On a system with an orifice tube rather than an expansion valve, the receiver/drier is substituted for by a component called an accumulator. The accumulator is usually located near the outlet of the evaporator. Refrigerant flowing out of the evaporator enters the accumulator to be stored, especially if it all has not vaporized in the evaporator. Because a liquid cannot be compressed, the accumulator ensures that only vaporized refrigerant reaches the compressor. The accumulator also contains a desiccant to keep the system free of moisture. Accumulators are similar in appearance to receiver/driers but usually bigger. Also, they never contain a sight glass. Their function is similar to that of receiver/driers in most every way, but they are located in the low pressure side of the system.
The accumulator should be replaced if it is leaking, if the system has been left open for a period of time, or whenever any other major A/C system components are replaced. The accumulator cannot be serviced and can only be replaced as a unit. To replace an accumulator, first recover the refrigerant from the system. Disconnect the negative battery cable and disconnect any electrical connectors from the accumulator. Disconnect the refrigerant lines from the accumulator and cap the lines to prevent system contamination. Remove the fasteners that secure the accumulator and remove it from the vehicle. If equipped, remove any switches or sensors that are mounted on the accumulator. If necessary, install any switches or sensors that were removed onto the replacement accumulator. Measure the amount of oil found in the accumulator that was removed, and then add the same amount of fresh refrigerant oil into the replacement accumulator. If oil has leaked out of the system, use the manufacturer’s recommended amount. Position the accumulator in the vehicle and install the mounting fasteners. Tighten all fasteners to specifications. Using new, lubricated 0-rings, connect the refrigerant lines. Reconnect the electrical connectors to the accumulator, if necessary, and reconnect the negative battery cable. Next, evacuate and recharge the A/C system and check for leaks. Finally, check system performance.
Refrigerant entering the evaporator must be metered, or sprayed in as a low pressure liquid at a controlled rate. If too much refrigerant enters the evaporator at too fast a rate, it can cause evaporator icing. If too little refrigerant enters the evaporator at too slow a rate, it can cause poor cooling performance. Also, the refrigerant’s pressure must be kept low enough so that it will vaporize (boil) around the temperature of the vehicle’s interior. The orifice tube acts as a restriction in the system, which creates the pressure drop. On its outlet side, pressure is lower than its inlet side. The orifice tube is usually located at the evaporator inlet. As the low pressure liquid refrigerant exits the orifice tube, it sprays into the evaporator. As the refrigerant’s pressure drops, so does its temperature. The refrigerant is now colder than the air inside the vehicle so it readily boils, or vaporizes. It absorbs heat from the passenger compartment, which it carries away as it is drawn out to the compressor. However, the orifice tube has a fixed inside diameter and cannot change in response to evaporator temperature. When the cooling load is low, an orifice tube flows too much refrigerant and floods the evaporator with liquid refrigerant. Because of this, an orifice tube system always has an accumulator to catch and store the liquid refrigerant.
Air conditioning systems with orifice tubes usually cycle the compressor on and off to control refrigerant flow. When the temperature/pressure in the evaporator drops, the compressor is stopped until the temperature/ pressure rises to a certain level, when compressor operation resumes. Because evaporator pressure and temperature are closely linked, the cut-out and cut-in signals can be provided by a temperature sensing thermistor located near the evaporator or a pressure switch mounted on the accumulator or low pressure hose. Most orifice tubes look very similar to each other externally. Internally, the difference is the actual diameter of the opening inside the tube that the refrigerant passes through. This tube, often made of brass, has a very small inside diameter. (It could be as small as 0.050-in.). The external ‘housing’ of an orifice tube is usually plastic with a filter screen to trap debris so it doesn’t plug the tiny inside diameter of the tube. There is also usually an 0-ring seal on the external housing. This wedges the orifice tube in tightly against the inside walls of the portion of the A/C system plumbing it is contained in, which prevents refrigerant bypass. The most common reason for replacing an orifice tube is if it is plugged. If there is a complaint that the A/C is not cold enough or not cold at all, and if the lowside pressure is very low and there is frost on the line between the orifice tube and the evaporator, suspect a plugged orifice tube. Note that on some vehicles the orifice tube cannot be removed from the line. If orifice tube replacement is required, a new line must be installed or a kit that provides a new line segment including the orifice tube can be installed.
