Nailing tough A/C problems: A review of new tools and diagnostic strategies

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Nailing tough A/C problems: A review of new tools and diagnostic strategies

Craig Truglia is an ASE A6, A8, and L1 certified technician who presently works as a service writer for Patterson Auto Body, a repair facility in Patterson, N.Y. A former shop owner and editor of several automotive repair magazines, Truglia combines his Columbia University education with the real-world experience he sees daily in the automotive repair field. Technicians Truglia and Fred Byron took part in diagnosing the different vehicles presented as examples in this article.

As labor rates climb, and serviceability gets increasingly convoluted, it is becoming more and more important to diagnose A/C problems as accurately as possible. Of course, being that air conditioning is generally diagnosed by interpreting pressures and looking at the “general vicinity” of where UV dye can be spotted, it’s less than an objective, exact science.

However, there are new tools and diagnostic strategies that make use of the tools already in most shops, that can greatly increase accuracy in diagnosing A/C problems. Chief among these is using thermal imagers, emissions analyzers, broad spectrum UV flashlights, CO2 gas and a smoke machine, and the Bullseye Leak Detector (which in principle works the same).

Using dye

The tried-and-true method to finding leaks is to use a UV dye and a broad spectrum UV light.

There is a good reason for this: It works almost all of the time.

The normal procedure for the technician is to use a UV light and scan the obvious trouble areas. This includes the evaporator, condenser, lines, compressor, expansion valve/office tube, Schraeder valves, and any associated seals.

A/C machine-safe sealants?

There are two kinds of technicians — those who use whatever A/C sealant they can find and hope it does not cause any issues and others who are really paranoid and use sealant detectors like the Neutronics Quick-Detect to find harmful sealants. If the technician wants to avoid potentially trashing their A/C machine, an in-line sealant filter such as the Charge Guard from Air Sept can be used to remove it.

The Society of Automotive Engineers has issued a standard (SAE J-2670) for sealants that can be used for mobile air conditioning that will pose no threat to standard air conditioning servicing equipment. U-View makes a sealant called “Leak Guard” that is oil-based, so it does not harden when it reacts with moisture like most other sealants. This particular product has a dye in it, so it can be used for the detection of very large leaks, as well as replacing the need to inject oil in the system because it is in of itself oil-based. In independent testing (by the author of the article) it has been found to work, even in a vehicle that has had the standard A/C sealant added to it to no avail.

Many shops make it a standard practice to inject oil, dye and then sealant in three different steps. Not only is this potentially risky to equipment, but it is time consuming.

New J-2670-certified products make it possible to mitigate the risk of A/C leaks, while at the same time lubricating the A/C system and putting in dye so larger leaks can be spotted.

Dye cannot always spot leaks for several reasons. Sometimes, it washes away or it is covered by road debris. In other instances, it is somewhere the technician cannot easily point an UV light at (i.e. behind the firewall where the evaporator is). Oftentimes, evaporators are sold without total confidence that they will fix the problem. Is there a way to be more certain about the location of an A/C leak?


CO2, a technician’s best friend

The methods covered here should be able to be replicated with any electronic leak detector that is SAE J-2791 certified, because they are rated to spot leaks as small as 1/4th of an ounce of refrigerant a year. It is recommended to try any approved “A/C sniffer” if one has it available, as it does not require putting CO2 gas into an A/C system. However, because the author of the article has successfully used CO2 to find leaks, the methods covered here focus on the methods that are verified to work.

The principle behind finding A/C leaks using CO2 is pretty simple. CO2 is an inert gas. It does not react with refrigerants, oils, or sealants, so it will not mess anything up. Plus, it can be detected using an emissions analyzer.

On top of this, atmospheric CO2 is so low it is essentially undetectable, so if an emissions analyzer picks any up is a dead giveaway that there is a leak. Further, CO2 is heavier than air, so it always falls below the leak, which is very useful for spotting leaking evaporators. Lastly, CO2 is so small (it’s smaller than the molecules that constitute refrigerant), it can leak out of any leak point that a technician would have to worry about.

