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Your semi truck’s air conditioning system uses refrigerant to absorb heat from the cabin and produce cool air for a comfortable ride. Once the AC system can’t control the refrigerant, it might lead to a complete AC failure. The part of your AC system that prevents this is the AC expansion valve. In this blog, you’ll learn how an AC expansion valve works, its functions, common issues, and how you can properly maintain it to ensure optimal cooling and a pleasant driving experience.

What Is an AC Expansion Valve?

The expansion valve is part of your AC system that maintains your unit’s optimal and consistent refrigerant flow. It regulates the pressure drop when the liquid refrigerant is transformed into vapor in the evaporator coil. 

sample-AC-expansion-valve

Sample of an AC expansion valve

If the optimal evaporator pressure isn’t maintained, the refrigerant will not be converted to vapor properly. This means the air released from your AC vents will be either too cold or too warm. Furthermore, the air will consistently shift back and forth between various temperatures, causing damage to other areas of your AC system.

How Does an AC Expansion Valve Work?

A fully-functional AC expansion valve controls the pressure in the AC system to facilitate the production of cold air. Here’s a quick breakdown of how an AC expansion valve works:

temperature-dial-ac-system-truck

Temperature dial on a truck’s AC system

1. Controls the Flow Rate of Liquid Refrigerant

The expansion valve controls how much liquid refrigerant flows into the evaporator coils. This will ensure the system has the proper refrigerant to produce cold air. An excessive amount of refrigerant would make the evaporator core freeze.

2. Controls the Pressure Drop

The pressure drop in the expansion valve is crucial to the overall operation of the AC system, as it transforms the liquid refrigerant into cold vapor. To produce cold air, the expansion valve must precisely control the pressure applied to the refrigerant.

3. Controls the Output Temperature

Superheating refers to the shift from the actual temperature of the vapor to the saturation temperature with the refrigerant. The AC expansion valve maintains 4 to 7°C of superheating before more refrigerant is allowed to enter the evaporator coils.

Once the expansion valve completes this process, the evaporator coils can properly release the cold air on the AC vent. This is why the expansion valve is always attached to the evaporator. 

Here’s an informative diagram, sourced from Ariazone’s automotive air conditioning training manual, showing the role of the AC expansion valve:

diagram-of-an-AC-system

Diagram of an AC system. (Source: Ariazone)

What Are the Different Kinds of AC Expansion Valves?

Most vehicles use thermal expansion valves, which can further be categorized into externally equalized valves and internally equalized valves. As described a while ago, these thermal expansion valves exert pressure on the refrigerant to regulate the air temperature. 

Here are the two kinds of thermal expansion valves: 

1. Externally Equalized Valves

Many types of vehicles use externally equalized valves. These thermal expansion valves use the evaporator outlet pressure to regulate the pressure drop through the evaporator. The most common kind of externally equalized valves found in automobiles is block-type valves, which use a sensing bulb on the suction line within the valve body. 

2. Internally Equalized Valves

Internally equalized thermal expansion valves use inlet pressure for the refrigerant by creating a closing force on the valve. These kinds of AC expansion valves are prone to excessive superheating as they may create a more enclosed position that will create a larger pressure drop to the refrigerant than needed. 

What Are the Common Issues of an AC Expansion Valve?

Frozen vents, inconsistent airflow, the release of warm air, and AC compressors working continuously are the most common signs of faulty AC expansion valves.

Let’s take a look at each of these issues:

1. Frozen Vents

As mentioned earlier, the expansion valves regulate refrigerant intake in the system. Once it becomes faulty, this will lead to a continuous flow of liquid refrigerant in the system, especially through the evaporators. This will then lead to the production of excessively cold air, which can cause frozen vents. 

2. Inconsistent Airflow

This is another sign that your AC system isn’t maintaining the optimal amount of refrigerant. The vents would release cold or warm air every once in a while. Excessive liquid refrigerant would also force the vapor to go back and forth in the system, producing inconsistent airflow. 

truck-ac-vents

Truck AC vents

3. Air Vents Releasing Warm Air

When the expansion valve can’t produce the optimal pressure to evaporate the liquid refrigerant, the AC system can’t produce cold air. When your vents release warm air, this might mean that the refrigerant can’t flow through the system properly.

