How to Tell If a Car Air Conditioning Compressor is Bad

Vehicle air conditioning is typically one of a car's most durable systems. Though it is often in use every day, a modern air conditioning system just keeps humming along providing cool air when you need it. And the heart of the air conditioning system, the AC compressor, is long-lived and hearty. 

signs your air compressor is bad

Photo: Christian Wardlaw

But when your air conditioning does fail, it can be a painful experience. Let's face it, we have become used to mechanically cooled air whenever we want it. The prospect of losing that, starting to perspire, and having to roll down the windows is almost too dire to contemplate. Because of that, you should always be on the lookout for signs that your car air conditioning compressor is bad.

The good news about that bad news is you almost always get clues that your air conditioning is about to fail. While sometimes A/C simply stops working, most often the failure comes little by little and is accompanied by signs that something is going south. The following hints will help you to recognize a problem.

How Does Auto Air Conditioning Work?

You might look at the air conditioning compressor as the heart of the air conditioning system in your car. In essence, the AC compressor performs for the system the same function that your heart does in your body — it provides the circulation of a precious fluid, the refrigerant, through the air conditioning system. 

The compressor also compresses the refrigerant from a gaseous state to a fluid state, sending it to the condenser that acts as a "heat-exchanger" removing the heat from the refrigerant. From there, the refrigerant goes to an expansion valve or "orifice tube," where it returns to its gaseous state. 

An accumulator removes moisture from the refrigerant, and it is then transferred to the evaporator in the cabin of your vehicle, usually under the dash. There the cold refrigerant cools the air that passes by and through the evaporator, which resembles a small radiator. The cooler, drier air is then sent out the vents into the interior of your car via a fan. Ah, no sweat.

Air Is Cool, Not Cold

One sign that the air conditioning system is having a problem is the relative coldness of the air coming from the vents in the interior of your car. If you have the air conditioning switched on, the temperature turned down, the fan cranked up, and the air coming out of the vents doesn't seem as cold as it used to, it may not be your imagination. It can be an early sign that something is wrong with your air conditioning system. The two most likely problems are a leak in the system that is causing it to lose refrigerant or a faulty AC compressor. 

Noises from Under the Hood

When your vehicle is making strange noises, it is never a good sign, and that is certainly the case when it comes to air conditioning. As we said, the AC compressor has the tough job of compressing and circulating the air conditioning refrigerant. It typically cycles on and off many times during the course of daily driving, and it is filled with components that can fail over time. 

Often, the AC compressor incorporates a sealed bearing, and that bearing can wear out or seize up, especially if the bearing lubricant leaks out. That could produce a high-pitched squeal or the grinding sound of metal-on-metal. If the bearing seizes (freezes up) it will typically result in the squealing of the drive belt that is suddenly unable to spin the compressor.

In some instances, AC compressors can be repaired. However, it is most often a better procedure to replace the faulty compressor with a new one. It is not only the most time-effective solution to the problem, but it will also most often result in a better, more durable repair. 

Compressor Clutch Issues

The AC compressor has a clutch that is connected to the engine by a belt and pulley, and it can provide problems, too. By having a clutch, the compressor can cycle on and off rather than being in use all the time you are underway. This results in less wear on the compressor and better fuel economy since the engine doesn't have to turn the compressor when it isn't needed to provide cool air.

A problem that can occur, however, is the failure of the compressor clutch. It can stick in the "on" position, which will result in the compressor turning all the time you are driving. Or it can cease to function at all, which means the compressor is never engaged. Neither is good. While in some instances the clutch can be repaired, again the preferred fix is to replace the compressor and clutch as a unit.