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auto maintenance February 08, 2018

Keeping Your Ride Cool

By Douglas Flint

During the exceptionally long cold spring we had, I reminded people, “Don’t worry about a cold spring! It will get hot enough soon enough and hotter still before it’s done!”   It will tax our endurance and our air conditioning systems to the max.

Automotive air conditioning systems are quite remarkable in that they must do everything a household unit does, only much faster, while contending with the enormous heat the engine generates, while bouncing down a road in a variety of hostile environments, while under pressure from the EPA to use less refrigerant, to use a less harmful refrigerant, and to do so efficiently enough to not dramatically affect the car’s efficiency or performance.

Although there are some new AC technologies on the horizon and possibly on the road, let’s stick to the basic AC system that 99% of today’s vehicles use.  Physics fact: there is no force in the universe known as “cool” (excepting Arthur Fonzarelli). So the way AC systems work is they remove heat and put it somewhere else. (Ever notice warm air blowing out from under your refrigerator?)  Heat is removed via evaporation.  Get out of a swimming pool on a 98 degree day.  You feel a chill on your shoulders, back, and arms even though the heat is sweltering.  That is because as the water evaporates it draws heat away from your body — the energy needed for the change of state from liquid to vapor.

There are chemicals that are way more efficient at heat transfer than water, and evaporate at much lower temperatures, and those are what we use in AC systems.  Old timers like me still like to call it Freon, the DuPont trademarked name for Refrigerant 12, which was banned because of environmental considerations in 1994. Since then the refrigerant used is 134A, which is now slated for phase-out over the next few years.  (And that means another $8,000 expense for the old repair shop.)

The compressor is the heart of the system, driven by a belt from the engine and coupled with an electromagnetic clutch that compresses and pushes the refrigerant on its endless cycle through the system. The compressed refrigerant gas enters the condenser, a black finned device in front of the radiator and about the same size. Here the refrigerant dissipates its heat and condenses back into a liquid.  The liquid travels through lines first to the receiver drier for filtration and then enters the evaporator (that’s the part inside the car, another finned radiator-looking device).  The refrigerant changes from a liquid to a gas in the evaporator, drawing all the heat away from it, causing the fins to become ice cold.  The blower blows air across the evaporator and the air give up its heat (and humidity), resulting in cool air blowing into your car.

Contained within this oversimplified explanation there are many subsystems to make it work as desired.  Left unchecked, the evaporator would literally become a block of ice, so valves and switches are used to regulate refrigerant flow and to cycle the compressor on and off.  Under the dash, a system of passages controls the direction of air flow (floor, face defrost) with electric motors opening and closing doors.  Temperature is controlled by a blend air door that mixes cooled air with non-cooled.  A resistor block or module gives you different fan speeds.  Additional cooling fans in front of the radiator may be necessary for additional cooling during AC operation.

But what does all that mean?

Automotive air conditioning systems lose a small amount of refrigerant every year.  It is not uncommon to have to service them every 3 to 5 years, although some really well-built systems have been known to go a decade or more without losing enough refrigerant to matter.  If your AC system performs well in the morning but seems to weaken or give up on a hot afternoon, it is most likely low on refrigerant and in need of service.

A compete service at Wiygul Automotive Clinic includes a physical inspection of all visible components and a check of their basic operation. The system is then evacuated, safely removing and storing any remaining refrigerant. The system is then placed under deep vacuum to remove moisture and contaminants.  This is also a crude leak test because a system that won’t go into or hold deep vacuum has a large leak that must be repaired before continuing.  If the system holds vacuum for the prescribed amount of time, the system is recharged with refrigerant and oil to lubricate the compressor.  A special dye that will help find small leaks is injected.  Once this is complete, the performance of the system is checked and evaluated and a UV light is used to look for any leaks.  If none are found, you will be advised to return in 10 days for a follow-up leak check.

Frequently asked questions

Can’t you just top off my system?

No! Modern AC systems carry much less refrigerant then the old systems.  The amount of refrigerant must be exact. One ounce too much or too little can hinder performance and even damage the system.  There is no reliable way to tell how much refrigerant is in a system. The only safe method is to completely empty it and add only the specified amount.

What can I do to maintain my AC system?

Have your cabin air filter replaced once a year regardless of manufacturer’s recommendations. The under dash part of an AC system is very hard to access. Once debris and mildew build up in there it is very difficult to get rid of the smell.

Where the windshield meets the hood-line on a car is called the cowl.  This is typically where fresh air is drawn into the AC system. This is also where leaves, pollen and other debris accumulate. I have found cars where the mulching process has already gotten well under way with genuine dirt with plants growing in it. This is no place to do organic farming. Open the hood to thoroughly clean it.

Fortunately most modern AC systems are designed to give you years of trouble-free performance. But if you do have a problem, be assured that the staff at Wiygul Automotive Clinic can solve it.

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