June 14, 2006

Air conditioning trouble-shooting
By Jim Kerr

Warm weather always brings a flurry of air conditioning work into the shop. "My air conditioning doesn't blow much air" or "there is a funny noise under the hood". Sometimes the owner is quite graphic, such as the one who expressed concern about an odour from the system. He described it as "vaguely agricultural smell with bouquets of baby puke." Of all the concerns I hear, the most common one has to be with the lack of cold air from the system. "It worked last Fall. Why doesn't it work now?" Let's try to find the answer to all of these concerns.

R134a, the refrigerant used in automotive systems is a very small molecule. It can leak through the smallest opening, so any leaks in the refrigeration system will allow the system to lose its charge. The quality of hoses and seals improved dramatically when R134a was introduced, so leaks occurred at a much slower rate, but every system seeps gradually. Over the span of 3 to 4 years, it is common for the system to become low enough on refrigerant that it won't cool properly. This is especially true on newer systems.

Newer vehicles have air conditioning systems that are sized to meet the expected cooling needs of the vehicle and nothing more. Systems that used to hold a kilogram or two of refrigerant have been made smaller and now hold less than half a kilogram in many vehicles. This downsizing of air conditioning capacity saves weight and cost of larger components and systems. Less weight translates to better vehicle performance and better fuel economy, but the AC systems must now be working more efficiently to cool the vehicle.

AC system efficiency has improved. By changing the way air and refrigerant flow through evaporators (in the heater housing) and condensers (in front of the radiator), engineers are able to get better cooling out of smaller systems, but if any refrigerant leaks then the system can't cool properly. You may simply need a recharge of refrigerant to make your system operate properly again, but before recharging any system, a technician will check for leaks to prevent unnecessary release of refrigerant to the environment and save you money for another recharge.

Noises from under the hood can be caused by small stones trapped in the ribs of serpentine belts, rough bearings on pulleys or tensioners, or it could be coming from the air conditioning compressor. An overcharged AC system will cause a compressor to hydraulic lock and create a hammering sound (because the compressor can't compress a liquid) but a low system can cause bearings to fail in the compressor. A bad bearing will knock when the compressor is running.

Lubricating oil flows through the AC system with the refrigerant, so there is no level to check, but a loss of oil occurs when a major refrigerant leak in the AC system happens. If the oil is not replenished, then compressor damage will occur. Anytime a system has a major leak or a component has been changed, the reference guides show how much extra oil to add to the system.

Sometimes poor air conditioning performance is caused by faulty controls. Electric motors or vacuum pots operate the temperature and air delivery doors on most systems. A blown fuse or a vacuum leak may be all that is wrong with the AC system. Another cause of poor performance is sticking doors inside the AC/heater housing. Pens, combs, paper clips and straws can fall down defroster vents and block a door from moving. Crayons melt and block a door, and soda pop spilled on the dash can gum up the works so bad the complete housing needs to be taken out and cleaned. This can be costly, so be careful.

Remember that vivid description of a bad odour. It is likely caused by mould growing in the moist dark fins of the evaporator unit inside the AC housing. If it starts to smell, then the system needs to be cleaned with a bacterial agent to remove the mould. This is a common problem and most repair shops are experienced at correcting this problem. To reduce the chance of mould growing again, turn the AC system off and let the fan blow for several minutes if you are going to park the vehicle for a while. Mould grows best in humid conditions, and operating the fan dries out the moisture on the evaporator fins so it doesn't grow.

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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