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A/C servicing — are you doing it right?

Wednesday, May 1, 2019 - 07:00
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System refrigerant and oil capacities are smaller, R1234yf 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.

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Let's start with R134a

In the early days of R134a, system refrigerant charges of two pounds or more were not uncommon. Over time, though, 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 ounces 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?

(Photo courtesy of Robinair) The newest machines are certified to recover 95% of the system charge. Preheating the system can help you recover even more on that first try.

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 and that means 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% of the vehicle's charge and recharge the system to within 1/2 ounce 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 R1234yf. 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 ten 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.

Heat exchangers (condensers and evaporators) are moving to these flat-tube designs. Note the small passageways and multiple flow paths. You are not going to flush these clean!

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), evap core and condenser. Today, most of the oil is supposed to remain in the compressor - up to 75% 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.

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