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Air Conditioning - An Idiot's Guide
So you want to switch on the AC? When you push the button it sends a signal to the Powertrain Control Module (PCM) which in turn engages the AC compressor clutch. As the compressor starts to turn it compresses the refrigerant (R134a) which makes it hot and turns it from a gas to a liquid. The hot refrigerant passes through the condenser mounted in front of the radiator. Outside air blows over the condenser and removes heat from the refrigerant, so now we have high pressure, cool refrigerant, condensed to a liquid. This liquid then passes through an expansion valve and is sprayed into the evaporator. This is another radiator-like component mounted inside the HVAC unit in the dash. As the refrigerant expands, it absorbs heat from the air, cooling the air which then enters the cabin. Blower settings on the dashboard simply adjust the speed of the airflow into the cabin. The refrigerant is now hot and gaseous again, so we need to cool it down. It is pumped into the condenser by the compressor and the whole cycle repeats.
What can go wrong?
The AC process is pretty simple, but there are a number of things that have to be working or else you won't get cold air out.
- Because dangerously high pressures can be generated in the system there is a pressure switch that will deactivate the AC when it reaches a certain pressure (around 35bar)
- Because the refrigerant also lubricates the compressor, it is important not to run the system when empty. There is a low pressure switch that will deactivate the AC if a certain pressure is not reached (6 or 7 bar)
- Because it takes power to turn the compressor, there is a danger the engine will stall at low revs. When AC is requested, the idle speed is normally increased slightly. If a minimum idle speed is not reached, the AC will be deactivated
- To cool the refrigerant in the condenser you need a certain amount of airflow. To get this the main cooling fan normally comes on when the AC is demanded. If the fan does not come on the AC will be deactivated
Q The little green light is on but I am not getting cold air
A This just indicates that the request for AC is being made. Any of the issues listed above could be stopping the AC working. The light really only indicates that the switch works
Q I get cold air when driving, but when I stop in traffic it gets warm
A Engine idle speed might be too low or the fan may not be working – see above
Q Water is dripping from underneath the car – is this normal?
A Perfectly normal. As the evaporator cools the outside air, it also dries it. The water condenses onto the evaporator and runs into a drip tray. When this is full it overflows onto the ground
Q What is the refrigerant specification
A R134a. This is the only legally allowed refrigerant apart from CO2? that is being developed as an alternative
Q How much refrigerant is in the system
A This should be noted on a label under the bonnet. When the car is under development, it is tested with less and less refrigerant in until the AC system stops working. This amount is then increased to allow for some leakage during the vehicle life, and that is the amount stated on the label – usually 600-750g
Q My AC works but the air isn't very cold
A The efficiency of the AC system is dependant on how much refrigerant is in the loop. Over time this can leak out slightly and you find that even on the lo setting the air isn't getting very cold. Unfortunately the only fix for this is to get the system professionally regassed
Q I've heard that I should run the AC from time to time
A Since the refrigerant is used to lubricate the compressor you should run the system once a month or so for around 15mins to ensure the bearings don't dry out. This also stops any seals drying out which is typically where leaks start. Because the AC system dries the cabin air as well as cooling it, it reduces fogging in winter conditions too.
Q Doesn’t it increase my fuel consumption?
A Yes it does, because you are taking power from the engine to drive the compressor. The power loss depends on the engine and AC system installed, but can typically be up to 5%. I've driven cars where the fuel consumption doesn't seem to change, and others where using the AC increases the fuel consumption by 5-6mpg
Also see: Climate_Control_Self_Diagnosis_Guide