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(AOL Autos) -- It's that time again, when you turn on the A/C to chill out from the summer heat and all you get is hot air!

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Air conditioning on the fritz? Taking your car to an A/C tech will keep you from getting hot headed.

Ughhhh! How do you restore that refreshing, cool air to your vehicle's interior cabin so you can survive the heat?

Well ... sit back and relax, and I'll tell you exactly what to expect from your shop.

System performance test

First, the tech should perform an A/C system performance test. He/she will first check vent temperature to confirm that the system is indeed inoperative.

Should this be the case, the tech will then perform a head pressure check. During this process, gauges are installed on the high and low side of the system to determine if there's any refrigerant in the system.

An extremely low (or no) pressure reading usually indicates a lack of refrigerant in the system, which means it has leaked out. Sometimes the pressure reading may be too high, in which case there is a restriction in the system, inhibiting the flow of refrigerant.

There are three diagnostic paths, depending on the initial evaluations. Should the system be low on refrigerant, the tech should run a leak test, identify the location of the leak, repair it, and recharge the system with refrigerant and oil.

If the pressure in the system is too high, the tech should locate the restriction, often caused by dirt that finds its way to the orifice tube, a small in-line filter designed to screen out any particulates in the system. (Restrictions can occur for other reasons that I will not go into here for the sake of space.)

Once the plug is found, it is removed, and dirt is flushed from the system. Finally, if the system seems to be operating properly (all head pressures are in line with factory specifications), then the tech will look to the duct system for problems.

The duct system

The engine in your car generates vacuum as a result of taking in air. This vacuum is used for the duct system.

How the system works: Vacuum is collected in a vacuum reserve chamber; this device usually resembles a plastic ball or a coffee can. The vacuum builds up inside this chamber and when A/C is called for, vacuum is channeled through the switch and small vacuum lines (capillary tubing) to the servo motor.

The servo motor is responsible for opening a special duct door (the air blend door), which directs the correct amount of cool air into the vehicle's cabin. Problems crop up when vacuum is lost due to a cracked vacuum reserve chamber, broken vacuum line, faulty vacuum servomotor, bad switch, or poor engine vacuum.

The tech must track down the cause of the vacuum loss and repair it in order to restore the system. Other causes of poor HVAC air volume are broken air blend door or door hinge, organic debris in the fan squirrel cage inhibiting airflow, worn blower motor shaft bearings slowing down the squirrel cage, or electrical wiring / component problems that control fan operation.