The Mobile Air Conditioning Society (MACS) has determined, by means of surveying its members, that the most frequent air conditioning (A/C) repairs involve servicing the compressor. Due to this fact, today's A/C technicians must possess the knowledge to diagnose root causes of compressor failures and employ appropriate service procedures to avoid premature compressor breakdowns. As a result, enhanced customer satisfaction can be achieved and costly comebacks prevented.

So what causes compressor failures today? Some causes are inherent design factors beyond the control of technicians. To meet stringent corporate average fuel economy (CAFE) standards, automotive manufacturers have to balance the design of their compressors between weight and durability. Additionally, a large majority of today's vehicles locate the compressor down low. Several experts believe this mounting location allows noncompressible refrigerant oil, instead of vapor, the opportunity to run back into the suction side of the compressor, causing compressor "slugging." Naturally, compressing a liquid guarantees premature compressor failure.

Another design concern, especially relating to a clutch cycling orifice tube (CCOT) system, is the high impact loads placed upon the compressor as it cycles on. Most compressors are driven faster than engine speeds (overdriven), due to a smaller diameter compressor pulley versus the crankshaft pulley. Engaging a compressor at engine speeds above 1500 rpm with high discharge (head) pressures places extreme loads upon the compressor's components. To add to this destructive process, most newer vehicle designs employ lower hood lines and little or no grill openings. As a result, compressor head pressures increase due to limited airflow over the condenser.

What compressor failures are within the control of the technician? Most failures fall under two primary categories: 1) inadequate compressor lubrication, or 2) excessively high head pressures. Let's investigate the origins of these failures and applicable diagnostic and service procedures.

Inadequate Lubrication

Because most modern compressors no longer contain an oil sump or pump, proper refrigerant circulation is crucial since it holds small portions of oil as it moves throughout the A/C system. Passageways and internal pressures distribute this oil-laden refrigerant when it enters the compressor. Because the lubricant continuously flows throughout the system and does not stay in the compressor, any fault that allows the oil to escape from the refrigerant's grip or impedes the proper flow of refrigerant within the system will cause premature compressor failure.

A/C systems that operate at exceptionally low pressures are susceptible to premature compressor failure. Low pressures frequently occur during two operating conditions: a) cold weather in defrost mode, and b) systems functioning with low refrigerant charge. During these two conditions, insufficient low side pressure in the evaporator will not allow system lubricant to properly circulate with the refrigerant. During cold weather, thickening of the refrigerant oil only worsens this situation.

To avoid compressor failure during cold temperature conditions, A/C systems use some type of electrical safety switch that must be operating properly to prevent compressor engagement. Switches performing this function include an ambient temperature switch that prevents compressor operation below 40 degrees Fahrenheit, a low-side pressure cycling switch that prevents compressor operation usually below 25 degrees Fahrenheit or an evaporator temperature switch that cycles the compressor off when the evaporator becomes cold.

Unfortunately, with an evaporator temperature switch, if there is a low charge the evaporator will never become cold. This leaves the compressor engaged, resulting in compressor failure. Conversely, a refrigerant overcharge can also decrease compressor lubrication due to the possibility of refrigerant oil pooling in the condenser or accumulator/ receiver-drier instead of circulating throughout the system.

Many times, inadequate compressor lubrication is simply due to an insufficient quantity of refrigerant oil in the system. Refrigerant leaks, which take out oil with them, are a major cause of low refrigerant oil levels in A/C systems. Beware, when servicing a vehicle that has a low refrigerant charge, if you only top up a system with refrigerant you may be sending the vehicle back into service with an inadequate amount of refrigerant oil. Locating these leaks used to be simpler with R-12 because mineral oil leaves an oily, telltale residue that collects dust around leaking fittings. On the contrary, today's hygroscopic polyalkaline glycol (PAG) and polyester or ester (POE) oils associated with R-134a tend to dissipate and wash away dirt, leaving fittings solvent clean.

Also keep in mind when replacing leaky o-rings on all A/C systems, including R-134a systems, that replacement o-rings and their fittings should be lubricated with either mineral oil or an application-specific, silicone-based lubricant instead of PAG or POE oils. PAG or POE oils should not be used due to their hygroscopic nature that can promote moisture attraction, possibly causing o-ring deterioration or thread fitting seizure.

When servicing a compressor, technicians must follow manufacturers' recommendations relating to both oil quantity and type. Recently (August 2002), General Motors Corp. released a new PAG oil that went into production in certain 2002 model vehicles. This new GM Universal PAG Oil, U.S. Part No. 12378526, is compatible with all GM R-134a A/C systems (refer to GM Bulletin No. 02-01-39-004). Also, in December 2002, GM issued a new compressor oil balance procedure, used when replacing a compressor, which supersedes all previous information in their service manuals (Refer to GM Bulletin No. 02-01-38-006).

