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The cars Air Conditioning system and how to repair it

Since temperatures are getting hotter, it's nice to get the A/C on a car running again. Chances are that the A/C isn't working anymore and has lost most or all it's refrigerant. Also, many classic cars come with an old A/C that used a different refrigerant and can't be easily refilled. The A/C system is something many don't bother with anymore and write it off on their cars. Some even remove the entire system because it is not working anymore. There is no reason to do that, and I will show how you can easily get it running yourself without going to the A/C workshop. Even professional workshops struggle with old air conditioning systems and just give an expensive fill for $100 without even first checking for leaks. After one week, the system is empty again, and your investment is gone.

car air condition pressure gauges
DIY A/C pressure gauges to check and refill the system at home

How Air Conditioning works

The A/C is a closed-loop system that works exactly like any fridge or freezer. The refrigerant gas is pumped through the system by a strong multi-piston compressor. The gas gets compressed and changes its aggregate state to a liquid in the condenser where it is cooled down by the airstream and a fan. From there it flows through the dryer to remove moisture, gets metered by the expansion valve and is then expanded into a gas again in the evaporator core. When a liquid changes its aggregate state to a gas, it absorbs a lot of energy which it removes from the environment. Imagine a pot with boiling water on a stove. You keep pumping energy in the form of heat into the pot, but the water keeps boiling at 100 °C or 212 °F without increasing its temperature. The water absorbs all the heat energy from the stove and uses it to turn the liquid into a gas. It does that for a very long time until all the water has evaporated. Another great example is a wet towel which you place onto your face. The water starts to evaporate slowly and in the process, cools down the towel and your face. In the same way, the refrigerant liquid cools down the surroundings (evaporator core) drastically. Water starts boiling at 100 °C or 212 °F, but the A/C refrigerant R134a starts boiling already at -14.9°F or -26.1°C. The refrigerant, just like a pot with water on a stove, uses energy from its environment to change its state to a gas. To liquify it again, it needs to be highly pressurised and then cooled down. The gas gets pressurized by the A/C compressor, which drastically heats the gas. This hot gas then flows into the condenser, where it gets cooled down by the air and an electric fan. To become a liquid again at 30°C or 86°F, it needs to be pressurised to 7.7 bar or 110 psi.

Car air condition system
How a cars air conditioning system works

Finding Leaks

The A/C will start to leak eventually due to many components and pipes, hoses and gaskets failing. The reason a fridge rarely fails is that the refrigerant closed loop system is sealed and doesn't have any screw connections. The copper pipes are all soldered to the compressor and there are no gasket and points where it could start leaking. With a car, the system is more complex and powerful and pipes need to run throughout the vehicle. All connections and the compressor have different rubber seals and gaskets to contain the refrigerant. The A/C system needs to keep the gas sealed off and pressurized to at least 4-6 bar / 60-90 psi when off. The refrigerant is also infused with lubrication oil to keep the compressor working and to lubricate seals. The entire system is designed to lose tiny bits of refrigerant because the oil needs to creep underneath seals to keep them intact and working. The shaft seal is kept lubricated the entire time the compressor is engaged and running. Many compressors lose their charge when the A/C system is not being used. This happens as an under-lubricated seal around the compressor's shaft dries up and develops an oil and refrigerant leak.

car air condition compressor
A/C compressor with swash plate and pistons Photo: VW Group

Over time, a car's A/C system always loses small amounts of refrigerant. To get the A/C working again, you have to find the leak and repair it. This can be done with a UV dye or a "sniffer". The better option is the UV dye since it exposes the leak via UV rays and yellow goggles. You will be able to see exactly where the gas is escaping. Many systems are already factory filled with the leak-dye so you can straight away go and inspect all pipes and connectors with the UV lamp and the goggles. Open the cap which protects the Shrader valve and press it briefly in with a screwdriver so a tiny bit of gas escapes. If you can see a yellowish oil residue, the system has been filled with the dye already. Closely follow all the pipes with the lamp and wear the yellow goggles which expose the fluorescent gloom of the dye.

air condition leak detection
A/C dye, yellow leak goggles, UV lamp and dye injector