A simple and easily workable way to put CO2 gas into an A/C system is to remove the Schraeder valve and pipe in CO2 gas at full blast using a smoke machine. It does not matter whether it is via the high or low side.

Dye can be added to these smoke machines to help find leaks the tried-and-true way, while the CO2 gas in the smoke can be picked up by an emissions analyzer.

Smoke machines (at least, the ones not used for smoking turbos) operate at a very low pressure. As a result, they will take a long time to fill up an A/C system. Nonetheless, they can still be used to find evaporator leaks that do not require operating pressure to lose refrigerant.

Another method, without a smoke machine, is to devise a way to connector the shop’s CO2 tank or cartridge to the A/C system and regulate the air pressure. If there is not enough time to build a contraption, it can be purchased by a company such as Automotive Test Solutions. Theirs is available with the Bullseye Leak Detector, which regulates the pressure the CO2 fills the system to 140 psi. An A/C system can be filled with up to 200 psi safely. The benefit of filling an A/C system with higher PSI is that many A/C leaks in the real world can only be found under operating pressure.

In order to find an A/C leak with an emissions analyzer and CO2 gas, instead of pointing an UV light at all the suspect areas, slowly run the emissions analyzer below the suspect areas. An emissions analyzer generally takes a few seconds to update and CO2 is heavier than air, so it tends to fall onto the bottom-side of the culprit part.

Spotting a leaking evaporator with CO2 can be done by plugging the evaporator condensation drain hose and after 30 to 60 minutes of filling the system with gas, removing the plug. If there is a leak, CO2 at this point would have descended to the bottom of the evaporator housing, nailing that evaporator leak. This method, with the right tools and CO2 under high pressure, works beautifully.

Diagnosing problems using pressures and thermal imagers

A lot of the above strategies help find A/C leaks, but how about systems with a full charge and yet they do not blow cold air? In a situation like this, the technician will be looking for a bad compressor, or some sort of blockage that prevents normal low and high side pressures.

In a known good A/C system at 70 degrees Fahrenheit ambient temperature, pressures should be about 35 to 40 psi on the low side and 145 to 160 psi on the high side. Psi tends to increase up by 1 or down by 1 with each degree Fahrenheit of ambient temperature.

What do the pressures tell us? An internal restriction typically creates low pressures on the low sides, and high pressures on the high sides (that’s easy to remember!). Expansion valves and receiver driers often get clogged with debris, sealant, or moisture that freezes up.

If pressures are the opposite (high on the low side, low on the high side), the compressor is likely not working.

If the compressor still does not turn on even when jumped with a Power Probe (generally by grounding it), it should be replaced.

Understanding pressures can help the technician find blockages in the system using a thermal imager. Here’s what the temperature of the refrigerant should be in a known-good A/C system while operating:

  • Enters compressor cool, leaves hot.
  • Then enters condenser relative hot, but slightly cools along the way, maintaining a relatively stable temperature until it reaches the receiver-dryer.
  • After the receiver-dryer the refrigerant enters the expansion valve hot and it should leave it cool.
  • After this point, refrigerant enters the evaporator, which is not viewable with a thermal imager.

A restriction should lead to a large temperature difference from what should be expected in an unexpected area.

Thermal imaging makes this problem quickly visible in a way that was before not possible.

Also, when in used in tandem with system pressure it can help narrow down problems.


For example, if system pressures indicate a restriction of some sort and not an under-performing compressor, but the receiver dryer is operating normally, look at the expansion valve. If it does not rapidly cool the refrigerant, then it needs to be replaced.

The temperature of the refrigerant should only slightly cool as it leaves the compressor, so that it is still relatively hot when it enters the receiver-dryer. This A/C condenser has an internal restriction, which radically cools the refrigerant.

How do we find a parasitic draw when pulling fuses wakes up more modules than it puts asleep?

In extreme situations a thermal imaging tool can catch a relay stuck on or a fuse in which something on its circuit is on, because power running through a circuit draws power.   ●

For more by this author, see:

Parasitic draw diagnostic strategies

OBD-II Mode 6: Making it part of your diagnostic arsenal

ECU soft resets: Lilling power to on-board modules does wonders in diagnosis and repair

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