Air Conditioning for Cars

Air Conditioning for Cars

Refrigerant is the gas/liquid that is used within the air conditioning system. There are only two types of refrigerant that have been approved for automotive use and they are called R12 and R134A. R12 refrigerant was made illegal in January 2001 due to its harmful effects on the earth’s ozone layer. Cars fitted with an R12 system can still continue to operate but it is illegal to refill a car with R12 refrigerant. Any servicing to an R12 system which involves the removal of R12 refrigerant cannot be refilled unless it is converted to R134A or a drop-in gas (more on this later). This process of conversion is called retro-fitting. Any R12 refrigerant removed must be destroyed by an approved disposal site. It is the job of the engineer to ensure this happens.

R134A refrigerant is not harmful to the ozone layer but is still a greenhouse gas so it is now illegal to vent the refrigerant into the atmosphere. Any person caught doing this can be fined up to £20,000 or receive up to a six month prison sentence. This includes any vehicle owner that knows his air conditioning system is leaking but fails to have the fault rectified. This has not been strictly enforced as yet in this country but will be in the future.

There are essentially two types of system used on cars and each system requires 5 essential components for the air conditioning to function. Manufactures may add on there own small components for protection or just to make the air conditioning function a little bit differently, but its these 5 major components that allow the air conditioning to work. If any one of these 5 components fails, then the air conditioning will not work adequately or at all.

TXV System

The first system and the one that the majority of cars including MG-Rover use are the TXV type system (Thermal Expansion Valve). This system is made up of the following 5 components:-

Receiver Drier
Thermal Expansion Valve

We will start the process at the compressor. The compressor’s job is to take low pressure refrigerant vapour and turn it into high pressure vapour. Under pressure, the refrigerant will give up its heat more readily (I won’t go into the physics of it). The high pressure vapour is then passed to the condenser.

The job of the condenser is to remove heat from the refrigerant. The condenser is located at the front of the vehicle to gain maximum airflow. Because the airflow is substantially cooler than the refrigerant vapour, the vapour gives up its heat to the cooler air (heat always moves from a hotter environment to a cooler environment).

During this process the refrigerant turns into a liquid. R134A refrigerant boils at -26.3 degrees centigrade hence the reason that it is a vapour before it reaches the condenser; it is essentially refrigerant steam at that stage. Obviously the temperature is not -26.3 degrees at this stage so how comes the refrigerant is not boiling? The answer is because the refrigerant in under very high pressure (As high as 18bar), the boiling point is raised considerably (again I wont go into the physics of it unless you want me to).

Still under pressure but with vast quantities of heat removed, the liquid refrigerant is now passed onto the receiver drier. The receiver drier is responsible for cleaning and drying the refrigerant. The receiver drier also acts as a storage facility so keep the refrigerant running smooth around the system. At this stage the refrigerant is still in liquid form. The next step is the thermal expansion valve. This valve usually sits inside the car under the facia with the evaporator. The job of the expansion valve is to control how much refrigerant can flow through the evaporator. The valve does this by sensing the temperature and the pressure of the refrigerant in the evaporator. Usually this uses a bulb and capillary system. The bulb on the end is connected to the output of the evaporator and this passes the information back along the capillary tube to the expansion valve. The valve will then open and close to either increase or decrease the amount of refrigerant flowing. The expansion valve will also reduce the pressure of the refrigerant changing it back into a saturated type of vapour thus making it extremely cold ( as the heat was removed earlier at high pressure). The evaporator then takes over the process.

The evaporator then removes heat from the vehicle interior. Remember that heat moves from a hotter temperature to a cooler one so because the refrigerant in the evaporator is cool, the heat from the cabin moves into it. The heater blower fans are behind the evaporator thus blowing cool air from the vents. The evaporator also dries and cleans the air by this process of air blowing across it. This process produces water droplets which are expelled via a drain tube from the evaporator. This is why you sometimes see water under the car when the air conditioning has been on. The process of the refrigerant taking on heat also ensures that the refrigerant starts to boil turning back to a vapour at low pressure (it boils at -26.3 under normal atmospheric pressure). This is a very important task as the next stage is to pass this back to the compressor to start all over again. The compressor will be destroyed if liquid is given to it as liquid cannot be compressed so it is important the expansion valve and evaporator boil all refrigerant into a vapour. This is why the valve has to regulate the amount of refrigerant going through the evaporator to ensure that only vapour gets passed back onto the compressor.

