This article is part of a 6-part series on automotive air conditioning and heating theory. Part 5 covers the flow control devices that meter refrigerant into the evaporator, the distribution of air through the HVAC case, and the electrical circuits that control blower speed and compressor engagement. Seeing how the expansion valve, orifice tube, blower motor, and electrical switches work together shows why a failed sensor or a clogged filter can stop the system from cooling properly.
Pressure Drop and Atomization at the Restriction Point
The refrigerant flow restrictor, which is an expansion valve or orifice tube, creates a pressure drop by forcing high-pressure liquid through a small calibrated opening. This transition into the low-pressure evaporator allows the refrigerant to atomize and begin its boiling process. The system utilizes a temperature-sensitive thermal bulb to modulate flow. This bulb senses the temperature of the evaporator outlet and adjusts the valve opening to ensure the refrigerant is completely vaporized before exiting the core.
Expansion Valve Metering Forces and Thermal Bulb Contact
The valve admits a precise volume of refrigerant based on the current heat load. The control variables include internal spring pressure, which is pre-set to maintain a specific baseline resistance. Evaporator pressure acts against the bulb pressure. Thermal bulb pressure is connected via a capillary tube; as the evaporator outlet warms, the bulb pressure rises, forcing the valve further open to increase cooling. The capillary tube must remain intact and the bulb must be securely clamped to the outlet pipe with high thermal contact. If the bulb loses contact, the valve will hunt or flood the evaporator with liquid.
Compressor Discharge Pressure and Component Placement
The compressor elevates refrigerant temperature above ambient air. If the compressor discharge pressure is too low, the temperature differential at the condenser will be insufficient for condensation. The receiver-drier removes moisture and debris that could otherwise freeze or clog the small orifice of the expansion valve. Airflow is a mechanical requirement for heat transfer. Low blower speeds or a clogged cabin filter lead to insufficient heat absorption, causing the evaporator to freeze over. The receiver-drier is installed on the high-pressure side between the condenser and the expansion valve to act as a reservoir for liquid refrigerant and ensure a solid column of liquid is supplied to the restriction point. The evaporator is placed within the passenger compartment’s ventilation housing to maximize the cooling and dehumidification of cabin air while minimizing thermal loss through the lines.
Fixed Orifice Restriction and Compressor Cycling
Unlike a Thermostatic Expansion Valve (TXV), a fixed orifice tube uses a non-adjustable, calibrated hole to create the pressure drop. It does not modulate based on temperature; instead, it provides a constant restriction. Because the orifice cannot adjust flow, the system prevents evaporator freeze-up by cycling the compressor on and off. A pressure-sensitive switch or thermostatic switch monitors the evaporator and disengages the compressor clutch when the temperature reaches approximately 32 degrees F (0 degrees C).
Accumulator Location and Desiccant Function
In CCOT or fixed orifice systems, the storage tank called the accumulator is located on the low-pressure suction line between the evaporator and the compressor. Since the orifice tube may allow liquid refrigerant to exit the evaporator during certain operating conditions, the accumulator acts as a catch-can to ensure only pure vapor reaches the compressor. The accumulator contains a desiccant bag to remove moisture, which is critical because moisture combined with refrigerant creates hydrochloric acid, which corrodes system internals.
Orifice Tube Screens and Cycling Switch Calibration
Orifice tube screens are fine mesh filters located on both the inlet and outlet of the orifice. Clogging of the inlet screen by metallic shrapnel typically indicates internal compressor failure, a condition known as Black Death. The thermostatic cycling switch is calibrated to close the clutch circuit when evaporator temperatures rise above 45 degrees F (7 degrees C) and open to disengage when temperatures drop to 32 degrees F (0 degrees C). A calibrated hole at the bottom of the accumulator’s internal U-tube, called an oil bleed hole, allows a small, metered amount of compressor oil to return to the compressor along with the refrigerant vapor.
Local Shop Note:
This brings back a story I picked up from a technician out on NY-28 in Cooperstown, N.Y. He was telling me about it at an OEM dealer shop mechanic A/C training seminar. A newer SUV rolls in with a complaint that the AC blows cold for about ten minutes, then gradually turns warm — but if you shut it off and restart the engine, it blows cold again for another few minutes. Customer had already been to two other shops. One replaced the compressor, another replaced the expansion valve. Still had the same problem.
He hooked up his manifold gauges. Low side was dropping into a vacuum, high side was normal. That told him the system was starving the evaporator. He pulled the orifice tube and found the inlet screen packed with tiny gray metallic debris — the telltale sign of internal compressor wear. But the compressor was new. So where was the debris coming from?
He cut open the old accumulator that the previous shop had left in place. The desiccant bag had ruptured, releasing desiccant beads into the system. Those beads traveled downstream and lodged in the orifice tube screen, partially blocking flow. When the system ran, the blockage would slowly starve the evaporator, pressures would drop, and cooling would fade. Shut it off, the pressure equalized, debris shifted, and it worked again briefly. The previous shops had replaced the compressor and valve but never flushed the lines or replaced the accumulator.
He replaced the accumulator, flushed the entire system, installed a new orifice tube, evacuated and recharged by weight. That system stayed cold.
