Air Con/Climate control FAQs

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is a retronaut
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Oct 26, 2006
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What is the difference between 'Air Con' and 'Climate control'?

Air conditioning is where air is passed through an evaporator to reduce its temperature. Also moisture in the air is reduced as it is condensed back into water and that's what drips under your car when you park it.

Climate control is a system that maintains the cabin temperature to whatever you set it at. It uses the air con system, the heating system or a combination of both to keep the temperature constant.

The climate control is only enabled when in 'Auto' mode. When not in Auto mode the output air temperature is fixed at whatever you set it at (as opposed to changing to keep the cabin the same temperature).

What does the ECON button do?

The ECON button will turn off the air con system. The climate control will still work when in auto mode however without the use of conditioned (cold) air, that means that it?ll be harder and sometimes impossible for the climate control alone to reach lower temperatures if the outside temperature is too warm.

When in ECON mode the air con compressor is not being used. The compressor takes its drive from the engine via a belt and therefore takes some power from the engine and uses extra fuel.

Please note that on newer models the ECON button has been replaced with an AC button. It performs the same fuction however the button has to be switched on to enable the air con rather than the other way around. This FAQs talks in terms of ECON for ease of understanding.

Will ECON mode really save me money?

If you can live without cold air then you can use the ECON mode as much as possible. It will save you a small amount of money in fuel costs. The climate control system in Audis actually uses a mixture of cold and warm air to maintain cabin temperature. Even if the temperature is not that hot outside the climate control may still use conditioned air to keep the cabin at the desired temperature. At certain temperatures the unconditioned air taken from outside will keep the cabin at the desired temperatures perfectly well.

How can I save money even when I use the air conditioning?

Keeping the windows closed is the most obvious answer to this question. There is no point making air cold only to let it blow straight out of the window. It is a complete waste of energy and money.

Another method is by selecting the air recirculation mode. The warmer the air passing through the evaporator (the cold matrix that the air in the cabin blows through) the harder the air con system has to work. The end result of this is more load on the compressor and then on the engine, this wastes money in fuel because the engine is working harder than it needs to be. By selecting the re-circulate mode the air passing through the evaporator is already at a lower temperature and the system doesn?t have to work so hard to cool that air again.

The light on the ECON button is on permanently, what does this mean?

This indicates a fault in the air con system. The most likely cause is too little or too much refrigerant in the system. The latter would only really appear shortly after the system has been recharged, albeit incorrently.

Air con systems in cars are designed to take only a certain amount of refrigerant called R134a. You can find the amount your car needs on a plate under the bonnet. It is measured by weight and on an A3 for example the required amount is 525 grams +/- 25 grams.

If this warning light comes on then take it to an air con specialist ASAP.

Please note that if the outside temperature falls below 4 degrees the air con will be automatically disabled. This is to stop a build up of ice on the evaporator.

The air con guy says the pressure is ok but there is still a fault, why is this?

The pressure reading at a service point cannot tell you how much refrigerant is in the system. It can only tell you that there is some in there. To measure the quantity of refrigerant, the system needs to be fully recovered and the weight is measured by scales under the recovery cylinder.

The ECON light isn't on permanently but there still doesn't seem to be any cold air, why?

You can check of the air con is running by checking that the condensor fan is working and that the magnetic clutch on the compressor is pulling in and out.

You will need someone to sit in the car and turn ECON (AC) on and off with the temp setting at low. Under the bonnet you should see the condensor fan at the front of the engine bay start and then go off after a certain time as the button is switched on and off. You should also be able to hear, and perhaps see, the magnetic clutch on the compressor popping in and out.

If these seem to be working fine and you still don't have any cold air then take the car to a specialist.

Can I leave the air con off all the time?

Although air con units are sealed systems they rely on rubber seals at certain joints. These seals need to be kept oiled for them to maintain integrity. The refrigerant in the system contains this oil and as such the entire system needs to be run at least once a month for at least 10 minutes to keep these seals working at their best.

Why does the fan under my bonnet stay on after I have switched off the engine?

If you have been running the air con whilst driving there will still be an amount of refrigerant vapour in the system. This can be dangerous as it is under pressure so the air con system is designed so as much of this vapour is turned back into a liquid. This is done by keeping the condenser fan running to cool the refrigerant back into a liquid.

There is no definitive answer on how long the fan should stay on for after switching off. Typically it should only be a few minutes, but it will be longer if the ambient temperature is very high. The fan should not remain on for more than an hour and this would normally indicate that there is a fault.

If you are unsure whether it is the condenser fan that is running or the main engine cooling fan you can check it yourself.

Turn on the engine
Turn on the air con (i.e. exit ECON mode if it is on)
Identify which fan is running (ensure the engine is cold so the cooling fan is not on)
Get someone in the car to switch the ECON button on and off
You should see this fan switch on and off
This is the condenser fan

Audis also use the condenser fan together with the cooling fan so if both fans are running after switch off that would indicate a cooling fault.

Why can't I control the temperature in both sides of the cabin from the drivers side anymore?

Press and hold the Auto button for a few seconds and this will allow you to control both sides from the drivers switch. It will remain this way until the passenger switch is operated.

What are the main parts of an air con system?



This is the pump of the system and is driven by the engine via a belt and pulley. It takes low pressure vapour from the evaporator, pressurises it and pumps it into the condenser.

The pulley on the compressor is driven permanently by the engine. When the air con is switched on an electro-magnetic clutch is pulled in to connect that drive to the compressor.



This is a radiator type unit that is normally connected to the main vehicle radiator. It receives the vapour from the compressor and then cools it by the use of a permanently running fan. When the refrigerant is cooled enough it will turn into a liquid and give up its heat energy.


Thermal/Thermostatic Expansion Valve (TEV)

The TEV is the key to the whole operation. It allows only a small amount of liquid refrigerant through a nozzle and this action super cools the refrigerant.

There are other methods in use on refrigeration systems (your fridge freezer uses a different method for example) but this is the one used on cars.

The TEV is constantly adjusting flow by the use of a thermal phial attached to the evaporator. This changes the load the compressor is under and therefore is the controlling method for load on the engine.


This is another radiator that is situated under the dash. It receives cooled liquid from the TEV and passes it through its matrix. The car?s cabin air is passed through it by the use of a fan which cools the air and heats the refrigerant turning it back into a vapour ready for the compressor to pump back around the system.


Please PM me if you find any inaccuracies.
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