FOT System

The other type of system used in cars if the FOT (Fixed orifice tube) system. This works on exactly the same principle as the TXV system above with a few subtle changes. The compressor and condenser work in exactly the same way but there is no receiver drier after the condenser. Instead we have a component called a ‘fixed orifice tube’. The fixed orifice tube reduces the pressure and changes the liquid into an atomised vapour. This process makes the refrigerant cold. Again the evaporator now comes into play but nothing is regulated going through the evaporator. Once the refrigerant has gone through the evaporator, it is partially liquid and partially vapours.

Since we cannot put liquid into the compressor, another device has to come after the evaporator to ensure that only vapour goes back to the compressor. This device is called an ‘accumulator’. Essentially, this has the same job as the receiver drier that is used in the TXV system but it stores the refrigerant long enough to ensure that it all boils back to a vapour. An accumulator is usually fitted to an area of the car that is very warm. The compressor then starts the process all over again.

A lot of people ask which system is better. The answer is there is no clear advantage for either system Most Ford cars are fitted with a FOT system. The FOT system does usually have an advantage that the fixed orifice tube is usually located in the pipe work under the bonnet of the car so can be removed easily, and also the component cost is only a few pence where as a TXV is around £30. The orifice tube is colour coded so that the exact type of orifice tube can be fitted during replacement. The colour coding is to do with the size of the tube and thus how much restriction is placed on the refrigerant for the vehicles design and is chosen by the manufacturer of the vehicle. A lot of air conditioning engineers do claim that a FOT system tends to produce cooler air but I have not really noticed any difference having used and serviced both systems or measuring them with a thermometer. I guess its personal choice for the answer to this question…

Low Pressure Switch

This is usually fitted to the receiver drier or accumulator (depending on the system) and its job is to switch the compressor off if the pressure in the system goes low (usually below 1.4bar). This normally happens if refrigerant has leaked out of the system. The refrigerant also carries oil around the system to lubricate the compressor so to protect the compressor the system has to shutdown under low pressure just in case.

High Pressure Switch

This is usually fitted to the compressor and does the reverse of the low pressure switch. If the pressure goes high (usually above 15-18bar) the compressor will switch off. This usually happens if there is a blockage in the system. This is important because too higher pressure could cause the system to burst and also damage components long before it bursts.

Wide Open Throttle Switch

This is usually fitted to low powered cars but any car can have it, it is usually controlled by the ECU. Its job is to turn off the compressor when you need extra power. The compressor is driven via a belt from the engine (usually the crankshaft pulley) and typically takes 10-15BHP away from the engine. If you push hard on the accelerator the ECU will turn off the compressor so you gain that power back. This would typically be necessary when you need to quickly overtake for example.

Trinary Switch

The trinary switch incorporates the low and high pressure switch above. It also incorporates a temperature control so that the condenser fans turn on automatically when the temperature reaches a set level.

Ambient Temperature Switch

The compressor will not switch on when the ambient temperature is below a certain level (usually 4 degrees centigrade). It does this because at very low temperatures the refrigerant or any moisture in the system can freeze. This would cause a blockage so to counter act this possibility the system will not switch on when it’s cold outside. If on a cold day you find that the air conditioning system does not switch on, it’s probably because of this switch.

There are many other switches that manufacturers fit. A lot of these are usually only fitted to low powered cars but they are becoming common on even higher powered cars. A power steering switch is incorporated on some cars which switch off the compressor when the steering is turned. This is done because power steering also takes power away from the engine so the compressor is switched off to reduce the amount of power taken from the engine.


The compressor actually incorporates two items. There is the compressor itself and also the magnetic clutch. Since the compressor is driven from the engine it means that it is turning all the time. Since we need to somehow turn the compressor on and off, the system employs a clutch system. When the compressor needs to be turned on the clutch engages thus allowing the actual compressor to turn. There is usually an audible click when this happens so you can hear if the compressor clutch is working. If there is no click it is usually that either one of the switches mentioned above has come into action and is disabling the clutch, or there is a possibility that the clutch has seized. Obviously, the clutch seizing off is better than the clutch seizing on but it can happen both ways. You don’t usually hear the clutch disengage as the click is nowhere near as audible.

