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Your car’s expansion valves and orifice tubes keep your A/C flowing

Expansion Valves

The expansion valve’s place in the system is at the evaporator inlet. Like any other valve, its job is to control flow; in this case, the amount of refrigerant entering the evaporator. Since system operating conditions vary (sometimes high cooling demand, sometimes low cooling demand) it is necessary to be able to adjust the amount of refrigerant entering the evaporator. For any given operating condition, if we were to allow too much refrigerant to enter the evaporator, it would get too cold, and the moisture collected on it could freeze. This would not allow the hot interior air to pass through its fins, and the refrigerant flowing inside the evaporator would not be able to absorb the heat from the air. This would eventually bring cooling to a halt. If we were to allow too little refrigerant to enter the evaporator, there may not be enough to properly absorb the interior heat, which would also result in inadequate, or no cooling. This process of varying refrigerant flow based on system cooling demand is referred to as “metering” the refrigerant into the evaporator.
So how does the expansion valve know how much refrigerant to meter into the evaporator, and how does it do it? First the “how it does it”, and it’s quite simple. Expansion valves contain a movable rod which travels up and down inside the valve. As the rod moves up and down, it can open and close the passage inside the valve that serves as the flow path for the refrigerant. The valve does not have to be fully opened or fully closed at any given time. Its position can vary, or modulate, between the fully opened and fully closed positions. Because of this, it can very accurately meter the precise amount of refrigerant needed to meet any given cooling demand.
This internal passage inside the TXV is much smaller than that of the refrigerant flow pipe that delivers the refrigerant to it. Because of this, as the refrigerant flows through this passage, its pressure drops, and it becomes the low-pressure liquid we referenced earlier. So as you can see, the expansion valve also serves as a “dividing line” between the high and low pressure sections of the system.
Now the how does it know how much part…
This is based on evaporator’s outlet temperature. The warmer the evaporator is, the more refrigerant flow needed, and vice-versa. The expansion valve has a temperature sensing device called a sensing bulb. The sensing bulb measures temperature at the evaporator’s outlet and sends a signal to the movable rod inside the expansion valve. This signal corresponds to the amount of refrigerant needed, the rod moves to the proper position, and the correct amount of refrigerant enters the evaporator.
An expansion valve could somewhat be likened to the thermostat in an engine cooling system:

  • The thermostat controls the flow of coolant from the engine to the radiator based on cooling system temperature.


  • The expansion valve controls the flow of refrigerant entering the evaporator based on evaporator temperature, or A/C system load/cooling demand.


Orifice Tubes
 Orifice tubes are used in systems that don’t use expansion valves. Like an expansion valve, the orifice tube is used to control the amount of refrigerant entering the evaporator. The diameter of the orifice tube is similarly small to that of the passage inside the expansion valve, but there is one big difference between orifice tubes and expansion valves. An orifice tube is a simple fixed device with no moving parts. It cannot vary the amount of refrigerant flowing into the evaporator the way an expansion valve can. So in systems that use orifice tubes, some additional method of refrigerant flow control must be employed. Two of the most popular methods used to do this are turning the compressor on and off at appropriate times, or cycling it, or installing a valve inside the compressor that can actually cause an adjustment to the pumping capacity of the compressor. This, of course, regulates the amount of refrigerant leaving the compressor.


Orifice tubes also serve as a dividing line between the high and low pressure sections of the system.


Things that can go wrong with expansion valves and orifice tubes
Expansion valves may fail in these three ways:

  • Clogging or blockage


  • Sticking either open or closed, or partially open or closed.


  • Loss of proper metering ability due to wear, or an internal failure.

Of course, any of the above conditions could cause the wrong amount of refrigerant to be metered into the evaporator, which could lead to improper system operation. Any of these problems will require replacement of the expansion valve.
An orifice tube is a very simple component with no moving parts. About the only thing that ever goes wrong with them is clogging from debris, which always requires orifice tube replacement.
When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.  
You can E-mail us at or visit to find a Mobile Air Conditioning Society repair shop in your area. Visit to find out more about your car’s mobile A/C and engine cooling system.

0 responses to “Your car’s expansion valves and orifice tubes keep your A/C flowing”

  1. car says:

    thanks for post.

  2. mantu says:

    its a very helpful post. thanks a lot

  3. Virgil says:

    Thank you for your help it was very useful.

  4. Dean says:

    Valuable information.

  5. len strong says:

    why do the engineers use expansion valves on passenger cars and orfice tube type on trucks or large r vehicles . is based on evaporator size and demand of ac

  6. jfpitbul says:

    very helpful thanks

  7. John Estes says:

    my ford van has both,expansion valve in back and orifice tube in front. Now I understand why

  8. Wes Smith says:

    Very helpful

  9. Rudy Rojas says:

    I have a Nissan altima i have replace the relay switch, pressure switch, expansion valve. My A/C is still blowing hot air what els can it be .can u please help it’s a 2000 altima

  10. You don’t mention if you are a professional technician, please visit professional MACS member service and repair shop and get a proper diagnosis.
    You may have a refrigerant issue and it needs to be professionally recovered.

  11. […] a good article explaining what happens when the expansion valve (TXV) gets clogged or just fails. Your car’s expansion valves and orifice tubes keep your A/C flowing | Mobile Air Conditioning Soci… Here's what I would do (sorry if it seems scatterbrained,its the cold medicine): Take it to a […]

  12. iyke says:

    change your condenser unit.

  13. Dee says:

    I put my orifice tube in backwards in my 1989 Lincoln Town Car, & I think I overfilled it will refrigerant..
    I was driving & the engine began racing off & on, then I heard a loud BANG!!, then the whole car died, lights & all.
    Once I pulled over, I noticed power steering fluid had sprayed….
    My car was running great before I put in the refrigerant & backwards orifice tube..
    Any answers???
    Tow Truck guy said my battery may have blown, to go have it recharged, & top off my power steering fluid, turn around my orifice tube, release some of the refrigerant, & I should be okay…
    Does anyone have any idea what I may have done, & how I could fix it??
    Thanks in advance..

  14. Much the way I do not recommend operating on yourself if you need surgery take the car to a professional shop where the skilled and trained techs will fix your car using the correct equipment.

  15. Robert Hook says:

    My low side pressure is at 35psi, and the high side at 150psi. The high side is supposed to be at around 250psi. My ac system has failed twice, could this be a result in an expansion valve not working properly? The high and low side tubes and accumilator also frost up, and after awhile the compressor works hard.

  16. LATOYA BARKER says:

    I am having issues with my A/C. What was your ending issue to resolving the problem?

  17. Please take your car to a service professional we cannot diagnose over the internet

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