Air Conditioner

Would you like to know how a vehicle’s air conditioner works and the first things to check when it doesn't? The system looks complicated, but it's not, once you know how it delivers cold air you will be better informed when problems occurs and how to fix it, or understand more on what a garage is telling you when having it repaired.

This step by step guide will help you know if you can fix the problem yourself, or when a repair garage is best, let's begin.

A climate control system is designed to operate the various settings to improve occupant comfort, these controls may differ in appearance, but perform the same task: fan speed, vent mode (defrost, mid and floor) and temperature control.

Control Panel
Let’s go over the operation of the system from start to finish.

Step 1

Refrigerant: We will start with the fluid/gas the unit has inside which is used to produce the cold air, this liquid/gas is called R134a. This refrigerant is specifically created for the automotive industry sometime in the late 1980’s. Some call it Freon which is a brand name like Kleenex.

The optimum property of this gas is its ability to go from a hot gas when it's compressed (250 psi at 180° F or 82° C), and when cooled in the condenser (located in front of the radiator) it transforms into a warm high pressure liquid which is then released through an expansion valve or orifice tube (small hole) the high pressure liquid instantly transforms to a low pressure gas vapor (35 psi at 32° F or 0° C) which creates the coldness.

This entire action is basically a high pressure liquid, then being released into a low pressure gas. If you take a can of hair spray and then release the product along with the propellant the can will get cold. This is the exact same chemical reaction used the only exception is the gas is recovered in the evaporator and recycled into the system.

Step 2

System: The diagram below shows the entire system and refrigerant flow; these systems are closed (sealed) which continuously re-circulates the refrigerant. The receiver dryer or accumulator is where the refrigerant is cleaned and dried.

Once compressed, the R134a travels to the condenser where it is cooled and transformed into a liquid state from the compressor which takes the spent gas from the evaporator and feeds it into the compressor via the low side hose or tube.

Once entering the compressor the R134a is compressed and then transferred into the high side hose which connects from the compressor to the condenser. This condenser sits right in front of the engine's radiator so it can get air drawn through it much like the radiator by the radiator cooling fan.

The R134a is then pushed through the condenser which looks kind of like a radiator only a bit smaller and then cooled back into the liquid form and still under high pressure form the compressor.

Now the cooled R134a still under high pressure is forced through an expansion valve or an orifice tube where the magic cold happens by the pressure drop.

This pressure drop occurs inside the evaporator core which looks like a small squat radiator which is located inside the vehicle. Once the refrigerant has spent its cold charge it is gathered into the low side hose and onto the compressor once again being processed.

Step 3

Maintenance and Operation: As your vehicle ages the refrigerant level should be maintained and added too, insuring that air flowing into the occupant cabin is cold, especially on those hot summer days. Because this system is pressurized, kind of like a tire, it will need to be recharged as it ages. Refrigerant charging kits are available at many auto parts stores and Amazon. Recharging the system is not to difficult and can be done in about 20 minuets.

The low side pressure port is used to charge the system and is always larger in size than the high side pressure port. You can see the high side pressure line is always smaller than the low side pressure line in the image below. Some refrigerant lines will have an insulation wrap on the low side line; this is usually due to the location of the engine exhaust being in close vicinity.

Charge and Pressure Ports
The compressor is the main mechanical part of the system with internal workings inside the unit that is driven by the engine's serpentine belt. When there is a weird noise when the a/c is on the compressor or blower fan is probably the problem and most likely due to a mechanical failure. When the compressor stop working the system will blow warm air and when this happens the compressor needs to be replaced.

When you turn the system on a voltage supply is given to the climate controller, the blower motor fan and the compressor clutch or internal valve. These components then begin to operate connected through a climate control wiring harness fuse and relay.

Some condensers are designed with a receiver dryer attached which acts as a refrigerant cleaner/moisture remover which helps the system last longer. Some systems will have an accumulator that does the same job, each car maker is a little bit different, but achieve the same goal.

Leaves and dirt can get logged in the condenser hindering its performance. I use a garden hose to clean the condenser with a high pressure nozzle from time to time and it works really well which helps the air conditioner work more efficiently.

Some air conditioner systems have a separate fan for the condenser instead of the radiator cooling fan, or they will have an additional fan to help the radiator fan cool the refrigerant. These fans are controlled by the climate control computer which gives a command to the control relay to power the fan on. When these fans fail the system will not cool as well and should be replaced.

A serpentine belt is used to rotate the compressor pump by the engine's serpentine belt and belt tensioner, once the engine is running the air conditioner can be turned on. The internal workings of the compressor include reed valves and an eccentric vane pump.

A hybrid and electric vehicle’s compressor is powered by the vehicle's battery pack not the engine, a small electric motor is fitted inside of the compressor which makes it operate. These compressors have a pair of large gauge wires form the compressor controller and not a serpentine belt. Check out the picture below.

Hybrid Engine Compressor

Step 4

Non-Operation: If the compressor is not working the best place to start is with the state of charge or fuse of the system, it can blow at any time, so it's worth checking. Below is the order by popularity in which the system will not work. Learn more
  • Low on charge (R134a)
  • Compressor failure
  • Blown fuse
  • Bad climate controller
  • Shorted control switch
  • Plugged orifice or expansion valve
  • Failed radiator or condenser fan
  • Blower motor failure

You can visually see most system leaks because it uses a peg oil along with the refrigerant, which are mixed together as they travel throughout the system. This oil is used to lubricate the internal vanes and reed valves of the pump. See what a refrigerant leak looks like below.

If the system needed a full repair and has been opened, a vacuum down and recharge is needed to remove moisture which gets into the system naturally and can cause the inside of the system to rust which will cause damage such a plugged expansion valve or orifice tube or ruin the compressor. In the video below you will find one of the best air conditioner full vacuum down and recharge videos.

If you need more information about this article please visit our forum which has thousands questions about air conditioner systems answered by our online mechanics.


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