Sometimes you will hear the clutch clicking and then clicking again a few seconds later and then again and then again. This could be for a number of reasons. One reason could be that the pressure in the system is low so the low pressure switch is turning the system off, but then the pressure builds up so it is turned on again. The pressure then drops again so the low pressure switch turns it off again and so on. There are also systems that incorporate a cyclic system which turns the compressor on and off to regulate the pressure flow through the system (maintained by the high pressure switch).

Compressor Oils

For the compressor to work efficiently, it needs oil to keep it lubricated. The old R12 system used to use mineral oil which was pretty stable. Mineral oil is both non-toxic and virtually non-hygroscopic (does not absorb water). Unfortunately, mineral oil will not mix with R134A refrigerant so could not be used. Instead synthetic oil had to be developed which was called PAG oil (Polyalkeleneglycol oil). This, unfortunately, has some distinct disadvantages. First off it is toxic and can be absorbed through human skin. Although a little bit wont really hurt you, if you do get it on your skin you should wash it with soap and water immediately. If you do begin to feel sick then seek medical advice. PAG oil is also very hygroscopic. It absorbs water fast. A bottle left open to the air for even as little as a day could become unusable. PAG oils come in three main types (viscosity). PAG46, PAG100 and PAG150. Before using the oil, you should check which flavour the compressor on the car is designed for. There is universal PAG oil available that is supposed to replace all three and is really PAG68 but I have always stuck to using the recommended oil.

Each system will have a recommended amount of oil (usually around the 200ml mark. Most of the newer Rovers are around the 135ml mark). It is important to make sure the amount of oil in the system is as correct as possible. Too little oil could cause damage to the compressor over a period of time and too much oil will reduce the efficiency of the air conditioning system. Having said this, if you have to guess, it is always better to slightly overfill the system with oil. The oil gets distributed around the system and there will be some oil in all the components although the majority of oil will remain in the compressor. Some of the oil will be absorbed by the receiver drier/accumulator so it is always difficult to accurately put oil in after major servicing. Some compressors have a sump to hold the oil; other compressors do not have a sump and just retain the oil in their moving parts. There is a rule of thumb guide as to how much oil should be added when replacing a component but I am not going to list them here as different manufactures can give different figures. It really is guesswork when it comes to adding oil.

When doing major servicing work, it is often better to drain the compressor, flush the whole system using special solvents so there is no oil at all in the system, and then add the correct full amount of oil. It only needs to be added to the compressor because that will distribute unused oil around the system.

More about refrigerants

R12 cannot be used anymore. This presents a problem to anyone that has an older car as if it was built before 1995 it is very likely to have R12 in it. Engineers are only allowed to recover R12 from the car and dispose of it, we are not allowed to refill with R12. Ok, why do we not just put R134A in there? Unfortunately, the two refrigerants behave slightly differently. For a start, the oils are different. R12 uses mineral oil and R134A uses PAG oil. Also, R134A has much smaller molecules (going back to chemistry class now) so standard R12 hoses could well let R134A refrigerant easily leak through. Various companies have come up with products to solve this problem that work but even after this many years they are still not 100% sure that it works fully. They don’t know what long term damage it may cause for example. To change over, the whole system has to be flushed with solvent (after removing the refrigerant of course) and then oil has to added (PAG compatible of course) and usually the products have other additives to help seal the hoses and o-rings. The receiver drier/accumulator also has to be replaced and then the system can be filled with R134A. If other components are being replaced then they would be replaced with their R134A compatible counterparts. Finally the valves that are used to fill and empty the system have to be replaced as R12 valves are different to R134A valves. The valves also have to be changed so that any future work uses the correct equipment for the type of refrigerant installed (R12 equipment valves will not fit R134A equipment valves).

