A/C servicing — are you doing it right?

May 1, 2019
While A/C system service is fundamentally unchanged, changes in technologies and system designs require you pay more attention to the details.

System refrigerant and oil capacities are smaller, R-1234yf is more common and loss due to refrigerant leakage impacts system performance more now than ever. The fundamentals you learned in school or on-the-job still apply when it comes to troubleshooting and servicing these systems, but they are increasingly less tolerant of errors, so it's important to pay attention to the details you may have forgotten.

Let's start with R-134a

In the early days of R-134a, system refrigerant charges of two lbs or more were not uncommon. Over time, system capacities have become smaller, with the current average hovering just over a pound or so, and there are several models in production (and have been for the last few years) that use a little more than 10 ozs of refrigerant to cool the cabin. When you consider that the industry standard variance is only +/- 10 percent, that means that an overcharge or undercharge will result with a variance of as little as one ounce and what does that mean in the real world?

Overcharged systems run hotter than they should, experiencing increased compressor head temperatures that can lead to breakdown of the lubricating oil and accelerated wear in the compressor. Undercharged systems are unable to keep the oil circulating through the system, meaning oil starvation to the compressor with the same results. Of course, neither condition will be able to cool the cabin as efficiently as a properly charged system.

The R/R/R (Recovery/Recycle/Recharge) machines in use at the time did not have the capabilities needed to insure the full recovery of the existing charge or the accurate refill of the vehicle after the repairs were completed. Roughly 10 - 12 years ago, the SAE established new standards for servicing mobile air conditioning systems under SAE J2788. R/R/R machines made to these standards had to be capable of removing at least 95 percent of the vehicle's charge and recharge the system to within 1/2 oz of the desired amount. Unfortunately, there are still shops using their older equipment to service late model systems, and those that did invest in the newer machines are bypassing some of the features in the interest of saving a few minutes on the job.

Today, we're faced with new challenges as more and more vehicles come equipped with R-1234yf. The cost of the refrigerant makes recovering as much as possible more important than ever. Even with the recovery capabilities of the latest machines, you can help the process by preheating the A/C system before hitting the "start" button. Simply close the hood and run the engine for five to 10 minutes to raise the temperature of the components (and pressure as a result) prior to evacuation. You can also turn the heat on full blast in the cabin to coax more gas out of the evaporator. You'll know you did the best you could if you open the system and don't hear the "hiss" of escaping vapor!

Along with refrigerant capacity, system oil charges have also been on the decline. Here, too, accuracy is critical to a correct service. Too little oil will bear the obvious consequences while too much can actually coat the heat exchangers (condenser and evaporator) internally, reducing their ability to dissipate the heat taken in by the refrigerant.

And it's not just quantity. It's where you initially add the oil. When we completed a major repair in the old days (compressor and components), we would add half the oil charge directly to the compressor and split the remainder between the drier (accumulator), evaporative core, and condenser. Today, most of the oil is supposed to remain in the compressor - up to 75 percent in a running system - so be sure you follow the OEM service procedures to the letter.

Keeping compressors healthy

Leaks in the A/C system are inevitable over time. As the refrigerant is lost, the amount of liquid charge available in the evaporator drops and is less able to carry off any oil that has collected there. In the meanwhile, normal wear in the compressor is causing the accumulation of fine, abrasive wear particles to collect in the oil, which are then transported throughout the system. Left unchecked, these factors eventually lead to the demise of the original compressor but replacing the compressor without addressing the root causes is only asking for a repeat of the failure — often, a repeat that occurs much sooner than it took for the first compressor to die.

I'm sure that many of you have seen that little written warranty notice many compressor manufacturers have been including in the parts box. It states the manufacturer's require the system be flushed, and the accumulator (and orifice tube) or the receiver/drier be replaced at the same time the compressor is being replaced. Either that, or immediately void the warranty.

I absolutely insist that the accumulator/orifice tube (or the receiver/drier) be replaced whenever I perform an A/C system service; whether it be a new compressor install or a simple leak repair. The main reason I do so is to insure that what little remaining moisture left in the system after I've had it under a vacuum is contained. I do not, however, flush the system - at least, not all of it - when performing a compressor replacement.

For a long time now, the industry has been using flat tube, multi-path multi-pass heat exchanger designs. There simply is no way to flush these components and be successful in getting all that abrasive debris out. The only right way to insure that the debris is entirely gone is to replace the condenser and the evaporator as part of the repair.

The same is true of any line that has an inline filter or muffler in it. They cannot be flushed and must be replaced.

I've already talked about oil a bit, but I do want to reiterate here that following the OEM procedure for adding oil to the system must be followed. It is also critical that you use the correct oil. If your R/R/R machine has an oil injection mode on it, do yourself a favor and don't use it. When your machine is being used to service a number of different vehicles, the oil in it is almost surely cross-contaminated and filled with moisture.

Another commonly missed step is a critical part of the install process. Once the compressor is bolted up, connected, and the system charged, be sure to rotate the compressor through several times to prevent the possibility of "slugging" on initial start-up. On some models, this is a problem even in everyday use, and the ECM has a special logic in its programming to control initial compressor engagement. Nothing worse than suffering a hydraulic lock on that brand new compressor.

