A/C System Operation

Typical Orifice Tube System

CCT Operation

The temperature readings on the system shown above indicate a properly charged A/C system operating at maximum efficiency.

In an HVAC system the refrigerant changes state at the evaporator and at the condenser. At the evaporator the refrigerant liquid changes to a vapor. At the condenser the refrigerant changes from a vapor to a liquid. Each time this state change takes place a tremendous amount of heat energy (BTUs) is transferred with no measurable temperature difference.

If this state change does not take place at both the evaporator and condenser, the efficiency of the HVAC system will decrease significantly. While heat transfer would still take place, the refrigerant cannot remove enough heat from the air in the passenger compartment, and the refrigerant cannot efficiently get rid of the heat at the condenser.

In a properly charged A/C system there will be no difference in temperature across the evaporator.

This indicates that there is still some liquid refrigerant exiting the evaporator and therefore oil circulation is being maintained. Measuring evaporator inlet and outlet temperature is usually fairly easy on an orifice tube system. However on a TXV system, the valve may be inside the HVAC case and taking the evaporator inlet temperature reading after the valve may be impossible.

Typical TXV System

TXV Operation

In a TXV system the drier is called the receiver/drier. It is located in the liquid line between the condenser and the evaporator. It serves four functions – it contains a desiccant, which absorbs any moisture from the system. It acts as a liquid vapor separator to ensure that only pure liquid refrigerant reaches the TXV valve, it has a filter to catch any particulate debris and it also acts as storage vessel for refrigerant in the system.

The TXV valve performs the same function as the orifice tube. The key difference is that the TXV valve can vary the refrigerant discharge rate into the evaporator in direct proportion to the heat load on the system. The TXV valve is connected to the evaporator outlet through a capillary tube and sensing bulb filled with refrigerant gas. As the evaporator outlet warms and cools (a reflection of the heat load on the system) the gas trapped in the sensing bulb and capillary tube expands and contracts. This expansion/contraction acts on a diaphragm attached to a metering valve inside the TXV valve to precisely control the flow of refrigerant.

Because TXV systems more precisely control the flow of refrigerant in the system, in general, they are less prone to lubrication issues than orifice tube systems.

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Peter McArdle

Peter McArdle

Peter McArdle


Sr. Master Technician and Trainer, SMP

Peter has always been fascinated with the evolution of automotive technology and especially the growing complexity of a/c systems today. For over 30 years he has specialized in drivability and electrical repairs, and has devoted his career to providing first-class training to natural born technicians like himself. As an ASE Master and L1 certified technician, and a member of the American Society for Training and Development, Peter’s technical knowledge and commitment to training has contributed to the development of an award winning training program.

Peter has been a highly sought after presenter in the HVAC industry. His knowledge and enthusiasm go hand-in-hand as he engages with his audience, providing new and innovative tips, techniques and much more. 

“I feel privileged to have this opportunity to speak at the MACS Training Event. It’s a great way to engage with fellow technicians. The interaction and feedback from the audience at training events like MACS is often one of my best sources of diagnostic insights and great service and repair tips” – Peter McArdle on presenting at MACS 2016.

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At Last! Diagnostic Direction with Differential Temperature Testing!

The Concept

“Differential Temperature Testing” is a powerful air conditioning diagnostic and evaluation technique. It is also sometimes called “Maximum Heat Load Temperature Testing”. During the test, the A/C system is placed under maximum stress (heat load) and a series of temperature measurements are taken at specific points in the system. By testing the system under stress, any underlying weakness in the system is much more likely to be revealed. The results of the temperature measurements are compared to expected values. If any of the results are out of range, three easy-to-follow diagnostic flow charts provide clear diagnostic direction as to the likely cause of the problem, see charts A, B & C. After a repair, you can return the vehicle to the customer with confidence that the entire system is operating efficiently and will be unlikely to suffer premature Compressor failure or comeback.

“Temperature Testing allows us to evaluate the performance of each individual component in the system and check if it’s operating at peak efficiency – to see, for example, if the Condenser and Evaporator are maximizing heat exchange.”

Use Temperature Testing to:

  • Validate the repair after Compressor replacement and confirm that the system is truly fixed before returning the vehicle to the customer
  • Confirm and optimize the system charge level – especially when a replacement component such as a Condenser is significantly different from the original unit.
  • Measure overall system efficiency as part of a preventative maintenance evaluation of the A/C system.

Temperature Testing has several advantages over traditional O.E system performance testing:

  • The system is tested under maximum stress – at idle with the doors open. This setup creates the greatest demand on the entire system. An underlying weakness is much more likely to be exposed.
  • Temperature Testing uses the same basic vehicle setup and test parameters for all vehicles.
  • You can use the test both as a diagnostic tool to determine the root cause of a system problem, or to confirm that the system is truly fixed and operating at peak efficiency.

Want more information on Temperature Testing? Check out our Temp Testing on TXV Systems here:

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