4. AC Compressor Working Continuously

The AC compressor is part of your AC system that manipulates the refrigerant through pressure. Once the expansion valve can’t control the amount of refrigerant in the system, your AC compressor will be forced to work continuously. This domino effect of damage is one of the most common causes of AC failure among vehicles.

5. Evaporator freezing up

If the expansion valve is failing to regulate the refrigerate intake, the system can get too cold causing the evaporator to freeze. The evaporator dropping to a temperature around freezing will trigger the thermostatic switch or freeze switch to shut the system down to avoid further damage.

 

How to Properly Maintain an AC Expansion Valve

AC expansion valves are mostly wear-and-tear parts. This means its performance is expected to decline with continued use and age. However, there are some best practices when using your truck’s AC to help the AC expansion valve work properly:

  • Maintain optimal coolant levelUnderfilling or overfilling your coolant may affect your entire semi truck, including your AC system. A leak in your coolant system may allow refrigerant to escape, reducing the overall pressure in the AC system needed to produce cold air. 
  • Avoid playing with your AC dialPlaying around with your AC dial may cause problems with your compressor. When your AC compressor fails, it may create a domino effect of damage, resulting in a valve leak. 

How Long Does an AC Expansion Valve Last?

The lifespan of your AC expansion valve would depend on the age of your semi truck, your AC habits, your environment, and the quality of your AC system.

The presence of salt on the roads during winter can cause rust to form on your metal HVAC system. Once your system loses its protective layer, a ripple effect of damage will happen to your AC system, including your expansion valve. This is especially applicable to trucks, as they are mostly exposed to harsh environments and weather conditions.

3. Air Vents Releasing Warm Air

When the expansion valve can’t produce the optimal pressure to evaporate the liquid refrigerant, the AC system can’t produce cold air. When your vents release warm air, this might mean that the refrigerant can’t flow through the system properly.

4. AC Compressor Working Continuously

The AC compressor is part of your AC system that manipulates the refrigerant through pressure. Once the expansion valve can’t control the amount of refrigerant in the system, your AC compressor will be forced to work continuously. This domino effect of damage is one of the most common causes of AC failure among vehicles.

5. Evaporator freezing up

If the expansion valve is failing to regulate the refrigerate intake, the system can get too cold causing the evaporator to freeze. The evaporator dropping to a temperature around freezing will trigger the thermostatic switch or freeze switch to shut the system down to avoid further damage.

 

How to Properly Maintain an AC Expansion Valve

AC expansion valves are mostly wear-and-tear parts. This means its performance is expected to decline with continued use and age. However, there are some best practices when using your truck’s AC to help the AC expansion valve work properly:

  • Maintain optimal coolant levelUnderfilling or overfilling your coolant may affect your entire semi truck, including your AC system. A leak in your coolant system may allow refrigerant to escape, reducing the overall pressure in the AC system needed to produce cold air. 
  • Avoid playing with your AC dialPlaying around with your AC dial may cause problems with your compressor. When your AC compressor fails, it may create a domino effect of damage, resulting in a valve leak. 

How Long Does an AC Expansion Valve Last?

The lifespan of your AC expansion valve would depend on the age of your semi truck, your AC habits, your environment, and the quality of your AC system.

The presence of salt on the roads during winter can cause rust to form on your metal HVAC system. Once your system loses its protective layer, a ripple effect of damage will happen to your AC system, including your expansion valve. This is especially applicable to trucks, as they are mostly exposed to harsh environments and weather conditions.

premium-stainless-steel-hvac-box

Premium stainless steel HVAC box

Fortunately, here at Shoreline offer stainless Kenworth HVAC boxes to ensure the longevity of your AC system. These stainless HVAC boxes are corrosion proof, which means the box should last the lifetime. We also provide other premium aftermarket AC truck parts. 

Maintain Your Driving Comfort with a Premium AC Expansion Valve

The performance of a truck and all other vehicles depends on the driver’s comfort. Your comfort affects your decision-making, which dictates your safety on the road. To ensure you’ll get nothing less than a comfortable and safe ride, make sure your system has premium AC parts

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