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Diagnosing the AC System - Basic Visual Inspection

Auto AC system

Once you understand the basics of AC system operation, you can begin diagnosing issues with it. First, make sure there is a problem beyond just high humidity or bad airflow by doing an AC performance test.

Remember that working on your air conditioning system likely requires special certification. In most parts of the U.S., it's illegal to physically work on your AC system, specifically the coolants involved, without certification and fines for doing so are steep. The tests we're going to cover here do not require certification as they do not require that you breach the AC system in any way.

The first step in diagnosing an AC issue is to visually inspect the system. With the engine off, visually look over the AC system's lines and connections. You can use leak spray externally to find leaks, but you're likely to see serious leaks and line breaks easily.

Next, start the vehicle and disengage the AC (turn it off). Watch the compressor and listen for engine RPM changes to ensure that it's not engaged. On most vehicles, the AC compressor has a pulley on a clutch. This electromagnetic clutch only engages when the compressor is activated (AC is turned on). The pulley appears to be spinning all of the time, but if the clutch is separating it from the compressor, the center of the pulley will appear to be sitting still.

Now engage the A/C system (turn it on) and see if the pulley begins to turn fully. The engine's RPM should drop slightly when the AC is engaged.

If the pulley engages and seems to be putting some weight on the car's engine, then your compressor is capable of operation. This doesn't mean it's actually working, but it does mean that it should be able to. So your problem is not the AC switch in the car, the circuit to the compressor's pulley engagement, or the physical pulley itself.

Next, locate the condenser unit. This is usually located at the front of the car near the radiator. This condenser will usually have a fan attached to it. When the AC system is active, the condenser fan should also be on. If not, test its circuit. A non-functioning condenser fan means that the refrigerant passing through is not being cooled. That can greatly affect how efficient your AC system is.

Now visually inspect all of the hoses and fittings again, this time with the AC system active. Look for obvious leaks.

In the vast majority of AC system problems, leaks or compressor engagement are the cause. Finding either of these is the most likely reason your system has stopped working.