ACtion Magazine - June 2012 - (Page 22)

by Paul Weissler, MACS Senior Technical Correspondent State of the art detection mistake in leak diagnosis could mean unnecessary R&R. Further, even the refrigerant line joints (the sources of many leaks) may not be readily accessible, nor the places where a line vibrates against sheet metal and rubs through. It should be obvious that several leak detection methods will not work unless there is good access to the area. Today, all detection methods have to apply to much smaller leaks. Simple arithmetic: a 1980s car with a 3-4 lb. refrigerant capacity system and a large receiverdryer, had 8-10 oz. of refrigerant to spare. Even an accumulator system of that vintage (no receiver-dryer) had a capacity margin of 10% or more, so there was about 5-7 oz. of extra refrigerant. On the older systems with rubber hoses and barbed fittings, there could be what we considered a normal annual loss of that reserve, so a spring recharge was “preventive maintenance” and ensured the system would remain sufficiently charged to provide good cooling for the entire season. However, a modern system with one pound of refrigerant has perhaps 2 oz. of extra refrigerant, and can lose maybe 3-4 oz. before a performance loss can be measured. And with today’s superior seals and hoses, we don’t even think about an annual recharge and haven’t done so in many years. Recent studies of late-model cars show that 0.3 oz. (10 grams) for a front-only system, and 0.5 oz (15 g) for a front-rear system with a second evaporator, are the new annual leakage norms for a well-sealed system. Since the compressor shaft seal accounts for about half that amount, the leakage rate from individual joints must be really small—and it is, in some cases well under 1 g/yr. The previous generation of electronic leak detectors was calibrated to find leaks of 14 g/yr. and greater. That’s just too high for today’s systems. That’s why even if a system has a large leak that you locate, with most or virtually all of the charge gone, you have to continue to look for leaks of all repairable sizes. You wouldn’t want to replace a leaking compressor and have the customer back in a year or two with an A/C performance complaint because a few O-ring seals are leaking. here is no area of A/C service that is so filled with different techniques and technician opinions as refrigerant leak detection. The reasons are simple—the systems themselves have changed over the years, and A/C technicians with lots of experience have lots of experience with leaks. Some of the methods also have changed, and techniques that worked years ago just aren’t as effective on the latest systems. The SAE engineering standards committee that focuses on A/C, including all the service equipment, recently released SAE J1628, “Technician Procedures for Refrigerant Leak Detection Service of Mobile Air Conditioning Systems.” It covers test evaluations of almost all methods in current use, their relative effectiveness, and the most logical approaches to their use. The information is based on objective testing, not anecdotal “evidence” and represents the most up to date information available. An experienced technician may well have a streak of “stick with what worked.” He’s learned how to keep using techniques that have become inherently less effective, but still work for him because he combines them with that precious experience. He can almost smell a leak. (Don’t get any ideas; refrigerants do not emit a detectable odor.) The technician with long experience also may have started in this industry when problem leakage was large enough to be detectable with methods that are less effective today. That doesn’t mean he never finds a leak, just that he’s probably wasting a lot of time. The primary reason for changing leak detection methods is the important changes in the system itself. Refrigerant capacities have been coming down dramatically—veteran technicians may remember systems with capacities in excess of six pounds. Today, for many cars, 22-24 ounces qualifies as large and some systems are in the 12-18 oz. range. The need to use refrigerant to circulate oil has been sharply reduced so it’s hardly unusual to find system oil capacities in the 3-5 oz. range compared to the 8-10 oz. of not too many years ago. With less oil in circulation, and the use of more efficient condensers and evaporators, the amount of cooling from the reduced charge is maintained. The systems are more energy efficient as a result. T First step: Is there a leak? Leak detection should begin with a determination of whether or not there really is a leak—an “A/C not working” complaint could be from an electrical or mechanical cause and that includes failure of a low-pressure switch or pressure sensor. But if the circuit seems operative, the first refrigerant system check is a pressure gauge test. That doesn’t say you can tell normal from below-normal pressures with a static pressure check, but you can tell if the system has any significant amount of refrigerant in it or if the reading is so low that a lot of refrigerant must have leaked out. With today’s small capacity systems, however, a loss of say 30-50% of the charge can be difficult to evaluate so when you feel you have to find out how much refrigerant remains in the Everything is tighter Not only are the systems’ capacities much smaller, but access is more difficult. Oldtimers may fondly remember when the expansion valve was wide open under the hood, and when the HVAC evaporator case projected through the firewall; removeand-replace was a one-hour job. Today we’re lucky if we even see the expansion valve under the hood and access to the evaporator could take a day. Any 22 ACTION • June 2012

Table of Contents for the Digital Edition of ACtion Magazine - June 2012

ACtion Magazine - June 2012
Contents
Outlook
Expansive Valve
Technically RELAY-ted
Under the Southern Cross
Leonard's Law
Cooling Corner
Virtual View
News & Updates
State of the Art Detection
Running Short On Time and Info
The End of R-134A?
Association News
Quick Check
New Products & Services
Last Watch

ACtion Magazine - June 2012