Most replacement compressors today are being shipped without the proper amount of oil recommended by the automotive manufacturer. When replacing a compressor, add half of the recommended oil to the compressor, and the other half to the new accumulator/receiver-drier, which should also be replaced at this time. Before operating the A/C system, always rotate the compressor shaft by hand with a spanner wrench a minimum of 10 times to clear any refrigerant oil from a cylinder. Remember, too much refrigerant oil is also detrimental to compressor life and A/C performance. Excess oil will sit in the condenser, slowly heating up, and act as a heat sink - preventing proper heat transfer. Symptoms of an oil overcharge include an A/C system that begins to perform properly with a standard 20 degrees Fahrenheit to 60 degrees Fahrenheit drop across the condenser inlet and outlet, then subsequently exhibits slowly rising head pressures and less than a 20 degrees Fahrenheit condenser inlet to outlet differential. This is due to the additional refrigerant oil sitting in the condenser and holding heat.

When a compressor fails, the condenser becomes the debris field. To prolong a replacement compressor's life, it is imperative that either a filter be installed to catch the debris, or the system be adequately flushed to remove it. Vehicle manufacturers and mobile A/C experts all have differing opinions on which method is best, or if a combination of the two is necessary. A concern with flushing relates to solvent residue that might be left behind, later hindering the lubrication properties of the refrigerant oil. One aftermarket supplier's A/C flush solvent, "Dura 141," has seen successful acceptance by the repair industry. It has a boiling point below 90 degrees Fahrenheit, so leaving residual solvent doesn't appear to be a problem.

An additional concern relative to flushing is the design of today's condensers. Unlike earlier tube and fin condensers that utilized a large, 6mm (.273 inch outer diameter) round tube, modern high efficiency condensers now employ a flat tube made up of several individual ports measuring approximately .040 inch by .060 inch or .127 inch square (Photo 1). Totally flushing contaminants out of these flat tube condensers may be impossible. For best results, a condenser of this design should be flushed in the direction of refrigerant flow, and an inline filter installed to ensure any debris that might later become dislodged would not be allowed to enter the compressor. To prolong a replacement compressor's life and return the A/C system to like-new performance, replacement of the condenser may become the only option.

Keeping the Pressure Off

Like a high compression engine, abnormally high A/C head pressures place excessive loads upon internal components of the compressor. Many times when diagnosing higher-than-normal compressor head pressures, technicians overlook the possibility of refrigerants that are foreign (alternative refrigerants and unacceptable mixes), contaminated, or contain noncondensable gases (air). Some foreign refrigerants do not readily release heat, resulting in high head pressures. Likewise, air in the A/C system tends to migrate to the condenser, reducing its effectiveness. With the assistance of a refrigerant identifier (Photo 2), the percentage by weight of R-12, R-134a, R-22, hydrocarbons and air can easily be identified, thereby eliminating this potential problem.

Condensers can either be internally restrictive (refrigerant restriction) or externally restrictive (airflow restriction), causing excessively high head pressures. Internally restricted condensers will display a large, uneven (not gradually decreasing) temperature drop somewhere in their refrigerant flow path. As earlier stated, proper flushing and/or inline filter installation can prevent internally restrictive condensers.

An externally restrictive condenser involves anything affecting airflow through the condenser. Making sure the condenser fins are clean and the cooling system (especially the radiator fan) is functioning properly can go a long way in extending a compressor's life. Keep in mind, when servicing fan clutches, that installing the proper rotating assembly is critical to prevent engine overheating or possible breakage of the fan clutch. Viewing the water pump pulley and belt can easily identify the direction of these fan clutch assemblies. Clockwise rotating assemblies include V-belt pulleys and poly-grooved pulleys, if the grooved side of the serpentine belt is in contact with the grooved side of the pulley. When the pulley is flat and in contact with the backside of the serpentine belt, the direction will be counterclockwise (Photo 3).

As approaching spring brings A/C-related repairs to your facility, commit to the right repair procedure that provides a long life to your customer's A/C system. Charge properly, check for proper operation of low-pressure switches, and use the correct type and quantity of refrigerant oil. Keep in mind that a clean A/C system - clean both inside and out - provides you with the security of knowing you did the job right!

Keith Reinhardt is an assistant professor of automotive technology at Southern Illinois University Carbondale in Carbondale, Ill. He holds a master's degree in educational administration and is an ASE-certified master automotive technician/paint and refinishing. His e-mail address is kvette@siu.edu.

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