Keeping you and your machine healthy

Another issue that seems to be on the increase is contamination of the refrigerant charge in the customer's vehicle. Counterfeit refrigerants are one cause but not the only one or the most common in my opinion. There are too many YouTube "experts" that are showing untrained DIYers how to use alternative refrigerants in their systems, including the popular "Dust Off' aerosol designed for use as a computer/electronics cleaner.

Another source of potential trouble for you and your shop equipment is sealant. Stop by any big box retailer or automotive parts supplier, and you'll see shelves filled with do-it-yourself cans of refrigerant. Look even more closely, and you'll see that nearly every one of them also contains some kind of sealant additive.

I'm not going to argue the merits, or lack thereof, of sealants in this article, but I will say that I know of no OEM that approves the use of sealants of any kind in any of their systems, and that's good enough for me. Unfortunately, the DIYers aren't listening to the OEs, and if a little is good, more must be better!

In order to protect your own health and safety (from the possibility of blend or hydrocarbon-based alternative refrigerants that may be in the system), and your service equipment (from ingesting additives that may clog them up worse than eating ten pounds of cheese), I'm going to once again preach to the need to perform a sealant check, and a refrigerant identification prior to recovering the vehicle's charge. Basic identifiers can be had for a nominal investment, and if you've already taken the plunge into service equipment designed for R1234yf, you know it's not even an option. The high cost of the new gas makes it imperative that you don't contaminate it with any other during recovery, and the R/R/R machine requires this step before allowing you to do so.

Speaking of new machines and new refrigerants, it was brought to my attention at the recent Mobile Air Conditioning Society (MACS) conference that many of you are using R134a machines modified to recover R1234yf. I cannot stress how wrong that is, and I don't blame you guys since many of you purchased the refits from "reputable" sources. It is a pure matter of safety that you cease and desist today!

We all know that R1234yf is "mildly flammable", and there are certain safeguards built into machines certified to the proper SAE standards for servicing these newer systems that the old R134a cabinets simply do not have. Another big difference is the R1234yf machine's requirement that a refrigerant identification test be performed prior to evacuation and recovery of the vehicle's charge. Considering that you'll be servicing R134a systems for some time yet, and the number of vehicles you'll see fitted with R1234yf will just keep growing, bite the bullet and invest in the proper equipment to do the job.

Finding those lost dollars (leaks)

Locating and fixing even the smallest system leaks is also more important; in part, due to the high cost of R1234yf. It's also critical due to the lower system capacities. As I've already noted, even a 10 percent drop in charge will impact cooling capabilities and oil flow through the system.

The most commonly used leak detection method is fluorescent dye, so allow me to offer a few notes on its use. First, it may take a bit longer for dye to circulate through the system than it used to on some models. If you've fixed the big leaks and want to make sure you got them all, ask your customer to return after a few days for a recheck.

It also helps if you match the UV light you're using to the dye and wear the yellow lenses that the dye maker includes with their detection kit. Yes, UV lights operate in a range of frequencies and the dyes can also vary from maker to maker.

Also, be aware that many manufacturers are adding dye at the factory. It's not going anywhere unless there's a leak so check to see if dye is already in the system before adding any. If you do add some, add only 1/4 oz to the system. With the lower oil capacities of today, it is easier than ever to overdose the system with dye if you use too much.

A neat tip I also learned at the MACS event (in a class taught by Standard Motor Products' Peter McArdle) may be helpful when you suspect that small leak is in the evaporator core. Park the suspect vehicle in the sun and close up the cabin nice and tight. The idea is to build up the interior heat level to raise the pressure of the refrigerant in the evaporator core and to give the oil that may be coating the interior surfaces of the core to fall to the bottom. The heat increases the pressure of the gas, too, making it easier for the gas to escape.

After the vehicle has sit for a few hours, take a styrofoam cup and place it under the evap drain tube prior to starting the car and turning the A/C on "max cold". The idea is to capture the first droplets of the water condensing on the outside of the evaporator in the cup. Now, use your UV light to look for dye.

You can also use your sniffer to check for the captured gases in the evaporator case by sticking it up in the evap drain line prior to start up. Since refrigerant is heavier than air, the fumes should collect in the bottom of the case. On some cars, removing the blower motor resistor allows access to the base of the evaporator as well.

On the high side, hose leaks right at the hose crimps are common. To help you find those stubborn ones, get the system pressures high by running the system for a while and then checking for leaks immediately after shutdown. An option is to use the same tactic I discussed when trying to coax out every last ounce of refrigerant - preheat the system by closing the hood and running the engine until it reaches normal operating temperature. If you live in a warm climate, simply putting the vehicle out in the sun for a while may be enough to push up system pressures and reveal the leak. All of these are techniques you can try using the service equipment you likely already own.

Yes, the fundamental principles that allow us to keep our customers cool in the summer and warm in the winter haven't changed. But the way we have to service these systems to keep them at their peak efficiency has!

About the Author

Pete Meier | Creative Director, Technical | Vehicle Repair Group

Pete Meier is the former creative director, technical, for the Vehicle Repair Group with Endeavor Business Media. He is an ASE certified Master Technician with over 35 years of practical experience as a technician and educator, covering a wide variety of makes and models. He began writing for Motor Age as a contributor in 2006 and joined the magazine full-time as technical editor in 2010. Pete grew the Motor Age YouTube channel to more than 100,000 subscribers by delivering essential training videos for technicians at all levels. 

Connect with Pete on LinkedIn.

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