by: Tom Torbjornsen

    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! 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.

    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!

    'Til next time ... Keep Rollin'

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    1 - 8 of 8 Comments
    dmagnum888 Oct 02, 2009 8:43 PM
    try taking out all the fuses that pertain to the ac system and putting them back in also try diconnecting the battery.In response to ugot2bhappi
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    ramsesmax Aug 29, 2009 11:44 PM
    foreign cars? , I'm sure that helped the U.S economy.
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    ryanorbilly Aug 24, 2009 5:19 PM
    ugot2bhappi: There is a duct that connects to the inside of the air vents in the back and the duct could have come loose inside of the trim somewhere and therefore isnt blowing cold air through the vent but into the inside of the car itself, where it is heating up and being forced through the vent. That could be the cause because cars only have one system(that i have seen, i do commercial/industrial heating and air so cars arent my specialty) carolcarolverm: As stated in the beginning of this article, car systems work on a vacuum and if the vacuum line is off or the switches are bad, that could be the cause of the problem. We have units come from the factory with bad switches sometimes and this can be the same thing in your newer car. kmfrost69: This one was a little tricky, but being a '96 the seals in it could be bad from sitting or not being used too often, causing the charge to leak out and therefore no cold air. I have personally never heard of a filter in a cars system but that is just me, I would also assume that if it says there is a filter, it would tell you where it is and how to get to it, not just the fact that one exists. Check your manual and if there is nothing in it then i wouldnt worry about it. Also, the entire package pretty much will probably be level with the top of your engine, so the chances of something getting bumped around with enough force to knock something loose are very low. Get a hotshot kit for yourself at advance auto and charge it yourself and see what happens; if it works great for a few days and then stops, you know its a leak and has to be fixed, and if it works for good then thats even better.
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    kmfrost69 Aug 18, 2009 2:34 PM
    I read there is a filter I should check. But where is it?
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    kmfrost69 Aug 18, 2009 2:07 PM
    My newly bought 96 Bronco 5.8L with 6 inch lift and 36 inch tires blows only warm air. It rides rough, and wonder if the rough ride could have broken something in the AC? I don't have a clue the last time the AC was used but knew it did work as I bought it from a friend.
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    ugot2bhappi Aug 18, 2009 11:50 AM
    I own a 2001 chevy tahoe and the rear ac only blows hot air? the front AC works great it blows cold...Does anyone know what can cause this problem? We checked all the fuses...
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    bf157 Aug 18, 2009 10:54 AM
    I've experienced some problems with my Ford F150. The dealer told me I needed a complete replacement including the condenser. After the shock of the first quote, I took the vehicle for some 2nd and even 3rd opinions. The compressor wasn't "locked" up and one shop explained that an AC system cannot be even accurately diagnosed unless the tech knows that system is charged correctly. He emptied the system into a recovery machine, he found a small fitting leak and repaired it, and then recharged the system to factory specs. The AC system is working fine. The other shops showed me the guage readings and told me the system was charged. I thought the guage readings explained what was going on? Yet the last shop did fix the vehicle and for a LOT less than the other quotes. The other shops are very reputable shops and I did see the guage readings. Could you explain what normal guage readings are? I'm confused
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    carolcarolverm Aug 18, 2009 9:14 AM
    I own a 2008 Chrysler Town and Country. It has the worst air-conditioning sytem. I have taken it to two Chrysler dealers and still have no results. There is a lack of coolness and a lack of air pressure. Has anyone else had this problem with this car?
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    1 - 8 of 8 Comments
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