A special tool is usually required to remove an orifice tube. To replace an orifice tube, first recover the refrigerant from the system. Disconnect the negative battery cable. Disconnect the liquid line from the evaporator inlet line and plug the liquid line to prevent contamination. Pour a small amount of clean refrigerant oil into the evaporator inlet line to lubricate the line and orifice 0-rings during removal. Engage the tangs on the orifice tube with the special tool. Hold the tool stationary and run the sleeve on the tool down against the evaporator inlet line until the orifice tube is pulled from the line. Do not twist or rotate the orifice tube in the line as it can break. Lubricate the replacement orifice tube with clean refrigerant oil, then install it into the evaporator inlet line until it is seated. Using a new o-ring, connect the liquid line to the evaporator inlet line. Connect the negative battery cable. Evacuate and recharge the NC system and check for leaks, then check system performance.
Many AC systems use a Thermostatic Expansion Valve (TXV) to restrict the amount of refrigerant entering the evaporator. Systems with expansion valves operate differently from those with orifice tubes. Instead of cycling the clutch on and off to control refrigerant flow, the expansion valve opens and closes as necessary to maintain proper evaporator pressure and temperature. The TXV reduces the high pressure liquid refrigerant to low pressure liquid, which it meters to the evaporator at a controlled rate. There is a metered orifice at the expansion valve’s outlet, but there is also a ‘plunger’ that opens and closes to control the amount of refrigerant that goes through the orifice to the evaporator.The amount of TXV opening is determined by a signal it receives from the outlet side of the evaporator. A capillary tube bulb mounted to the outlet pipe of the evaporator is insulated against ambient heat. If outlet temperature is too high, the capillary bulb signals the TXV to allow more refrigerant to flow; if temperature is too low, the bulb signals the TXV to restrict the flow. Although all expansion valves operate on a similar principle, they are not all identical. Usually they are spring loaded devices with a diaphragm that is linked to the plunger. Some diaphragms are internally equalized while others are connected to the low pressure side of the AC system via an equalizer tube. Some expansion valves have a screen on the inlet side.
Expansion valves need devices to report to them what the conditions are at the evaporator. This is the job of the capillary tube. This tube is filled with gas that expands and contracts to move the diaphragm against its internal or external equalizing pressure. On the end of the capillary tube there is usually a sensing bulb that is in contact with the evaporator or the evaporator’s outlet pipe. It is insulated by a special tape to prevent ambient air temperature from affecting its operation. Regular electrician’s tape cannot be substituted for this special insulated tape. As the temperature at the evaporator outlet increases, the pressure in the capillary tube increases. This pressure acts on the diaphragm, which in turn opens the expansion valve and allows more refrigerant to enter the evaporator. Conversely, when the evaporator temperature decreases, pressure in the capillary tube decreases. Less pressure acts on the diaphragm, which in turn allows the expansion valve to close, and less refrigerant enters the evaporator. This dithering constantly controls the amount of refrigerant entering the evaporator, thereby controlling evaporator pressure and temperature. On evaporators with large pressure drops between the inlet and outlet, an equalizing tube may be used between the evaporator outlet and the TXV. In essence, this equalizer eliminates the effect of the large pressure drop on TX:V operation. These equalizers can be internal or external. Expansion valves are usually non seryiceable. If one malfunctions, yot1 must replace it. However, some are equipped with filter screens, and if this is the case, it should be cleaned whenever the system is opened for service of other components.
A TXV should be replaced if it becomes clogged or stuck in position due to debris, contamination or corrosion, or if the capillary tube is damaged. If the TXV is stuck closed, restricting the refrigerant flow, the air flow inside the vehicle will not be cold, the low and high-side pressures will be low and there may be frost on the valve. If the TX:V is stuck open, flooding the evaporator with excess refrigerant, the air flow inside the vehicle will not be cold and the system low-side pressure will be high . To replace a TXV, first recover the refrigerant from the system. Disconnect the negative battery cable. Remove the insulation covering the capillary tube and bulb and remove the capillary tube from its mounting. Disconnect the refrigerant lines from the TXV and cap the lines to prevent contamination. Remove the TXV from the vehicle. Install the replacement TX:V and connect the refrigerant lines using new 0-rings lubricated with clean refrigerant oil. Properly position the capillary tube and bulb along with the insulation. Connect the negative battery cable. Evacuate and recharge the AC system and check for leaks, then check system performance.
