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Auto air conditioning 101

  • Story Highlights
  • Problems with your A/C/ system can cost you money by wasting extra gas
  • Tech should first do a car A/C system performance test
  • Problems with vacuum system can also limit cooling in car

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By Tom Torbjornsen
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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!

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

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.

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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.

Proper A/C leak test procedure

The main cause of A/C system failure is refrigerant leak. This system is a closed system, so the refrigerant chemical and lubricant are sealed from the outside atmosphere. When a leak forms, the system drains of both refrigerant chemical and the lubricant vital to compressor life and function.

In addition, moisture and dirt can get in through the leak causing contamination. This contamination eats away at the inside of the system resulting in rust and scale buildup, corrosion and erosion in vital A/C system parts. Proper A/C system leak tests are necessary to identify the source of a leak.

There are three types of leak inspections: visual, halogen, and dye testing. The visual test includes inspection of all lines and external components (specifically condensers, hi and low pressure lines, compressors, air dryers, and expansion valves). Any indication of refrigerant oil on these components is an indication of a leak and the component must be replaced.

The halogen tester is designed to detect the presence of leaking refrigerant gas, which is odorless and colorless. The tech scans the system with the flexible probe on the tester. If there is a leak, then the tester will howl, light up, or click.

Finally, if the tech is convinced that there is a leak in the system and he cannot find it, then he performs a dye test. After a fluorescent dye is charged into the system, an ultra violet light is shown on the system. If there is a leak, it will show up as a bright yellow color under the light.

These are tried and true diagnostic methods that have been used in A/C system diagnostics and repairs for years and are guaranteed to track down the most stubborn A/C leaks.

A word about refrigerant chemicals

As a result of the Clean Air Act, all chlorofluorocarbons are no longer used as refrigerant chemicals because of their negative effect on the ozone layer. Consequently, all A/C systems now use an ozone-friendly refrigerant called R134A. Any vehicles that still run the road with the old R12 chemical have to be retrofitted to run R134A.

In addition, the tech is required by Federal Law to recapture the R12 into a reclaiming station for disposal (it cannot be released into the air). Venting of chlorofluorocarbons to the atmosphere is considered a Federal Offense. Should you have a vehicle that still has the old R12 system in it, have the system converted.

This procedure involves the installation of new schrader valves for system charging and testing, as well as a complete and thorough cleaning and flushing of the system. It is essential that all of the R12 be removed, because the mixing of R12 with R134A results in 'system meltdown.

Now that you know all about the air conditioning in your car, you can see that there are many sources of potential problems. Thus, proper diagnosis of problems with this system can save you a lot of money; so make sure you take it to a reputable shop that has the necessary equipment and experienced techs.

And enjoy the summer heat!

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