If the A/C system is empty and wasn't filled with the dye, you have to provisionally fill it with gas first before starting to look for leaks. The dye will be filled along with refrigerant into your A/C system. You can also do this yourself if the A/C system is still working. You will need a dye injector for this though. Unfortunately, this doesn't work for components that are hidden from sight, such as the evaporator core. But in over 90% of the cases, you will find the leak on one of the pipes, the compressor or the condenser. Just check really carefully and look at all connections from close. If you think you found the leak, spray some soapy water on it and check if it is bubbling when you turn on the A/C.

air condition leak detector
Refrigerant sniffer, exposed leak visible via the dye and a UV lamp

Repairing Leaks

If you have found the leak, you have to repair it with new components and seals. The condenser often brakes, and the dryer should be changed as part of regular maintenance. They are usually inexpensive, and aftermarket parts are available. Before opening the A/C system, you have to evacuate the refrigerant. You can vent the gas by opening the Schrader valve with a screwdriver (In some countries, this might be illegal), or you have to go to an A/C garage and get it evacuated. After that, you can open the system and replace the faulty components. Also, you have to replace the old o-rings and seals with new ones. The o-rings need to be made from a special material (Hydrogenated Nitrile Butadiene) to withstand the temperatures and the pressurised refrigerant. They usually are colour coded in green or purple and are available online as sets.

air condition dryer and condenser
Dryer, condenser and all other A/C components are connected via screws and sealed by special o-rings.

Retrofit from R12

If your car came with the old R12 (pre-94) refrigerant, you will have to retrofit it. Some say you can't convert R12 systems and would have to change all the seals, compressor etc. It's not true, and I have done the upgrade many times with just new connectors, and it was never a problem and is still working to this day. In order to convert your A/C system to an available and less environmentally harmful refrigerant, you have to change the high-pressure and low-pressure Schrader valves and fittings to be able to hook up the filler hoses and gauges. You can buy these universal adapters online. Once you have installed the new valves and fittings, you can fill the A/C system again with an alternative gas. Depending on the gas you might also have to change the compressor oil because it won't be compatible with the gas. You can read about this in the next section about the different refrigerants. Most of the oil is located in the A/C compressor and it can be drained there. Some compressors have a drain plug while others don't. If your A/C compressor doesn't have a drain plug you would need to take it off the car and drain it from the refrigerant hose connections.

R12 to R134a conversion
R12 conversion adapters, Low- and High-side ports Photo: Mercedes-Benz Group


The A/C shop will charge you $80-150 depending on if they have to evacuate first and how much refrigerant they have to fill. If you have a leak, you have to charge the system twice. You need to find the leak first with a charged and running system. Then you have to evacuate the gas to fix the problem until the system can be charged again with the permanent refrigerant. So you will be spending $160 - 300 just for refrigerant refills and evacuation. It might be worthwhile to invest in A/C gauges, hoses, an evacuation pump and refrigerant bottles. You can buy inexpensive special gauges online. They feature two manometers, one for the low-pressure side and one for the high-pressure side. The blue one measures pressure on the low gas side, whereas the red one measures pressure on the liquid high side. The yellow hose is used to hook it up to the refrigerant bottles.

air condition vacuum pump, hoses and gauges
A/C gauges, hoses, connectors, vacuum compressor and valves

The Refrigerant

There are many A/C refrigerants out there and according to an international agreement, they are represented by the letter R (as in Refrigerants) followed by a two- or three-digit number. In some cases, the number code is followed by another letter or two. The designation Rxxx (For example R290) is determined by the chemical composition of the molecules. You can check the engine bay for a sticker which states the type of gas and the kilos filled. It should normally be around a few hundred grams to 2 kilograms. You can buy refrigerant gas in bottles online or get them from a gas distributor near you. In some countries, it got increasingly harder to purchase refrigerants due to regulatory restrictions. It became necessary to have a commercial license to buy gasses which you need to be approved for. This is why many car garages don't offer A/C services anymore. Another factor is the current supply chain shortages that have emerged during the last pandemic and prices for some refrigerants (for example R410a) went through the roof. In order to understand at what pressures and temperatures the different types of refrigerants evaporate and liquify, you can refer to the Danfoss Refrigerant Slider app or website. There, you'll find all information on the different gasses, composition, pressures and much more information.

