AC Orifice Tube

Replace the AC orifice tube in the air conditioning system to restore proper refrigerant flow and cooling performance.

Warnings

Tools required

Parts

• AC orifice tube × 1 — Use OEM specification or equivalent for R-12 or R-134a system
• O-rings for refrigerant line fittings × 2 — Use correct material for refrigerant type
• PAG oil or mineral oil (depending on refrigerant type) × 1 — Match to compressor and refrigerant specification

Fluids

• R-12 refrigerant or R-134a refrigerant (if converted) — 0.75 qt

Preparation

1. Verify the vehicle's AC system refrigerant type (original R-12 or converted R-134a) to ensure proper equipment and replacement parts
2. Ensure AC recovery/evacuation/recharge equipment is properly calibrated and appropriate for the refrigerant type
3. Park vehicle on level surface and set parking brake
4. Allow engine to cool completely if recently run
5. Locate the orifice tube assembly, which is typically in the liquid line between the condenser and evaporator, near the firewall on the passenger side

Procedure

1. 1

   Recover refrigerant from AC system
   Connect the AC recovery machine to the high and low side service ports using appropriate adapters for the system's fitting type (R-12 flare/Schrader on original systems, or R-134a quick-connect if converted). Follow the recovery machine manufacturer's instructions to completely recover all refrigerant from the system. Record the amount recovered for comparison during recharge. Ensure recovery is complete before proceeding.
2. 2

   Locate and access orifice tube
   If an aftermarket orifice-tube AC conversion has been fitted, the orifice tube is located in the liquid line between the condenser outlet and the evaporator inlet, typically marked by a slight bulge or crimp at a coupling joint. Confirm the exact location against the conversion kit documentation, since the 1966 Corvette did not use an orifice tube from the factory.
3. 3

   Disconnect liquid line at orifice tube location
   Using the appropriate size line wrench for your system's fittings, carefully loosen the flare nut fitting on the evaporator side of the orifice tube housing. Support the line to prevent twisting. Once loose, disconnect the fitting completely. Some residual refrigerant or oil will drain out; capture with shop rags. Remove and inspect the O-ring.
4. 4

   Extract old orifice tube
   Insert the orifice tube removal tool into the open end of the liquid line where the tube is housed. The tool's fingers will grip behind the tube's rim. Gently twist and pull to extract the orifice tube from the line. If the tube is stuck due to corrosion or contamination, apply penetrating oil around the edges and retry. Avoid using excessive force that could damage the aluminum line.
5. 5

   Inspect removed orifice tube
   Carefully examine the removed orifice tube and its screen for metal particles, debris, or black contamination. Metal particles indicate compressor failure and the system will require complete flushing before proceeding. Minor debris on the screen is normal. Check that the tube came out intact with both screens present.
6. 6

   Clean orifice tube housing
   Use a clean, lint-free shop rag to wipe out the inside of the liquid line tube where the orifice tube sits. Remove any visible debris or old oil. Do not use compressed air as this can blow debris further into the evaporator. Ensure the sealing surface is clean and smooth.
7. 7

   Install new orifice tube
   Lubricate the new orifice tube's O-rings with a small amount of the appropriate refrigerant oil (PAG for R-134a systems, mineral oil for R-12 systems). Note the directional arrow on the orifice tube body, which must point toward the evaporator (downstream). Insert the tube into the liquid line by hand, ensuring it seats fully against the internal stop. The tube should sit flush or slightly recessed.
8. 8

   Reconnect liquid line fitting
   Install a new O-ring on the flare fitting, lubricating it lightly with refrigerant oil. Align the liquid line and thread the flare nut by hand to prevent cross-threading. Tighten the flare nut with a line wrench until snug. Do not overtighten the fitting; tighten to a snug fit plus 1/6 turn. Verify the correct torque specification against the factory service manual or conversion kit documentation, as flare-fitting torque varies by line size and material.
9. 9

   Evacuate AC system
   Connect the vacuum pump or evacuation function of the AC machine to the service ports. Evacuate the system to at least 29 inches of mercury vacuum for a minimum of 30 minutes. For best results, evacuate for 45 minutes to ensure all moisture is removed. Monitor for vacuum decay after pump shutoff; the system should hold vacuum, indicating no leaks.
10. 10

    Recharge AC system
    With the system under vacuum, introduce the appropriate refrigerant type and quantity through the AC machine. The refrigerant charge amount must be confirmed against the factory service manual or underhood label; if converted to R-134a, charge to approximately 85-90% of the original R-12 capacity by weight. Add the refrigerant oil that was lost during recovery if needed, following machine guidelines. Charge to the specification on the underhood label or factory service manual.
11. 11

    Leak test connections
    With the system charged, use an electronic leak detector or UV dye method to check all connections disturbed during this repair, especially the liquid line flare fitting at the orifice tube location. Check for leaks with the system both off and with the AC running. Address any leaks immediately by recovering refrigerant, tightening or replacing O-rings, and re-evacuating/recharging.

Reassembly

1. Ensure all AC line fittings are secure and properly torqued
2. Verify no tools or parts are left in the engine compartment
3. Confirm all service port caps are reinstalled to prevent contamination

Verification

• Start the engine and turn AC to maximum cooling with fan on high
• Allow system to run for 5 minutes and verify cold air from vents (typically 40-50°F at the vent with 80-90°F ambient)
• Check compressor cycling is normal (should cycle on and off appropriately, or run continuously depending on system design)
• Monitor gauge pressures: low side should be approximately 25-35 psi, high side approximately 200-250 psi at idle with 80°F ambient (actual pressures vary with temperature)
• Verify no unusual noises from compressor or expansion of lines
• Re-check all connections for leaks after system has reached operating temperature

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