By Jim Kerr

Spring is here and our vehicles’ air conditioning systems are going to be put to work again. For some of us, the cooling we want isn’t going to happen, but most of the time, all it takes is to prevent this is some easy maintenance.

The climate control systems in our vehicles are a blended-air type. Air is taken in from outside and passes through the evaporator, where it is cooled. Then, some of the air is directed to the heater core and heated. By mixing the cool air and heated air, the desired temperature is reached. Some manual climate control systems use cables to operate a blend or mixing door, usually called the temperature door. When you select full heat, the door blocks off the cold air; when you select full cold, the door blocks off the hot air. Any door position in between allows a blend of air.

One common problem with these systems is a binding or misadjusted cable from the control to the temperature door. When operating the temperature control, it should move smoothly and you will hear a slight thud when the temperature is moved to full heat. This occurs when the door seals against the housing. If you don’t hear it, the cable may need adjustment.

Many vehicles with manual AC now use electric motors to operate the temperature door. When you turn the system on after a battery has been disconnected, the system will typically cycle the blend door through its full range of motion. If the door is sticking, perhaps because someone has spilled a soda pop down the defroster ducts, or dropped wax crayons, paper clips or even credit cards into the duct work, then the door limit stops are set incorrectly and the temperature won’t be controlled properly. You may have cleaned up the soda from the vehicle interior, but not the mess inside the ductwork. Sometimes the complete heater housing has to be removed to clean it out. If you are lucky, some of it may be accessible for cleaning by removing some of the ducts under the dash.

Another common problem is a plugged air filter. Many vehicles are now equipped with dust or pollen filters to trap particles before they enter the vehicle. These filters can plug up quickly. I have seen filters in a truck plug after only a couple weeks of following other vehicles on gravel roads, though most will last several months. If the filter is blocking air, you won’t feel cool inside the vehicle even though the AC refrigerant system is operating correctly. Adequate airflow is important for proper interior cooling.

Airflow can also be blocked by bacterial growth on the evaporator fins in the AC housing. Water vapour condenses on the evaporator fins when the AC system is operating. The water droplets collect dust and drain out the bottom of the housing. This is why you will often see puddles of water beneath a stopped vehicle in the summer time. If temperature and humidity conditions are right, mould or bacteria starts to grow on the fins, which may create a musty smell inside the car when the fan is turned on. It also forms a fuzzy mesh on the evaporator, which blocks airflow just like a plugged air filter.

If you can smell a musty odour or airflow through the system, you may need to have the evaporator and ductwork cleaned with an anti-bacterial cleaner. There are some that can be sprayed into the system while it is operating, but this must be done with all the doors open, the interior vented and no one inside the vehicle. In the worst cases, the ductwork has to be removed and opened to clean the inside physically. Repair shops deal with this problem on a regular basis, so if your car has a musty odour, take it in for repair before it gets too bad.

Refrigerant can leak from AC systems over time. A very low charge will prevent the AC compressor from operating, and you will have no cooling. Systems can hold their charge for several years, so you may not need to have it recharged, but a slightly low charge will decrease cooling capacity. If your AC system doesn’t cool properly after you have confirmed there is enough airflow, then the shop may need to check your system for leaks and install the correct charge.

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