• Kathy

Lets talk Air Conditioning

My first car was a 1967 Chevy Nova with obviously no AC and black vinyl seats, I would have to pull the vents by the floor to let air in, which would also bring in a puff of dust, dirt and pollen with it. OH the good old days. Air Conditioning its a beautiful thing, what would do without it!

Thanks goodness most cars now a days come standard with AC, . But what do you really know about the AC System in your car? Well I am going to try to explain how it works and what can happen when you have a leak in the system and how it should be diagnosed.

The AC system in a car has the same components as your home AC system. You have a compressor, condenser, receiver dryer, expansion valve and an evaporator. Each component has a specific job:

The High Side: AC compressor -The compressor is a pump driven by a belt. When the refrigerant is sent into the compressor, it is in a low-pressure gas form. Once the gas is inside the pump the belt drives the pump, which puts the gas under pressure and forces it out to the condenser.

The Condenser: Is basically a radiator, it radiates heat. The refrigerant enters the condenser as a pressurized gas from the compressor. The process of pressurizing the gas and moving it to the condenser creates heat, but air flowing around the twisting tubes of the condenser cool the refrigerant down until it forms a liquid again. The liquid refrigerant is now a high-pressure liquid. As it moves out of the condenser, the liquid goes through a little reservoir installed in the line know as a receiver-drier. This receiver-drier contains desiccants, small granules that attract water. In the receiver-drier these granules remove any water that has entered the system. If the water is allowed to remain it can possibly form ice crystals, ice can be damaging to the air conditioning system.


The low side - Expansion Valve: The system changes from the high-pressure side to the low-pressure side. If you were to touch this part of the system, you'd feel it change from hot to cold. The high-pressure liquid refrigerant flows from the receiver-drier through the expansion valve, where it is allowed to expand. This expansion reduces the pressure on the refrigerant, so it can move into the evaporator. The valve senses pressure and regulates the flow of refrigerant, which allows the system to operate.

Expansion Valve

Some vehicles have an orifice tube rather than an expansion valve, but it serves the same purpose in allowing the refrigerant to expand and the pressure to be lowered before the liquid enters the evaporator. The orifice tube allows refrigerant to flow at a constant rate and has no moving parts, but it can become clogged with debris over time. Systems with an orifice tube automatically control the flow of refrigerant to the evaporator.

Orifice Tube

Evaporator: This is where the magic happens. While all the other parts of the system are located in the engine compartment, this one is in the cabin, usually above the foot well on the passenger side. It also looks like a radiator, with its coil of tubes and fins, but its job is to absorb heat rather than dissipate it.


Refrigerant enters the evaporator coil as a cold, low-pressure liquid, ideally at 32 degrees Fahrenheit (0 degrees Celsius), which is why you don't want any water in the system. The refrigerant doesn't freeze at this temperature, but it does have a very low boiling point. The heat in the cabin of the car is enough to make the refrigerant in the evaporator boil and become a gas again, just like water turning back to steam. In its gaseous form, refrigerant can absorb a lot of heat.

Air Flow Diagram

The gas moves out of the evaporator and out of the passenger compartment of the car, taking the heat with it. A fan blowing over the outside of the evaporator coil blows cool air into the passenger compartment. The refrigerant in gas form then enters the compressor, where it is pressurized and the whole process starts all over.

What happens when your Ac system is just not cold or it does not work at all.

Your mechanic will need to determine which component in the system has failed. Your Ac System is a sealed system the refrigerant other wise known as R-134A, should stay in the system unless you have a leak. With several components in the system as your vehicle ages something is bound to fail with time.

One of the first things your mechanic should do is to make sure the control unit in the vehicle is working properly because this is after all a piece of electronics and we all know how they can fail with time. Once this has been ruled out your mechanic should start to check the rest of the system to see if each of these component are doing their job.

If any of these components start to leak or has been contaminated by moisture it will not work correctly. Keep in mind that your ac system has a specific amount of oil to keep your system lubricated and if you have a leak in the system this means the oil is also leaking out, which means the system is not properly lubricated and this can cause damage.

Your mechanic has a set of gauges he can put on the system to make sure that the high and low side are working. If the system seems to be low the compressor may not even come on so the next proper step should be to recharge the system with refrigerant and a special dye, let the system run and then check for leaks with a special dye light. Once the refrigerant leaks out so will the dye and this will help determine which part is leaking in the system.

Ac Gauges

If for some reason all of the components under the hood seem to be leak free then its possible that the evaporator core is leaking which as I said earlier is located in the vehicle under the dash which can't always be seen without some disassembly.

Evaporator Core Leak

In some cases the leak is so minimal and slow that is may take some time for the dye to show itself, so driving your vehicle and waiting till your ac is just not cold anymore and returning to the mechanic to look for the dye is the only thing you can do.

Your mechanic should not just keep recharging your system and allow the refrigerant to leak out into atmosphere. The proper way is to determine the problem and replace the part that has failed.

Its never a good idea to add stop leak if you plan on keeping the vehicle because the stop leak goes through out the entire system, so if you only need one or so failed parts now, when you decided to replace the failed part your are replacing all the components because the whole system has stop leak in it. Stop leak can damage to ac compressor so its just not a good idea. Ac system repairs can get expensive especially if the whole system has become contaminated.

The recirculate button: - Most people don't even know what this button is let alone know that their car has one. Lets go over its use and why you should be using it. If its 100 degrees outside and you get in your car turn on the ac full blast, because lets face it its hot outside now the ac system is working as hard as it can to pull the hot air from outside to go through the whole process explained above to give you cold air. If you simply keep the recirculate button on or turn it on when you turn your ac system on it will only have to cool the air inside the car which is 1. a much smaller space 2. as the system starts to pull the air in it is already cooler from recirculating instead of continually pull hot air from outside the car. This will give cooler air much faster.

Don't be afraid to ask why you need a specific repair or what other options you have. I hope this help you understand your AC system. Feel free to reach out to me with any questions.

Thanks for reading !



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