There are other gases called drop-in gases that can be used in place of R12. You have to be very careful here because some of these gases are not all they seem. Some of these gases use hydrocarbons (like butane) as an additive. In some countries, like the US, this is illegal. In this country it is perfectly legal at present and is seen as an easy way to upgrade an R12 system to R134A. Most specialists don’t like this because when they recover this type of gas from a system it basically contaminates all their equipment. The good engineers will have special analyzers that can detect this and they may refuse to service your system if this kind of gas is detected. Most modern engineers will refuse to deal with R12 systems full stop.

What to expect when you have your air conditioning serviced

There are varying degrees of servicing that will be carried out when you have your air conditioning system looked over. This means there will be varying prices so the cheapest quote is not necessarily the best.

At the very minimum, the system should have the refrigerant taken safely out of the system and then a vacuum pump attached. The vacuum pump should run for at least 30 minutes (this is the very minimum). The reason a vacuum pump is used is to remove air and moisture from the system. So how does this work? Basically, only gas can be sucked out of the system so how do we get moisture out as well. Essentially, we make the water boil so it turns into steam which is a gas and can be sucked out. I hear you saying that at school you were told that water boils at 100 degrees centigrade. This is true but the teacher missed out one vital fact. Water boils at 100 degrees at sea level pressure. A vacuum pump lowers the pressure to well below sea level. In-fact, under vacuum, water can boil at around 4.4 degrees centigrade so it will boil at a low ambient temperature. For this to happen properly, half an hour is considered the minimum amount of time. If you have had major work done, the vacuuming process should be left on for a lot longer (a number of hours in some cases); especially if compressed air has been used for any processes (compressed air is sometimes used with solvents to flush the system). Usually the only gas other than refrigerant that can be used in the system is a gas called OFN (oxygen free nitrogen). This is safe with all the components and will not bring moisture into the system.

Vacuuming is a very important stage of the service because if any moisture is left in the system it can freeze and cause blockages. Also, moisture reacts with refrigerant and turns into acid. This acid can eat things like condensers and evaporators from the inside out. During the vacuum process it is normal from the engineer to turn the machine off and see if the system will hold the vacuum. This is a small test that can identify some leaks.

A good service will involve a lot more. If the system still has refrigerant in it, a full service will involve an electronic test where a special device that sort of sniffs refrigerant is run along all the components and pipe work. This will bleep in various forms if it detects refrigerant coming out of the system. They may even place bubble spray on common leakage point like the pipe joints, valves and condenser to see if they can detect any bubbles (bubble spray is just like soapy water).

Once they have then taken out the refrigerant, they should pump it full of oxygen free nitrogen (OFN). This is pumped in at about four times the systems normal working pressure (about 10bar). Don’t worry, your system can withstand about 30bar before it goes bang. They can then listen for leaks and their gauges will tell them if the pressure is going down. At this pressure, even a small leak will show up. Unfortunately, none of these tests are definitive as sometimes a leak only appears when the system is actually running. The system cannot be running during an OFN pressure test. They will then vacuum the system, refill it with refrigerant and do another electronic leak test. A good re-gas will also add dye to the system during the re-gas which can be seen under ultraviolet light if it leaks out. If they give you the option of adding dye to the system it is always better to have it done. Although it is a little more expensive, it could save a lot of money in the future if the system does start leaking.

It is also recommended that the receiver drier/accumulator is replaced every few years or if there is a major leak. These components absorb moisture and the process cannot be reversed. Once the component is saturated it will stop doing its job with no warning. Remember, moisture in the system can freeze and cause blockages. If there is a leak or the system has been opened up then assume that the receiver drier/accumulator has been destroyed. Most manufactures state that the receiver drier/accumulator can be open to the air for up to 4 hours, but considering the damage and expense it could because it’s not worth the risk. Always fit a new one after a major leak or major repair work.

You hear stories that receiver driers/accumulators can be dried out by putting them in an oven and then reused. This is total rubbish. The components work using a chemical process which is non-reversible. The only fix for a receiver drier/accumulator is to replace it. Some receiver driers/accumulators can be opened up and just the chemical inside replaced, but this is extremely rare now.

What about the DIY cans?

The DIY refill cans have been a real money spinner for the manufacturers. Most air conditioning retailers will happily sell you them but are dead against using them. There is no such thing as a top-up. If the refrigerant level has gone down it means you have air in the system (when the refrigerant leaks out, air leaks in). If you have air in the system it means you also have moisture in the system. Topping up from one of these cans does not fix the problem of moisture and it does not add the correct amount of oil that may have leaked out with the refrigerant. Basically, it may save you a few pounds now but start saving up for the future as it will cost a lot more when components start to fail for an unknown reason.