If there’s one thing to remember from that story, it’s that you can’t just replace the failed part — you have to address the root cause and remove the contamination. A new compressor won’t fix a clogged orifice tube, and a new expansion valve won’t fix a ruptured desiccant bag. The whole system is a loop. If debris is in one component, it’s in all of them. Always flush, always replace the accumulator or receiver-drier, and always inspect the orifice tube screens. Otherwise, you’re just installing new parts into a contaminated system.
Orifice Tube Orientation and Replacement Requirements
The orifice tube must be installed with the longer screen facing the condenser on the inlet side. This maximizes the filtering surface area for debris coming from the high-side components before it reaches the calibrated restriction. In many modern designs, the orifice tube is located inside the liquid line near the evaporator inlet and is not a separate bolt-on component. Reducing the number of threaded fittings minimizes potential leak points in the high-pressure section of the system. When a compressor fails, the orifice tube must be replaced rather than cleaned. The extremely small diameter of the calibrated orifice makes it impossible to guarantee the removal of all microscopic contaminants.
Blower Airflow, Condensate Drainage, and Muffler Design
The blower motor forces cabin or outside air through the evaporator fins. The refrigerant absorbs heat from the air, causing the moisture in the air to reach its dew point and condense on the evaporator surface. As water condenses on the evaporator, it collects in the evaporator case. Atmospheric pressure and gravity force this moisture through a drain tube to the exterior of the vehicle. High-pressure refrigerant pulses from the compressor create vibrations. A muffler uses internal baffles and volume expansion to dissipate these pressure waves, preventing audible noise from entering the passenger compartment.
Centrifugal Fan Design and Resistor Speed Control
The blower motor is a centrifugal squirrel cage fan designed to move large volumes of air relative to its compact size. Lower speeds are achieved by passing current through a series of resistors to reduce voltage to the motor. Modern systems use pulse-width modulation (PWM) or power transistors for infinitely variable speed control and higher efficiency. Inadequate blower speed reduces the heat load on the evaporator, which can lead to low suction pressures and evaporator icing.
Rigid Tubing, Flexible Hoses, and O-Ring Lubrication
Aluminum or steel tubing is used for rigid sections where components are solidly mounted to the chassis. Flexible hoses are used between the engine-mounted compressor and the chassis-mounted condenser and evaporator to absorb engine torque and vibration. Systems utilize specialized A/C O-rings. Unlike standard plumbing O-rings, these are engineered for compatibility with specific refrigerants and compressor oils such as PAG or POE. O-rings must be lubricated with clean compressor oil during assembly to prevent rolling or tearing when the fitting is torqued.
Sight Glass Indicators and Fusible Safety Plug
The sight glass is located on the receiver-dehydrator or high-pressure line. Clear flow indicates a full charge of liquid refrigerant. Bubbles or foam indicate a low refrigerant charge or the presence of air. A cloudy or streaked appearance indicates desiccant breakdown or internal contamination. The evaporator, heater core, and blower motor are often housed in a modular upper and lower or left and right case assembly. This compact packaging allows for the integration of air-mix doors to control temperature by blending cooled air from the evaporator with heated air from the heater core. Some receiver-dehydrators include a plug with a specific melt-point called a fusible safety plug. This provides a fail-safe to release refrigerant if extreme temperatures and thus extreme pressures threaten to rupture system components.
Resistor Bank Voltage Control and Relay Isolation
Airflow volume is regulated by a multi-tapped resistor bank. By switching current through different resistance values, the circuit alters the voltage supplied to the blower motor, thereby controlling its RPM. For maximum cooling, the resistor bank is bypassed via a dedicated high relay. This provides full battery voltage, nominal 12V to 14.4V, directly to the blower motor for maximum cubic feet per minute (CFM) output.
Ignition Interlock, Thermostatic Switch, and Pressure Cutouts
The A/C system is interlocked with the ignition switch IG2 or Run position to prevent battery drain when the engine is not turning the compressor. The thermostatic switch acts as a series gate. If the evaporator temperature drops to 32 degrees F (0 degrees C), the switch opens, breaking the circuit to the clutch coil to prevent core icing. Low-pressure or high-pressure cutout switches interrupt the clutch circuit if refrigerant levels are insufficient for oil transport or if head pressures exceed mechanical limits.
Fuse Ratings and Resistor Cooling Airflow
Main blower motors typically utilize high-amperage fuses, for example 40A or 50A, to handle the significant inductive inrush current during startup. Four-pin and five-pin relays are used to isolate high-current motor loads from low-current dashboard switches. Blower resistors are traditionally mounted within the evaporator or blower housing. The high current flowing through the resistors generates significant heat; placing them in the blower’s airstream ensures constant cooling of the resistor coils to prevent premature burnout. Common grounding points, which are BLK wires, are utilized for the blower motor, heater fan switch, and recirculation motor. Centralized grounding reduces electrical noise and ensures a stable reference voltage for electronic control modules. A dedicated motor called the recirculation control motor switches the intake from outside air to inside air. Closing off outside air allows the system to reach lower cabin temperatures more quickly by re-cooling air that has already been dehumidified.
The key takeaway from Part 5 is that the expansion device controls refrigerant flow into the evaporator, the blower motor moves air across the coils, and electrical switches protect the system from freezing or over-pressure conditions. The 6-part series continues with Part 6.