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Inner Workings of an A/C

Inner Workings of an A/C

Can you remember a time when all we had was cold iced tea and a pair of vent windows to keep us cool as we drove into Austin?  If you were lucky enough to have A/C in your car, you had to move the switches/levers on the dash to manually control the different parts of the A/C system.  Unlike modern vehicles, these A/C systems were very basic.  Today, vehicles are easier to keep cool while cruising down the road, but also much more complex!  The modern A/C system affords the occupants the luxury of automatic cooling functions and different vent temperatures throughout the vehicle.  Here is a little information to help you understand how an A/C system works and why they are so expensive to fix. 

To begin, an A/C system is composed of 3 separate systems that work together as one.  These systems are the A/C control unit (AKA, the computer), the ventilation system, and the refrigerant system.

The A/C control unit is one of many computers that are networked together throughout your vehicle.  Its responsibility is to adjust and control all the working parts of an A/C system.  It will also send messages to other vehicle computers to help with the overall system operation.  For example, when you turn on the A/C system, the A/C computer will send a message to the engine computer.  The engine computer will then send a message to the engine cooling fan computer that will turn on the engine cooling fans.   Another example is when you select the fan speed on the dash, the A/C control unit will send a message over a network wire to the fan speed control unit (another computer responsible for controlling fan speed).  This fan speed computer will then send the correct amount of power to the fan to allow you low, medium, or high air flow through the A/C vents.

Next is the ventilation system.  This system works with the A/C computer to get the air where it needs to go using actuators. These actuators, hidden deep inside your dash, are electronic or vacuum controlled doors that will open and close depending on the settings you choose.   Mode door actuators control where the air will flow to: the defrost, dash, or floor vents. Temperature blend door actuators control the air temperature, sending the air across the cold evaporator or across the hot heater core.  Vehicles that have different temperature controls for both the driver and passenger will have multiple blend doors; one for each set of controls.   Finally, the fresh air actuator, or recirculate function, will allow you to select air to be drawn from the outside or from inside the passenger compartment.  This really helps on hot days.  Once the air inside the passenger compartment has cooled off, select the recirculate function.  This will allow the ventilation system to draw cool air from inside your vehicle, instead of hot air from outside and will help cool things down even more.

The refrigerant system is what most people think about when they think about their A/C.  Whether the refrigerant is R-12 (old classic cars), R-134a (90’s – present), or R-1234yf (2013-present),  the major system components are the same:  an A/C system compressor, receiver/drier, condenser, thermal expansion valve (TXV), and an evaporator core.  As the refrigerant flows through the system, it is converted from a high pressure, hot liquid, to a low pressure, cold gas before starting the cycle all over again.  The refrigerant carries oil for lubrication as it travels through the system.  Although a sealed system should never run out of refrigerant, it is common to see small amounts seep out over several years of service.  Lower than normal refrigerant levels can cause the air from the vents to feel warmer than usual and cause long term wear of internal components such as the compressor. 

Now that you know the basics, why does A/C system repair cost so much?  Again, there are many reasons and exceptions for this, but here are a few fundamental truths. 

  1. Parts have become much more complex.  If the temperature controls stopped working on my Dad’s old Bronco, there was a good chance that replacing a $5.00 cable between the dash switch and the temperature blend door would solve the problem.  On modern vehicles, the same $5.00 cable has been replaced with 2 computers, wiring, and an electronic motor.  Unfortunately, “progress” is costing us more when it comes to repairing your vehicles A/C.
  2. More complicated systems require more work to find the problem and a higher caliber technician to perform the necessary electrical testing.
  3. Like many things over the last few years (oil, eggs, milk, and gas), the cost of refrigerant has literally tripled.
  4. Many of the A/C system parts are stashed away and hard to get to, making many repairs labor intensive. 
  5. Any time the refrigerant system needs to be repaired, it requires special equipment to remove all of the refrigerant from the system.  It is against Federal Law to open a refrigerant system without properly removing the refrigerant and storing it in a Federally approved container.

If you find yourself with cool air coming from your vents in the morning and warm air coming from your vents in the afternoon, contact your local automotive service professional.  Professional businesses, like Dripping Springs Automotive Center, provide courtesy A/C system performance checks to help narrow down the possibilities before recommending a course of action.  Texas summers are hot enough, don’t get caught having to drive into Austin with your windows down!