I hear that air conditioning has to be running regularly.

This is true. If the air conditioning if left off for very extended times, the o-rings and seals can dry out. Also, the oil in the compressor can settle thus when it is next used the is a short time when it is dry-starting. Running the system for 15 minutes every month is the best DIY service you can give the system. You don’t have to have the system on cold to do this, just turn it on so that the components run for a few minutes.

Service ports

Most, not all, car air conditioning systems have 2 valves on them called service ports. These are used by the engineers to remove refrigerant and re-fill. They will also plug gauges (called manifold gauges) into here to see how the system is running. Sometimes these are colour coded red or blue although more often than not there are just black covers fitted to them,

One of these service ports is under high pressure and the other is under low pressure. The high pressure port is the thicker port of the two if you look at the together. These valves are a type of needle valve. If you press the needle in the valve down then refrigerant will spurt out. You shouldn’t really do this as it allows refrigerant to escape into the atmosphere which is illegal. It is also dangerous as it could get on your skin and cause severe frostbite. Under no circumstances should you depress the high pressure port when the system is running. The high pressure bought is under very high pressure and the refrigerant is in liquid form in this section. Liquid form is the most dangerous form to be in when it comes to contacting bodily parts.

Common faults

The biggest single failure in the air conditioning system is the condenser. Condensers are subjected to extreme heat and extreme cold. Also, if there is moisture in the system which turns to acid, these can rot from the inside out. They are also at the front of the vehicle which can get hit by stones from the road and a whole manner of other debris.

Pipe work union joints can fail and leak refrigerant as can the service valves that are used by engineers to recover, monitor and refill the refrigerant.

A good way to detect faults with the above is to spray soapy water on them and run the system (if it will run. It will switch off if the pressure is too low).

Compressors hardly ever fail (it is extremely rare). If they do fail then it is usually something else in the system that caused them to fail. What does fail is the magnetic clutch that is on the compressor. Although this can be replaced, it is often more economical to get a used or reconditioned compressor as the time involved in taking some of them to bits makes it uneconomical.

The drain tube on the evaporator can very easily become blocked. This usually shows up as water inside the car. It’s usually the passenger foot well carpet that shows this problem.

Expansion valves can get blocked or stick open or closed. This can be a big job to replace as on some cars the whole dashboard has to be removed. This will be indicated by the air conditioning system switches on and off as the pressure will not be correct in the system. In the worse case, this can cause the compressor to fail as if this device is not correct liquid could be presented the compressor.

The high and lower pressure switches can fail which is usually indicated by the system switching on and off when it is not expected to. Without a set of manifold gauges this is very difficult to determine.

Fortunately, it is rare for the switches or a TXV to fail. The most common failures are usually a leak due to corrosion. This, in-turn, is usually caused by acid in the system caused by moisture. The receiver drier/accumulator can fail and you will not know until a long way down the line when moisture has damaged other components. Nearly all common faults are due to moisture or corrosion.

What are the legal requirements?

Up to about a year ago, anyone could service air conditioning with no regulations other than R12 was illegal and it was illegal to vent gas into the atmosphere. Last year on July 4th it was made illegal to recover refrigerant without official recognised qualifications’. Up until July 2009 an in-house certificate is accepted. Places like kwik fit use in-house certificates which are basically someone showing you how to use the machine. As from July 2009 it will be illegal to service or buy refrigerant without official qualifications and anyone with an in-house certificate will have to gain official qualifications to continue servicing. This is going to make things awkward for places like kwik fit and more expensive.

A lot of people think that the new law will prevent the sale of DIY cans. Some people are pleased that these could be banned and other people are not so pleased. Unfortunately or fortunately depending on how you look at it this will not be the case. Although it will be illegal to sell refrigerant to unqualified people after July 2009 this will only apply to quantities above 1kg. The DIY cans are substantially less than that so the law is unlikely to apply. Valid qualifications so far are City & Guilds and CITB. There may well be other qualifications added to this list nearer the time.

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