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Have you been sweltering in your car because of a broken air conditioner? Here's a short guide to how air conditioning (AC) works, why it might not work, and what you can do about it.
- Realize that auto AC is basically a refrigerator in a weird layout. It's designed to move heat from one place (the inside of your car) to some other place (the outdoors). While a complete discussion of every specific model and component is well outside the scope of this article, this should give you a start on figuring out what the problem might be and either fixing it yourself or talking intelligently to someone you can pay to fix it.
- Become familiar with the major components to auto air conditioning:
- the compressor, which compresses and circulates the refrigerant in the system
- the refrigerant, (on modern cars, usually a substance called R-134a older cars have r-12 freon which is becoming increasingly more expensive and hard to find, and also requires a license to handle) which carries the heat
- the condenser, which changes the phase of the refrigerant and expels heat removed from the car
- the expansion valve (or orifice tube in some vehicles), which is somewhat of a nozzle and functions to similtaneously drop the pressure of the refrigerant liquid, meter its flow, and atomize it
- the evaporator, which transfers heat to the refrigerant from the air blown across it, cooling your car
- the receiver/dryer, which functions as a filter for the refrigerant/oil, removing moisture and other contaminants
- the compressor, which compresses and circulates the refrigerant in the system
- Understand the air conditioning process: The compressor puts the refrigerant under pressure and sends it to the condensing coils. In your car, these coils are generally in front of the radiator. Compressing a gas makes it quite hot. In the condenser, this added heat and the heat the refrigerant picked up in the evaporator is expelled to the air flowing across it from outside the car. When the refrigerant is cooled to its saturation temperature, it will change phase from a gas back into a liquid (this gives off a bundle of heat known as the "latent heat of vaporization"). The liquid then passes through the expansion valve to the evaporator, the coils inside of your car, where it loses pressure that was added to it in the compressor. This causes some of the liquid to change to a low-pressure gas as it cools the remaining liquid. This two-phase mixture enters the evaporator, and the liquid portion of the refrigerant absorbs the heat from the air across the coil and evaporates. Your car's blower circulates air across the cold evaporator and into the interior. The refrigerant goes back through the cycle again and again.
- Check to see if all the R-134a leaks out (meaning there's nothing in the loop to carry away heat). Leaks are easy to spot but not easy to fix without pulling things apart. Most auto-supply stores carry a fluorescent dye that can be added to the system to check for leaks, and it will have instructions for use on the can. If there's a bad enough leak, the system will have no pressure in it at all. Find one of the valve-stem-looking things and CAREFULLY (eye protection recommended) poke a pen in there to try to valve off pressure, and if there IS none, that's the problem.
- Make sure the compressor is turning. Start the car, turn on the AC and look under the hood. The AC compressor is generally a pumplike thing off to one side with large rubber and steel hoses going to it. It will not have a filler cap on it, but will often have one or two things that look like the valve stems on a bike tire. The pulley on the front of the compressor exists as an outer pulley and an inner hub which turns when an electric clutch is engaged. If the AC is on and the blower is on, but the center of the pulley is not turning, then the compressor's clutch is not engaging. This could be a bad fuse, a wiring problem, a broken AC switch in your dash, or the system could be low on refrigerant (most systems have a low-pressure safety cutout that will disable the compressor if there isn't enough refrigerant in the system).
- Look for other things that can go wrong: bad switches, bad fuses, broken wires, broken fan belt (preventing the pump from turning), or seal failure inside the compressor.
- Feel for any cooling at all. If the system cools, but not much, it could just be low pressure, and you can top up the refrigerant. Most auto-supply stores will have a kit to refill a system, and it will come with instructions. Do not overfill! Adding more than the recommended amount of refrigerant will NOT improve performance but actually will decrease performance. In fact, the more expensive automated equipment found at nicer shops actually monitors cooling performance real-time as it adds refrigerant, and when the performance begins to decrease it removes refrigerant until the performance peaks again.
- If you suspect bad wiring, most compressors have a wire leading to the electric clutch. Find the connector in the middle of that wire, and unplug it. Take a length of wire and run it from the compressor's wire to the plus (+) side of your battery. If you hear a loud CLACK, the electric clutch is fine and you should check the car's wiring and fuses. If you get nothing, the electric clutch is bad and the compressor will have to be replaced. Ideally, if you can do this test while the car is running, you can see if the hub spins. Take care to keep fingers and loose clothes away from moving pulleys and belts. That would rule out a clutch that actuates properly but then slips so badly it won't generate pressure.
