Although air conditioning may not be a necessity to get from point A to point B, it sure contributes to the quality of the trip. If you find yourself in the less-than-ideal situation of driving a vehicle with non-functioning or under-performing AC, it serves you well to book a mechanic to fix it, so you can comfortably get through the heat.
How this system works:
At its most basic level, a vehicle’s air conditioning system works by intentionally manipulating the temperature and pressure of refrigerant (commonly called by the brand name Freon, which is a trademark of DuPont) and the direct relationship they have. The process begins with creating high-pressure gas by compressing the refrigerant inside your vehicle’s AC system to raise its pressure and temperature. The refrigerant then flows through the condenser, which turns it into a hot high-pressure liquid before going into the receiver. The receiver – sometimes referred to as a dryer – removes moisture and unwanted particles from the refrigerant. After that, it passes into the expansion valve, or orifice tube, which reduces the refrigerant’s pressure and causes the liquid refrigerant to change its physical state into a cold low-pressure gas.
To perform this change of state from liquid to gas, the refrigerant must have “latent heat,” and it must take that latent heat from the surrounding area inside the evaporator. As the latent heat is pulled from the evaporator core to almost below freezing, the blower motor sends the cold air through your vehicle’s vents. Think of a hot summer day, and you just got out of the pool: The liquid water on your skin starts to evaporate and turn into water vapor. The cooling of your skin is the result of the water pulling the latent heat away from your body to change its state into a vapor.
The air conditioning system is chiefly comprised of the following components:
Compressor: This is the core of your AC system. Powered by your vehicle’s serpentine belt, the compressor pressurizes the refrigerant. It is turned on and off by way of the compressor clutch.
Compressor clutch: Before the compressor turns on, a special electro-magnetic clutch called the compressor clutch is necessary to engage and disengage the compressor cycle. The compressor clutch tells the compressor when to turn on and off so the refrigerant is correctly pressurized for use by the condenser.
Condenser: Located in the front of the radiator, the condenser takes the hot refrigerant and cools it down while also reducing the pressure. This turns the refrigerant into a liquid that can pass to the next stage of the AC system.
Receiver/Dryer: Present in vehicles equipped with a thermal expansion valve, the receiver – or dryer – protects the compressor and other parts of the AC system from damage from unwanted particles and liquids. It separates gas from liquid (because the compressor can only handle gas), removes moisture by way of a desiccant, and discards contaminants.
Accumulator: Found in vehicles with an orifice tube, the accumulator has roughly the same function as a receiver. In fact, your A/C system will have either a receiver or accumulator – not both. In addition to filtering out debris and moisture, this part controls the amount of refrigerant that flows into the evaporator. The accumulator also stores excess refrigerant so that it cannot enter and damage the compressor. If you have an accumulator, then you don’t have a thermal expansion valve.
Thermal expansion valve or orifice tube: The thermal expansion valve and orifice tube have roughly the same function. In fact, your A/C system will have either an orifice tube or expansion valve – not both. Situated between the condenser and evaporator, either the expansion valve or orifice tube is responsible for regulating the amount of refrigerant that enters the evaporator, greatly reducing its pressure and temperature.
Evaporator: This component does the actual "cooling" in the vehicle. Set just behind your dash, the evaporator removes humidity from cabin air and cools the air that passes over it. A clutch cycling switch monitors and controls the temperature inside the evaporator’s core to prevent it from freezing, which can greatly damage your AC system as a whole.
Blower motor: This part moves the cool air that has passed over the evaporator through your vehicle’s vents. It is controlled by a central control head (or resistor) for fan speed. Also in the distribution system are a series of mode doors that control the direction of flow of cold air.
Note that all of these parts are universal for any vapor-cycle AC system, but the way the refrigerant is metered, controlled, and restricted will vary from manufacturer to manufacturer.
Common reasons for this to happen:
Although a malfunction in any part of your AC system can cause it to underperform or stop working altogether, there are certain issues that are common culprits. Low refrigerant is a common concern. However, since the refrigerant is recycled through the air conditioning system, the most likely way it can be lost is through a leak in one of the parts. Simply replacing the refrigerant will not solve the root of the problem.
The most frequent causes of a faulty air conditioning unit are:
Worn compressor: If your air conditioning suddenly stops working or starts making strange noises, the compressor may need to be replaced. A faulty compressor can also cause oil or refrigerant leaks, which a qualified mechanic can detect with dyes or a special infrared sniffer that can identify chlorofluorocarbon (CFC)-based gases.
Broken condenser: When your air conditioner blows air but it’s not cool enough, this can indicate problems with the condenser, although issues with the electric cooling fan or fan clutch could also be responsible. Typically, road debris causes damage to the condenser’s cooling fins and passage tubes, while internal debris prevents proper condenser function by restricting the flow of refrigerant and increasing high-side pressure. All of these cases result in a diminished ability for the condenser to transfer heat.
Faulty blower motor or resistor: If the AC system makes noise when you turn it on, but doesn’t push air through your vents, or if the blower only operates at maximum speed, the blower motor or resistor may be responsible.
Damaged receiver/dryer: If, in addition to your air conditioner not blowing cold air, moisture is accumulating on your windows that isn’t resolved by defrosting, the receiver is likely in need of replacement. This should be done as soon as possible because an excess of moisture will quickly damage other AC system components.
Blown fuse: Sometimes, the simplest thing to fix can wreak the most havoc. So, it’s wise to check the fuse that controls your air conditioning before assuming the worst. However, simply replacing the fuse without exploring other symptoms can mask the root problem and cause further damage. If you find a blown fuse, take your car in. The mechanic working on your car may need to perform electrical tests to find the high resistance in the circuit and solve the problem that resulted in the blown fuse.
With that said, air conditioning problems can be summarized within 4 basic categories:
Hydraulic problems under the hood, such as a lack of refrigerant, too much refrigerant, a lack of adequate compression, or a lack of a refrigerant control device.
Electric problems under the hood, such as with the AC clutch coil, clutch relay, high-pressure cut-off switch, low-pressure cut-off switch, or circuit protection device.
Electrical problems in the dash, such as the AC switch not working, the computer control head or module going bad, or issues with the blend door command motor.
Mechanical problems in the dash, such as the blend door being physically broken, which prevents it from forcing air across the evaporator; the mode door being broken, which prevents it from forcing air though the vents; or the recirculation door being broken and falling into blower motor.
What to expect:
A top-rated mobile mechanic will come to your home or office to determine the source and cause of the AC malfunction. After the inspection, the mechanic will provide a detailed report that describes the air conditioning issue, along with the scope and cost of the necessary repairs.
How it's done:
The mechanic will perform a visual inspection of the air conditioning system, check the refrigerant for the proper charge, use a sniffer to identify any leaks of CFC gas, test the heater controls to make sure the air is being directed across the evaporator and sealing off the heater core, and then check other system components as specific symptoms and issues are identified.
How important is this service?
Air conditioning systems use extremely high pressures and temperatures. If any of the above symptoms happen, please do the safe thing: have your system inspected by one of our expert mechanics.