Air Conditioning Season: A Quick Reference

Outside temperatures are rising, and a faulty air conditioning system will bring in overheated customers. Look out for these common causes to turn your bays before things get too hot.

Lack of Air Flow:

A dirty cabin air filter, which filters the air entering the passenger cabin, trapping dust, pollen, and other airborne debris, is most likely the cause of lack of air flow. If it is not replaced regularly, the filter becomes clogged and reduces cooling airflow. The next most likely cause is a faulty blower motor or the blower motor speed control. Testing the blower motor for voltage and ground will diagnose it.

Poor Cooling:

If airflow is OK, the next most common reason for lack of cooling is loss of refrigerant, usually accompanied by the compressor frequently cycling on/off or never turning on. Finding the leak can be time-consuming. You can purchase a cheap refrigerant detector that will be of little help, often misidentifying a leak, but even expensive ones can often lead you astray. Dye sometimes helps, and some systems these days come with dye installed. The trouble is, the dye sometimes is not easy to find because it is covered by oil and dirt, so you need to clean everything and then run it for a while before searching for it to show and hope it will be somewhere you can see it.

A good method is to pressurize the system and listen for leaks. A spray bottle with bubble fluid is a big help. You can make some of your own using a mixture of 6 cups of water, 1 cup of dish soap, and 1 tablespoon of glycerin (or 1/4 cup of corn syrup). You can use regulated shop air with a good drying system, but nitrogen is a much better and safer way to pressurize the system and listen for leaks, as well as look for bubbles. Once any remaining refrigerant is reclaimed, use a nitrogen bottle, regulator and gauge set to pressurize the system. This does not waste or release refrigerant into the atmosphere and can often make leak finding much easier. Usually, with the engine off and compressor and fan not running, you'll hear the hiss and locate the leak quickly. If you do not, use a hose to your ear or a stethoscope on the evaporator plenum. Spray all areas and watch for bubbling.

The next most common cause is the fan not providing enough airflow over the condenser and radiator. This can be a fuse, a relay, or a defective fan. The problem may also be due to a condenser clogged with leaves, bugs, or other debris. In either case, you will see higher-than-expected pressures on the gauge set.

A less common issue is a clogged expansion valve or orifice tube that is clogged or damaged. In the case where either is clogged, the low side will be way lower than expected, possibly negative, and freezing. If the orifice tube is broken (open), it will act like a faulty compressor, where there is little to no difference in high and low side pressures. In either case, it will require replacement. Your best tools are a gauge set and a thermometer.

These quick references are solid basics, but as always check for TSBs on the specific vehicle before employing them. This will help turn your bays and keep your customers satisfied.