The evaporator is a heat exchanger not much different in appearance or design from a heater core, but with one main operational difference. Cool air passing over a heater core picks up heat from it and becomes warmer. Heat from the air passing over an evaporator is absorbed by it and the air becomes colder. The evaporator is named the way it is because it reflects the fact that inside of it the refrigerant turns from a liquid to a gas (boils). Liquids boil sooner under low pressure than high pressure and refrigerant inside the evaporator has a pressure of about 30psi. Evaporator temperature should hover at 32°-40°F. At temperatures above this range, it does a poor job of cooling the inside of the vehicle. At lower temperatures, ice will form from condensation on the evaporator’s fins, blocking air flow and rendering it ineffective as a heat exchanger. The key is to keep the evaporator temperature within the narrow 32-40 degrees Fahrenheit range. The evaporator is housed in a case, often the same one housing the heater core and blower. This case is usually called the plenum or I call it the HVAC box. The plenum also contains a routing and/or temperature blend doors, and is essentially a ductwork system. At the bottom of the evaporator case is a drain hole with a hose that directs the condensed water outside the vehicle. Make sure the drain is kept clear or stagnant water can collect and allow bacteria to grow, causing odors. A clogged drain can also cause the case to fill up with water and leak into the passenger compartment.
Replace the evaporator if it is leaking, clogged or corroded from moisture. Depending on the vehicle, it may be possible to remove the evaporator from the plenum with the plenum installed in the vehicle, or the entire plenum may have to be removed to access the evaporator. Begin by recovering the refrigerant from the system. Disconnect the negative battery cable. Disconnect the refrigerant lines from the evaporator and plug the lines to prevent contamination. If the plenum is being removed and it also houses the heater core, drain the cooling system and disconnect the heater hoses. Label and disconnect the necessary electrical connectors and vacuum hoses. Remove all other components necessary for evaporator or plenum removal, then remove the evaporator or plenum. If the plenum was removed, disassemble it as necessary to remove the evaporator. Measure the amount of oil found in the evaporator that was removed, and then add the same amount of fresh refrigerant oil into the replacement evaporator. If oil has leaked out of the system, use the manufacturer’s recommended amount. If necessary, install the evaporator into the plenum. Install the evaporator or plenum into the vehicle. Install all components that were removed for access and connect the necessary electrical connectors and vacuum hoses. If the heater core was removed with the plenum, connect the heater hoses and add coolant to the proper level. Connect the refrigerant lines using new 0-rings lubricated with clean refrigerant oil. Connect the negative battery cable. Evacuate and recharge the AC system and check for leaks then check system performance.
Other Evaporator Pressure/Temperature Controls
Before the advent of orifice tubes and cycling compressors, some older vehicles used evaporator pressure controls like the Suction Throttling Valve (SVT) and the Pilot Operated Absolute (POA) valve. These devices maintain pressure in the evaporator by regulating the flow of refrigerant out of the evaporator, thereby controlling evaporator temperature. They are located in the suction line between the evaporator and compressor and are used in systems where the compressor operates continuously when the NC is on. The valve should be replaced if it sticks due to debris, contamination or corrosion. Symptoms of a valve that is stuck closed include high low-side pressure, poor cooling and high evaporator pressure combined with low suction pressure. A valve that is stuck open is indicated by low evaporator pressure or a frozen evaporator. To replace a valve, first recover the refrigerant from the system. Disconnect the negative battery cable. Disconnect the refrigerant lines from the valve and cap the lines to prevent contamination. Remove the valve from the vehicle. Install the replacement valve and connect the refrigerant lines using new 0-rings lubricated with clean refrigerant oil. Connect the negative battery cable. Evacuate and recharge the AC system and check for leaks, then check system performance.
High Pressure Relief Device
A high pressure relief valve is used to release pressure in the system before excessive high pressure can damage system components. When a predetermined pressure is reached, the valve opens, allowing refrigerant to escape until the pressure drops below the discharge point, when the valve closes. The high pressure relief valve is located in the high side of the system and often you will find it on the compressor itself through the high side circuit.