45 PSI
* Inches of vacuum
For R-134a refrigerant, add ten (10) degrees.
Story and photos courtesy Chris Petris