May 23, 2007

Poor air conditioning performance

Press here for cold air. Click image to enlarge

By Jim Kerr

Warmer days are here at last. Outside temperatures may be pleasant but vehicle interior temperatures soon become very uncomfortable if the vehicle has been sitting in the sun: yes, it's air conditioning season again.

I often hear the words: my air conditioning worked fine last summer but it just doesn't seem to keep things as cool this year. They may not be imagining things: it's likely the air conditioning isn't cooling as well as it did last year. There may be several reasons.

The most likely cause of poor air conditioning performance is a partial loss of refrigerant. R134a, the refrigerant used in automotive air conditioning systems will leak out of the smallest openings. In fact, it is normal for slight amounts of R134a to leak out. The seals on the compressor shaft, connections at pipe fittings and hose couplings are potential sources of leakage. Even the hoses used on the a/c system can let refrigerant leak through the rubber, although hoses used for more than a decade use a barrier layer in the hose to reduce leaks. Over the course of several years, more refrigerant may need to be added.

Before a service technician adds any refrigerant, they will inspect the system for leaks. A sophisticated electronic leak detector is used to "sniff" for any leaks. It will detect the R134a even if there are only a few parts per million in the sample. The technician slowly moves the probe around all joints and across the bottom of air conditioning components. Refrigerant is heavier than air, so leaks can be detected easier at the bottom of components. If no leaks are detected, then the system can be recharged. If a leak is found, it must be repaired before refrigerant can be added.

Large leaks, such as occurs when a compressor seal fails rapidly, or the vehicle is in a collision and the a/c condensor is punctured, may require more than just replacing the damaged part. A/C systems contain a drier agent to remove residual moisture from the system. If moisture were in the system, it could freeze and prevent the cooling cycle from occurring. The drier agent is located in either the receiver/drier or the accumulator, depending on the type of air conditioning system on the vehicle. If the system has been open for a few hours, the drier agent will absorb moisture from the air and it is no longer effective. The receiver/drier or accumulator must be replaced too.

Cooling the vehicle interior is done by removing heat to the outside. This heat is dissipated to the outside air by the condensor located in front of the radiator. Anything that prevents air flow through the condensor will reduce the cooling ability of the a/c system. Remember to remove winterfronts, and inspect the condensor and radiator for leaves and debris plugging the fins. Look closely between the condensor and radiator, as this area can trap a lot of fuzz and it is often overlooked. While inspecting the condensor, also check for missing rubber and foam seals that direct airflow into the condensor. Many vehicles take cooling air in from below the bumper. A small air dam lip below the front of the car helps direct air up into the condensor and radiator. If this air dam is damaged or missing, the system won't cool as well.

Many newer vehicles have pollen filters in the ducting of the air conditioning system. This filter traps pollen and dust before it enters the passenger compartment. These filters are wonderful for those with allergies, but they can plug up quickly if there is a lot of dust in the air. A plugged filter will slow airflow through the ducting. Although the refrigerant system may be fine, there isn't enough airflow into the interior to keep the vehicle cool. Directions for changing the pollen filter are often found in the owner's manual. Some are accessed from the engine compartment, but most are changed from under the dashboard on the passenger side of the vehicle.

Even if your vehicle doesn't have a pollen filter, airflow into the interior can be reduced by partially blocked evaporator fins inside the ductwork. The evaporator cools the air as it flows through the fins, but dirt, moisture and pollen on the evaporator fins can cause mould to form. You may notice a musty smell when the air conditioning system is first turned on. This mould can grow enough that it forms a mesh-like layer on the evaporator, blocking airflow through it. The air coming out the vents may feel cool, but there just isn't enough airflow to cool the interior properly. Moulds and dirt can be removed from the evaporator with chemical treatments but most of these should only be used in well ventilated environments and involve removing some parts for evaporator access, so this job is best left for the repair shop.

So this spring, don't lose your cool. A few easy checks and possibly a little refrigerant, and your vehicle will be ready for the hottest days of summer.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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