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Auto A/C Systems – A Quick Review

A Quick Review

As a review, lets put it all together in one place, and once more trace the path of the refrigerant starting at the evaporator outlet. For our example, we’ll use an expansion valve type system equipped with a receiver/drier. (The process is similar in an orifice tube/accumulator system.)

The compressor draws the heat-laden refrigerant vapor from the evaporator. It is then compressed and sent under high pressure to the condenser. Since at this stage of the cycle, the refrigerant is much hotter than the surrounding air, it gives up the heat it absorbed in the evaporator to the cooler outside air flowing through the condenser fins. As the refrigerant vapor gives up its heat, it changes back to a liquid inside the condenser. The liquid refrigerant then moves into the receiver/drier, where moisture it may contain is absorbed by the desiccant. Some refrigerant may also be temporarily stored in the receiver/drier until it is needed in the evaporator, dictated by system cooling demand. The liquid refrigerant leaves the receiver/drier, and is metered back into the evaporator as a low-pressure liquid. The refrigerant picks up heat from the warm interior air passing through the evaporator, and changes back into a vapor. The warm air is moved through the evaporator, and cooled air circulated throughout the interior of the vehicle, by the blower fan. The now-heated vapor travels to the evaporator outlet, returns to the compressor, and the cycle begins again.

A Word About Automatic Temperature Control Systems

ATC systems contain many complex components, with comparably complex service and diagnostic procedures, all beyond the scope of this publication.

Things that go wrong with Auto A/C Systems

The upcoming section will help you gain an understanding of the types of failures that can occur in automotive A/C systems, and procedures that may be necessary to correct them.

Things That Go Wrong:

with Evaporators:

Evaporator failures can usually be summed up in one word: leakage. Leaks can occur for a few different reasons. Usually, either a seam or weld has gone bad, creating a leak point, or corrosion has occurred, causing an “outside-in” failure. This often happens because leaves or other organic material enter the evaporator case through the exterior air intake vents and come in contact with the evaporator’s surface. This moist atmosphere causes decomposition of the organic materials, and can form caustic corrosive substances which can eventually perforate the surface of the evaporator. These same substances can also sometimes lead to an odor problem inside the vehicle, most noticeable when the system is first operated. Various de-odorizing biocide treatments and drying modules are available to help with this problem. The evaporator fins are also susceptible to clogging from leaves and other debris.

Whichever the type of evaporator failure, it must be replaced. And while an evaporator in and of itself may not be (for most vehicles) a very costly part, depending upon its location (usually like heater cores, deeply buried beneath the instrument panel) this is often a very time consuming, and therefore costly repair.

with Compressors:

Compressors are generally reliable components, but can catastrophically fail due to a lack of lubrication, just like your car’s engine. Lubricating oil can leave the system if a refrigerant leak occurs, because the leaking refrigerant can carry lubricant out with it. Such a failure can cause the compressor to lock up (seize) or wear out prematurely. A catastrophically failing compressor can also load up the inside of the A/C system (particularly the condenser) with debris, usually small chips of metal.

A compressor can also fail because of too much oil in the system, which can cause its internal valves to break. Compressors themselves may also develop a refrigerant leak, which usually necessitates their replacement.

Compressors may also be damaged by things such as an overcharge of refrigerant, excessive amounts of air in the system, the use of the wrong refrigerant or lubricant, inadequate condenser airflow, or even engine cooling system problems. And of course, just like any other component that contains moving parts, compressors may sometimes fail simply due to wearing out or breakage. Unlike in years past, with the exception of clutch replacement, compressors are not generally serviced in the repair shop any longer.

with Condensers:

Like evaporators, condensers are also susceptible to external blockage, mostly from things like leaves, insects, or even dust. A good cleaning with water, maybe even using soap and a soft-bristle brush, often does a good job of clearing debris from the outside of a condenser. But as previously mentioned, condensers are also susceptible to severe internal blockage after a compressor failure. Metal particles and other debris from the failing compressor move into the condenser with the refrigerant, and can quickly block the very small passages inside the condenser. In some instances, it may be possible to flush this debris from inside the condenser, but in many cases, the blockage is so severe that the condenser must be replaced.

Also like evaporators, condensers can suffer a seam or weld failure, resulting in leakage. But condensers must contend with something that evaporators don’t; because of their “up front” mounting location, condensers can easily suffer physical damage, from debris like small stones and such kicked up off the road, or from front-end collisions.

In the vast majority of circumstances, internally clogged, leaking, or damaged condensers are not repaired, but replaced with new units.

with Receiver / Driers and Accumulators:

Receiver/driers and accumulators rarely fail themselves, but as mentioned previously, need to be replaced whenever the system is opened for any other type of service. When a failure docs occur with a receiver/drier or accumulator, it is usually due either to clogging from debris inside the A/C system (like from a failing or failed compressor), or that the bag containing the desiccant has broken open, allowing desiccant material to circulate throughout the system with refrigerant and lubricant. Sometimes, the desiccant material will disintegrate into small sand-like particles. This can cause possible clogging in other system components.

with Expansion Valves and Orifice Tubes:

Expansion valves may fail in these three ways:

  • Clogging or blockage
  • Sticking either open or closed, or partially open or closed.
  • Loss of proper metering ability due to wear, or an internal failure.

Of course, anv of the above conditions could cause the wrong amount of refrigerant to be metered into the evaporator, which could lead to improper system operation. Any of these problems will require replacement of the expansion valve. An orifice tube is a very simple component with no moving parts. About the only thing that ever goes wrong with them is clogging from debris, which always requires orifice tube replacement.

with Pipes, Hoses, Gaskets and Seals:

Of course, hoses, gaskets and seals throughout the system are prone to leakage over a period of time. Metal tubing can suffer a rub through, cracking, or breakage. Very often, failed hoses and/or metal tubing can be repaired, but many times replacement is required.

with Air Distribution Systems:

Case/duct systems have proven to be fairly reliable and relatively problem free.

Failures in this area will usually fall into the following categories:

  • A binding, sticking or broken air routing door. (Sometimes foreign objects can fall down inside vents and cause problems like this.)
  • Breakage of door control linkage mechanisms. These are often made of plastic.
  • A slipping or binding control cable.
  • Vacuum motor diaphragms can develop leaks.
  • Electric control motor can fail either electrically, or mechanically. These motors contain gear drive assemblies that can wear out over a period it time.
  • Cabin air filter needs changing. These often neglected components eventually become dirty and clogged. This can reduce airflow, or even possibly stop it altogether.

Because of their location or limited working space, replacing some of the above components can often be difficult or time consuming. And therefore (you guessed it) costly.

A Final Word on Things that Go Wrong

As with many other systems on todays high-tech computer controlled automobiles, getting to the root of an A/C system problem is sometimes not a quick or easy task. In some instances, thorough troubleshooting aided by complex information an sophisticated test equipment will be necessary. If this situation should arise, your repair shop may tell you that it could take them some time (with an associated labor charge) to perform a proper diagnosis. Please be patient with your service provider if and when this should occur. Allow them the time they need to find out why your A/C ism keeping you cool.