- If your system is empty and you're refilling it, be sure to first replace the receiver/dryer and evacuate the system. If you don't have access to a vacuum pump (like what they'd use in a lab or shop), it's best to get a shop to suck all the air out of the system before filling it. Air contains moisture, and both are unacceptable in AC systems for a multitude of reasons.
- Your system will have a light oil in it. If you vent off any refrigerant, be prepared to wipe some oil off things nearby.
- Another possible replacement refrigerant is HC12a which is used quite a bit more in Europe. It performs better than R-134a or R12. It is more flammable. HC12a is illegal in many U.S. states, including Arkansas, Arizona, Connecticut, Florida, Idaho, Iowa, Indiana, Kansas, Louisiana, Maryland, Montana, Nebraska, North Dakota, Oklahoma, Texas, Utah, Virginia, Washington, Wisconsin, and the District of Columbia. HC12a has a lower impact on global warming and ozone depletion than R12 or R134a, but since HC12a contains hydrocarbons, it could contribute to volatile organic compound (VOC) emissions. Must be ordered on the Internet as local shops do not seem to stock it. The issue is that shops will not work on a car that has other refrigerants in it. Special equipment is needed for each type of refrigerant's recovery. Standard R12 or R134a is a safer choice.
- Be extremely cautious about converting your old R-12 system to R-134a. The R-134a conversion kits sold at Auto Parts stores and even WalMart, are called "Black Death Kits" by some AC repairmen. Frequently, the new R-134a refrigerant will not circulate the R-12 oil and you will burn up your compressor. The R-12 mineral oil has chlorine contaminants that will destroy the R-134a PAG or POE special oil. The only way to reliably convert from R-12 to R-134a is to remove the compressor and flush out all the old oil with the new type of oil; then replace the old Receiver-Dryer or Accumulator with a new one; then flush out all the lines, the evaporator, and the condensor with special cleaner then vacuum to a steady vacuum; and finally charge with 70-80%, (by weight) of the original R-12 weight, with R-134a; and expect poorer cooling ability. It is much easier to keep the old R-12 system running with R-12 that is readily available via ebay.
- If you have any reason to suspect that your refrigerant has leaked out completely (the DIY pressure gauge you bought at the parts store reads 0 psi; the compressor won't engage because it may be sensing no pressure in the system; you stuck a pen in one of the service ports and no refrigerant hissed out; etc) then you are best off taking it to a professional unless you are familiar with what you are doing. As a general rule of thumb, you are probably not familiar enough if you are reading this article. The reason for this is that a completely depleted refrigerant system has no pressure to keep air and moisture from coming in through wherever the refrigerant leaked out. Air and moisture are possibly the two biggest enemies of an air conditioning system. The ways in which they can do harm are outside the scope of this article, but suffice it to say that it is NOT ok to have a significant amount of either in the lines. The receiver/dryer unit MUST be replaced in a case like this as you can almost guarantee that it has absorbed enough moisture to be completely useless, and before refilling the system it MUST be evacuated (pulling a vacuum on the system, effectively removing nearly all air and moisture from inside) with the proper equipment, which few DIYers are likely to have access or knowledge to use. Let a professional handle it, and you may get away with paying for a fixed leak and an evacuate/recharge. Try to fix it yourself, and a few months down the road you might wind up having to pay for the same evacuate/recharge service, plus replacement of a seized compressor, as well as a new condenser, evaporator, and expansion valve/orifice tube because the compressor sent shards of metal throughout the entire system when it died.
- Venting refrigerant -- even R-134a -- is illegal in the United States, so act accordingly.
- NEVER connect refrigerant cans, oil or leak-detector cans to the "high pressure side" of the system. This is often marked with H or HIGH, or a red connector cap. Cans can explode, and that would hurt.
- Stay away from major leaks of refrigerant. As it vents it will get cold enough to freeze your skin.
- Look out for moving fan blades and fan belts!
- HC12 is a hydrocarbon, usually some mix of butane or propane. It will ignite with an ignition source (as will R-134a). Professionals don't use it because the EPA has not certified it for automotive use. And it could blow up in your face if you aren't careful.
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