Auto Air Conditioning Repair

1. 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.

2. Become familiar with the five major components to auto air conditioning:

    • the compressor, which compresses the refrigerant in the system (on modern cars, usually a substance called R-134a)
    • the refrigerant, which carries the heat
    • the condensor, which gets hot when the compressed refrigerant goes through it
    • the expansion valve, which isn’t really a valve at all but more like a nozzle
    • the dryer/evaporator, which adds heat to the refrigerant, cooling your car

3. 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 provides two results: First, the gas gets quite hot. When it is very hot, the outside air is cooler and heat can leave the gas. When the gas has cooled, remember that it is under pressure; it condenses into a liquid and this phase change makes it hotter. The refrigerant now loses huge amounts of heat. The liquid is then sent to the evaporator, the coils inside of your car. It is still a bit hot, but it is a liquid under pressure. There it goes through the expansion valve, finds a low pressure and evaporates. Evaporation requires a lot of heat energy, so the vapor gets cold. It is this phase change from liquid to gas that really causes the temperature to change. This cold gas vapor chills the evaporator, and your car’s blower blows air through the cold evaporator and into the interior. The refrigerant goes back through the cycle again and again.

4. 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 poke a pen in there to try to valve off pressure, and if there IS none, that’s the problem.

5. 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).

6. 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.

7. 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!


  • 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. 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, and have access to a small vacuum pump (like what they’d use in a lab or shop), it’s best to suck all the air out of the system before filling it. Air contains moisture, and moisture is bad in AC systems because it corrodes things.
  • 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 more eco friendly than R12 or R134a. Venting HC12a is not believed to cause environmental damage. 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 regrigerants 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 availabe via ebay.
  • 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 explode with an ignition source. Light up a cigarette if you have an evaporator leak and your car becomes a bomb. Professionals don’t use it because of this very reason.