R12 / R22

The R12 and R22 gasses are so-called CFCs and HCFCs which stands for Hydrochlorofluorocarbons. These are chemicals that were mainly used as refrigerants back in the 20th century. The release of HCFCs depletes the earth's protective ozone layer and contributes to climate change. The gas has a Global Warming Potential (GWP) of 2400 points. Due to this fact, it is not allowed anymore to produce and import these gasses which means if your car was running on these, you will have to use a substitute. Also, your A/C system will have old connectors which prevent it to be charged with other gasses. You can read below how to retrofit this.


R134a is a substitute gas for R12 and is many times less harmful than CFCs. It has been used since the early 90s in cars as the default refrigerant. Most A/C shops offer this gas and can refill the system with it. Unfortunately, this gas also contributes to global warming (GWP of 1300) and is therefore phased out again. You can use R134a to charge old R12 systems. The gas is unreactive and doesn't contaminate the compressor oil. Although, it can deteriorate gaskets and lead to leaks. This is why customers were quite upset in the 90s because the old R12 was banned and car manufacturers advised garages to retrofit A/C systems by changing all gaskets which cost thousands in labour. Pipes and connectors run throughout the engine bay and cabin and it's not an easy task to replace all seals and gaskets. But from experience, it isn't likely that you get any problems with failing gaskets. Old R12 A/C systems can safely be charged with R134a. One downside though is that the refrigerant is not as effective as R12 meaning that your A/C will work only at around 80% - 90% of its previous cooling performance.


This new refrigerant is the new go-to gas that replaced the old R134a. It has a GWP of <1 and its greenhouse potential is neglectable. The European Union banned R134a in cars manufactured from 2011 forcing them to adopt the new refrigerant. The downside of R1234yf is that it needs to operate under higher system pressures which means the A/C compressor, gaskets and systems will be under more stress leading to an earlier failure and gas loss. Also, the refrigerant itself is mildly flammable and needs to be handled with special care. Older R134a systems can't be converted to R1234yf due to the different gas pressures and incompatibility with seals and the oil. The R1234yf system refrigerant circuit is accessed using service couplers that are a different size to those of an R134a system.

R290 / R600a

R290 is also known as propane gas and is often used as a refrigerant in home-appliance units such as fridges and stationary A/C units. Service technicians often charge systems with this gas. Another very similar gas is called R600a which is Isobutane. Both gasses are highly flammable and it is often used with camping and bbq equipment to fire stoves and burners. Camping gas is popular for outdoor activities and comes in various bottle shapes. Often, it is a mix of propane and butane. If you can't get any refrigerant gasses such as R134a or drop-ins anywhere, you can theoretically use a Propane-Butane mix to charge the A/C. These gases are readily available, and it is sold at every supermarket, gas station and camping shop. When using a Propane-Butane mix, you will only need about half of the charge. Don't rely on the weight though but monitor the pressures in the system. Refer to the Danfoss slider to get the right pressures and temperatures. Using Propane or Isobutane is not as dangerous as some people say it is. Many household A/C units are being charged with it and they even take more kilos than a car's A/C system. The refrigerant R1234yf found in modern cars is also flammable. The problem with R290 / R600a is that you need to change the compressor oil which is a pain. The gas contaminates and deteriorates the oil which will eventually lead to a compressor failure. Using this gas can be a temporary solution but to make it a permanent one the compressor oil would need to be replaced by a compatible one.

Drop-in Refrigerants

Many gas suppliers offer so-called drop-in gasses which replace the old R12/R22 or R134a gasses. For R12 there is a substitute refrigerant called R24 which can be used on old A/C systems without any conversion or changing of the oil. Here is a link to the R24 substitute refrigerant with an explanation and where it can also be ordered:

Drop-in gasses come in all sorts of different compositions and consist of a mix of traditional refrigerants. The downside is that they can be quite expensive costing around €200 per kilogram. But drop-in refrigerants are by far the best option since they were developed to replace the old gas.

