A basic description of Automotive AC Systems and how they work
Automotive AC systems can be intimidating if you are to jump in headfirst without a little background on exactly what is happening inside the system. I am writing this article to give some insight into the workings of your car’s AC system, and hopefully allow you to properly diagnose and repair your own AC system.
The first thing I need to talk about is the proper equipment used to service an air conditioning system. Most of the system can be serviced using common hand tools, but there are a few specialty tools needed for proper service of your AC system.
The three tools shown above are critical to a proper AC repair. Before any repairs can be done on the AC system, the old refrigerant MUST be recovered. The only way to properly do this is with a recovery system such as the one above. This is actually required by law, so don’t skip this step. The vacuum pump is used after the repairs have been made, it pulls all air and moisture out of the system, so that when the system is refilled, only refrigerant goes into the system. The tools shown above are not tools that the average at home mechanic has, so if you don’t like the idea of shelling out big bucks to buy this equipment, you can always bring your car to a repair shop and pay a small amount to let them do the Recovery before you begin repairs, and the vacuum after the repairs.
The most common misconception about car AC is that cold air is created. Technically heat is being removed from the air. Although it sounds like a little nitpicky detail, it is actually quite important in understanding how the system in your car keeps you cool during the summer.
In my explanations I am going to use the word “Refrigerant” a lot. Many people call it “Freon” but Freon was technically R12, which has been all but eliminated from use today due to environmental reasons (remember the ozone layer?). R134a is the most commonly used refrigerant today, but there is talk of changing to another version soon. So to make things simple I will simply say “refrigerant” in reference to all types.
If you look at the diagram above, you will see the arrows which show the direction that refrigerant flows through your AC system. There are slight variations between some systems, but this diagram is a good overall view of a generic AC system. This setup is typical for most imports and a few domestic cars.
Follow along with me as I follow the flow of refrigerant and explain what happens as it goes through your system (if you have an orifice tube system, there will be some slight differences which I will explain in a bit). Starting at the compressor, the refrigerant is “compressed” into a high pressure, high temperature gas, typically about 200psi. From there it immediately flows into the condenser. The condenser has a single function, to “condense” the gas. It does this by cooling the high-pressure, high temperature gas. When the refrigerant is cooled, it turns from a gas into a liquid. The high-pressure refrigerant, now in liquid form then flows into the receiver dryer. The receiver dryer has two functions, the first thing it does is “Dry” the refrigerant. Inside the receiver dryer is a desiccant bag, this absorbs any moisture that may have contaminated the refrigerant. The receiver dryer also stores liquid refrigerant as it comes out of the condenser. Once the refrigerant leaves the receiver dryer, it flows to the expansion valve. This little device is absolutely critical to the function of the system. And it is very simple in how it does it.
The expansion valve is simply an adjustable hole for the refrigerant to flow through. There is a small temperature bulb which attaches to the outlet pipe from the evaporator, and will open and close the opening inside of the expansion valve depending on the temperature of the evaporator. If you look at the diagram above, you will see that the color of the tubing turns to blue after the expansion valve. That is because the hole for the refrigerant to flow through is so small, only a little amount can pass through at any one time. The blue lines indicate low pressure, one side of the expansion valve will be around 200psi and the other side will be around 35psi. This now low pressure liquid refrigerant flows from the expansion valve directly into the evaporator.
The evaporator does exactly what you would think, it evaporates. As the now low pressure refrigerant enters the evaporator, it literally starts to boil. Unlike water, which boils at 100 degrees Celsius, R134a boils at -26.3 degrees Celsius. As soon as the refrigerant drops to a low pressure, it is free to boil. Inside the evaporator intense boiling takes place as the R134a changes from a liquid back into a gas. A side effect of boiling is the absorption of heat. Heat is absorbed into the boiling refrigerant which causes the evaporator itself to become very cold as the heat around it is sucked into the boiling refrigerant. As the refrigerant leaves the evaporator, it is now a low pressure gas at which point it returns to the compressor to begin the journey again.
Meanwhile there are a couple other components which are critical to the proper operation of the AC system. The blower motor and fan push air through the cooled evaporator. Heat from inside the car is absorbed by the cool evaporator and then pushed back out into the car through the AC vents in the form of cold air. The condenser fan draws air from outside the car, and passes it over the condenser. This allows the condensation process which converts the high pressure gas back into a liquid.
Inside the dash of your vehicle are many air ducts. These ducts direct the airflow to the various vents in your car such as the defroster, or towards your feet when the heat is on. The direction of the airflow is controlled by doors inside the ductwork called mode doors. Your vehicle also has a temperature blend door which varies the amount of air which passes through the cold evaporator and the hot heater core to change the temperature of the air coming out of the vents. Some cars have computer controlled AC systems that can automatically adjust the position of the blend and mode doors depending on the desired AC temperature setting.
If the system is not cold, the most common reason, but definitely not the only reason is that the system has run low on refrigerant. When the system gets low on refrigerant, the ONLY cause can be that you have a leak somewhere. It may be a very small hard to detect leak or it could be a gaping hole in one of your hoses. Using a sniffer is the best way to detect a refrigerant leak. Most individuals don’t have one personally, so often it is best to have a shop find the leak for you.
Numerous other causes can cause the system to not be cold enough to your liking. These causes can be anything from a blend door not operating correctly, to a radiator fan going bad and not pulling enough airflow through the condenser.
A VERY common problem in modern cars is that the fan speed selector only works on high speed. When this happens, 98% of the time it is caused by a failed blower motor resistor. The blower motor resistor has various paths for the electrical current to take, each of these paths has a different level of resistance, that level of resistance causes the fan to run at different speeds depending on what path the electrical current has to take.
In the picture above, you can see the various curled wires on this blower motor resistor, the varying thickness and length of the wire is what causes different fan speeds depending on which setting the fan switch is currently on. When the fan is on HIGH, electrical current completely bypasses the resistor and goes straight to the blower motor, which is why HIGH is often the only setting that works.
Even with today’s outstanding technological advances, car AC systems are still prone to the same problems that have always been present. When working properly you will never think about what a complex system exists just to keep you cool on those hot days. Above are just two of the most common problems with car AC’s, I cannot get into every one as every manufacturer has its common quirks. There are so many variables in an AC system that if you are not 100% sure in what is causing the problem, I HIGHLY recommend bringing it to a shop with an ASE certified AC technician to diagnose your problem. This can save you headache and money and help get your AC working like new again.
Written by Joe Stepnicka
This article may not be reproduced without the author’s written permission.