1973-1977E Corvette VIR Eliminator Installation & VIR Maintenance

by John Pfanstiehl

The VIR, Valves-In-Receiver, assembly was used in the air conditioning systems of 1973 to 1977 early-model Corvettes. It has a POA suction throttling valve similar in function to what was used on earlier Corvettes but also includes a thermal expansion valve, receiver-dryer and a site glass.

The expansion valve meters the liquid refrigerant into the evaporator. The POA valve controls the flow of refrigerant out from the evaporator to maintain approximately 30 psi for an evaporator core temperature about 32 F. Each valve is in a removable capsule that was adjusted during manufacture and has no external adjustments. This is an issue when switching to R134a refrigerant because that requires a different pressure.

The lower portion of the VIR assembly has a removal shell. This is accessible from under the car and permits installing a new desiccant bag whenever AC system parts are changed. The desiccant removes any water or moisture inside the AC system. A new 73-77E VIR Maintenance Kit is available from Zip Corvette with the necessary O-rings, pickup screen and XH-7 desiccant. This should be installed when parts are replaced and the AC System is opened up for a while because any moisture or water that enters is severely detrimental to the metal parts of the system.

The VIR assembly also has a sight glass which is intended to enable viewing the condition of the liquid refrigerant during operation. However the location of the VIR assembly in a Corvette makes that close to impossible.

Follow along to see the steps in removing a VIR assembly, or just the desiccant, along with tips to make removal easier.

Step 1

01: Removal of the coolant overflow tank improves access to the VIR assembly. Loosen the adjustment bolt to swing the compressor as far as it will go toward the engine. Remove the three attachment screws and wiggle the tank out.

Step 2

02: It’s now a lot easier to see and work on the VIR assembly. There’s no particular order needed in removing the lines, but we started with the top evaporator line. Hold the line when breaking the nut loose to minimize stress on the evaporator. A 1-1/16 inch wrench or large adjustable wrench is needed.

Step 3

03: The tubing nuts are not flare nuts and therefore only need to be minimally tightened. The sealing is accomplished by its O-ring. Tip: apply antiseize or grease to the threads when installing to make future removal easier. Heat and moisture can eventually corrode the threads and make these nuts very difficult to break loose.

Step 4

04: Other fittings need to be reached from below. The small oil return tube is removed with a 7/16-inch wrench. Other wrenches needed in removing the AC lines are 5/8, 11/16 and 3/4-inch.

Step 5

05: The last hose to be disconnected was just too stuck to be easily broken loose while in the car. Tip: If that happens, disconnect the other end of the hose and work the assembly out from under the car.

Step 6

06: Once out of the car, the VIR assembly was held in a vice while a pipe was slid over the wrench for more torque. Tip: If fittings won’t break loose and the hoses are being replaced, cut the hoses in a convenient place to enabled removal of the VIR assembly.

Step 7

07: Take a photo of the alignment of the upper cover if disassembling the VIR assembly because the cover can be installed in any of four positions.

Step 8

08: The POA valve and expansion valve are visible when the cover is removed. These parts are free from corrosion or debris, which is a good sign for the condition of the rest of the system.

Step 9

09: Contamination was visible on the lower end when the lower cover was removed. The desiccant bag was swollen and there was debris on the mesh filter.

Step 10

10: Crud was also accumulated in the bottom of the cover. It appears that the filter prevented this contamination from getting into the system. However, to be certain, it’s wise to flush the hoses, evaporator and condenser with an AC system cleaner.

Step 11

11: It’s a good time to clean the coolant recovery tank while it’s out. The plastic is darkened on this tank but is otherwise in good condition. Zip Corvette offers new tanks, like the 73-77E Radiator Coolant Recovery Tank, and also offers new caps, hoses, clamps, fasteners and decals.

Step 12

12: An AC cleaner solvent needs to be used to flush the mineral oil and any debris from the evaporator, condenser, and hoses if switching from R12 to R134a. The PAG oil that is used with R134a should not be mixed with the mineral oil used with R12.

Step 13

13: Flush at least a quart of cleaner through the condenser and another quart through the evaporator. Check the discharge and continue flushing until the discharged fluid is as clear as the cleaner. In some cases, four quarts may be needed to thoroughly flush the system.

Step 14

14: After flushing, use an air compressor or air tank to blow air through the evaporator and condenser until no more cleaner odor is detected. A rubber hose placed temporarily on the lower evaporator tube directs the cleaner into a drain bottle.

Step 15

15: Zip Corvette offers the 73-77 AC VIR Eliminator Kit which contains the accumulator with desiccant inside, the eliminator (orifice) block, adjustable pressure switch, connector and O-rings. It’s made in the USA.

Step 16

16: Connect the oil drain line to the eliminator block and then connect the lower evaporator tube. Use the new O-rings supplied with the kit and coat them with the supplied oil.

Step 17

17: The VIR eliminator accumulator should be the last item to install because it has desiccant inside which will absorb moisture from the air when left open. Remove the plastic sealing plugs and make sure to remove the black O-rings.

Step 18

18: Because the large compressor hose is harder to reach, connecting it first may be easier. Then connect the upper evaporator pipe to the accumulator.

Step 19

19: The pressure switch has an adjustment screw between the terminals. It’s set to cycle off at about 20-22 psi and on at about 42 psi at the low side for R134a. If R12 is used, turn the screw ¼ to ½ turn clockwise per the included instructions until the switch cycles off and on at the specified pressures.

Step 20

20: Install the switch on the most convenient of the three ports. Attaching the gauge set’s low pressure line is easier if the port facing the engine is not used for the switch.

Step 21

21: Tip: Installing the included R12 to R134a adapter and using a R134a quick disconnects reduces losing refrigerant and oil when connecting & disconnecting the gauge sets hoses or a fill hose.

Step 22

22: Vacuum the system to remove air and any residual moisture for at least 30 minutes; the longer the better. Turn off the pump to see if the vacuum drops to check for leaks. Vacuum pumps and gauge sets are loaned for free by some major auto parts chain stores.

Step 23

23: It’s necessary to jump the compressor clutch terminal with 12 volts to keep it engaged while adding the first 2 lbs of refrigerant. The alternator and horn relay are easily accessed by a jumper wire.

Step 24

24: The center duct air temp was 45 degrees at 1400 rpm on an 87F, 77RH day at 30 mph. The temp decreased even further when driving faster. Mission accomplished.

1973-1977E Corvette VIR Eliminator Installation and VIR Maintenance

Source: Zip Corvette Parts
8067 Fast Lane | Mechanicsville, VA 23111 | (800) 962-9632

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