air condition refrigerants
Different types of refrigerants

The Compressor Oil

The A/C compressor features multiple pistons inside of it which pressurise the gas. Like the pistons of a combustion engine, they need to be lubricated in order not to seize. Inside the compressor is a swash plate and drive hub which moves the pistons up and down in its cylinders. Also, the refrigerant gets infused with small oil droplets which transport it around the system to prevent corrosion. These compressor oils are often called "PAG" which stands for Polyalkylene Glycol. It is a synthetic oil to lubricate the compressor. Most newer model cars with R134a and R1234yf systems use PAG as the lubricant. More or less all refrigerants with R1xx and R2xx can use the same PAG oil. The exception is if it’s R1xxx (four-digit numbers) or if you use drop-in gasses. R290 is another exception which doesn't work with PAG. For R290 and R600a a "POE" or Polyolester oil should be used. The right oil for the right refrigerant and type of compressor is science in itself. You will hear and read different things about what oils are best and should be used. The gas and oil compatibility information was provided by A/C expert Mathias from Motorsport Madness.

air condition compressor oil
A/C compressor oil

Evacuating the System

Hook up the blue hose to the low-pressure side of the A/C system. In your car, this is usually a thick and bulky hose to carry the volume of the gas. The high-pressure side is usually a thin metal line and doesn't need the same diameter as the low side. Hook the red hose up to the high-pressure side. Open the valves and switch on the vacuum pump which is connected to the yellow hose to start evacuating the system. If you don't want to invest in a vacuum pump, just use the one in your car. I didn't see a need for an extra pump and just used the one on my car, which powers the brake booster and vacuum system. Not every car has a vacuum pump though. Let it run for 15-30 minutes, and make sure both manometers show an absolute zero pressure reading. This is to evacuate the air and moisture from the A/C system. Any trapped water will start to boil at an absolute vacuum and will be drawn out from the system. Close all valves and wait for another half an hour to check if it kept the vacuum and didn't suck in any air from the outside. It's not 100% necessary to evacuate the system, but it will let the A/C run more efficiently and prevent corrosion. Trapped water moisture can remain in the system if its not evacuated. Some people do it without the vacuum and it still works.

evacuating the air condition system
Evacuating the system from air and creating an absolute vacuum

Filling the Refrigerant

Find out how many grams or ounces the system takes. If it takes 1000g, you will need five 200g bottles. Before you hook up the bottle, fill in a couple of sips of the UV dye and some sips of the compressor oil into the yellow hose so the refrigerant will drag it in. For the first charge, the engine and A/C is turned off. Pre-fill the system with two bottles first to give it a pre-charge and pressure. Once you load the A/C with around 40% of the total refrigerant weight and the gauges are showing about 2-3 bar / 40-60 psi, you can turn on the engine and the A/C. But don't forget to close the high-pressure valve, or you will blow the gas bottle. When the engine is running, keep filling the A/C through the low-pressure port until the pressure reaches around 3 bar / 43psi on the low side and around 17 bar / 250 psi on the high side. The needles should be fluctuating slowly depending on if the compressor is pumping or not.

car air condition pressures
A/C system pressures when off and when running

Monitoring the A/C

The compressor will engage and disengage all 20 seconds or so. On the pully of the A/C compressor is a clutch which gets electronically engaged. During the cycles, you can see on the manometers how the refrigerant gets pressurised and loses pressure again. The high side will go up when the compressor is running and the low side will go down. This reverses when the compressor switches off. It will become harder to fill the 3rd, 4th and 5th bottle because the system starts to become more pressurised. You won’t be able to completely empty the last bottles anymore but that is as expected. When you have completely filled the system, close all valves, switch off the engine and fit the caps on the A/C fittings. Monitor your A/C with a thermometer over the next few days if it keeps the temperature at a low level. A well-functioning A/C should blow out 4°C or 40°F air but it also depends on the outside temperature. If the system is running low on gas you'll hear hissing coming from the air vents and the evaporator core. It's an indication that you have to top up the system.

air condition thermometer
Checking the A/C temperature with a thermometer

If you want to learn more about classic car maintenance and repairs as well as tips and trick, consider the book "The Ultimate Classic Car Guide". It will give you everything to take care of your own classic car without having expensive garage bills.

Nils Willner

Nils is a Swiss-German engineer who is obsessed with old cars and engines. He is the author of "The Ultimate Classic Car Guide - How to Buy, Maintain & Repair Classic Cars" and the founder of EVC. His passion has always been with old cars and everything that has wheels and an engine.