Tool Briefing: The No-Cold Zone

Finding the reason for poor A/C performance


Next you’ll want to identify the refrigerant in the system. If it’s not pure R134a, your pressure gauge readings may not match your pressure/performance chart.

A new generation of electronic refrigeration system analyzers is available. They display things like low-side and high-side pressures, min/max pressures, vapor and liquid saturation temperatures, calculated superheat and subcooling, and a pressure/time chart of what’s happening in the system. There are other features too, but one of the most important is data logging of all these readings and calculations. The data can be downloaded to a computer for printing or importing to a work order.

If system pressure or charge is low, it’s time to look for leaks. There are two tools for this; dye and electronic leak detectors.

Dye Leak Detection

Dye added to the refrigerant will show up at the site of the leak under ultra-violet (UV) light. In most systems, only 1/8 ounce of dye is required, but don’t add dye before checking to see if it’s already there. Some vehicles have dye installed at the factory, or dye may have been added during a previous repair. Look for a “dye added” sticker somewhere under the hood. If you used a sealant detector as noted earlier, dye will show up in the flow meter too.

Excess dye is bad for two reasons. First of all, dye is carried by oil, not refrigerant, and some dye oils interfere with lubrication. The easiest way to avoid this is to use dye that meets Society of Automotive Engineers (SAE) Specification J2297. Secondly, dye fluoresces at its maximum brilliance at a specific dilution ratio, so excess dye won’t shine as brightly under UV light. Your dye injector must be accurate and precise, and you really should use it as instructed.

UV lights, whether powered internally or by the vehicle battery, are quite powerful. Most UV lights and/or dyes come with colored glasses. These not only protect your eyes from UV radiation, they also enhance the fluorescence of the dye by as much as 40 percent. Use the glasses.

With dye in the system, start the engine and run the A/C for at least 5 minutes, then start looking for leaks. Check carefully around all joints and fittings, especially the service port caps, and don’t forget the evaporator case drain. A very small leak may not appear for a few days, so you may have to ask the customer to come back next week. If you find a leak, don’t assume it’s the only one; check the entire system.

After completing your repairs, use the dye cleaner supplied in the kit to remove any dye stains that are visible, and don’t forget to affix a “dye installed” sticker somewhere under the hood. This will prevent confusion if A/C service is required in the future.

Electronic Leak Detection

An electronic leak detector works with the engine not running, as long as system pressure is at least 50 psi. The newest detectors built to SAE Standard J2791 can find leaks as small as 0.15 oz (4 grams) per year.

Using this tool effectively takes time, patience, and attention to detail. The tool must be kept clean and the replaceable sensor tip and batteries must be in good condition. Many are sensitive to some common underhood fluids and require periodic calibration/reset during use. Some will reset automatically.

Refrigerant is heavier than air, so it’s important to check below the line/fitting and, if possible, inside the evaporator case drain tube. By carefully following the user manual, and with practice, this important tool will tell you the size of the leak as well as location.

An automotive air conditioning system is designed to blow air that’s 25 to 35 degrees cooler than the outside ambient temperature. When everything is working correctly, you should see the same vent temperatures on both the front and rear A/C systems. If front and rear performance are not the same, these tools can help you find out why.

Thanks to Jeff Prickett of Bright Solutions International, Paul DeGuiseppi of the Mobile Air Conditioning Society, and Ritchie Engineering for their contributions to this article.

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