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Your Expansion Valve (AC Expansion Valve) worn out? » Read more about common malfunctions • Repair • Replacement manual
Generating coldness by pressure and heat with the expansion valve
Expansion is the main physical process, responsible for generating the desired cooling by the air conditioner. The expansion is made possible by the expansion valve.
Generating coolness through evaporation
The essential principle for generating coolness is the evaporation of a liquid. When the fluid condenses, i.e. converting from a liquid into a gaseous state, it draws temperature from its environment. The physical expression used for this is "cooling by evaporation". In order for a liquid to maintain its aggregate state, two conditions must be met: the air pressure and the temperature must be right. In normal air pressure, water remains liquid. Dripping it on a hotplate causes its immediate evaporation. An air conditioner does not change the coolant's temperature, but its pressure. This is done with help of the compressor. The compressor compresses the gaseous coolant in the air conditioner until it turns liquid. Subsequently, the compressor presses the liquid stream through the expansion vave. This is a small, mechanical component, directly placed in front of the expansion tank, the so-called evaporator. Initially, the liquid heats up strongly. Therefore, the expansion valve is equipped with an opening, letting through the coolant from the engine cooling system. This prevents heat accumulation in the expansion valve. The side on which the liquid coolant is pressed into the expansion valve, is called its high-pressure side. The coolant of the air conditioner flows out on the other side of the expansion valve through a fine nozzle, allowing it to expand. Now the coolant immediately evaporates and generates the desired effect, i.e. drawing temparature from the environment. This side of the expansion valve is called the low-pressure side. A controlled air stream is now flowing through the evaporator, and the cooled air can be directed into the car interior.
Expansion valve defects
An expansion valve can clog or leak. The only thing the driver will notice, is a non-functional air conditioning. Repairs of the air conditioner should exclusively be performed in professional garages. Especially siphoning the coolant and its professional disposal is reserved for special garages with the required equipment. An expansion valve can only be replaced. A repair is not worth while as it is a cheap spare part of ca. 40 EUR (± £35). An expansion valve is a component which is installed simultaneously with other air conditioner components and therefore ageing also occurs simultaneously. It is always worth while to check all other components of the air conditioner profoundly and if necessary replace or repair them. Otherwise you run the risk of having to change the coolant again, costing 60 EUR (± £53) on each occasion. These additional costs can be avoided when you both repair AND check the air conditioner thoroughly.
Critical component compressor
The expansion valve is a durable component on account of its mechanical simplicity. The compressor is considerably more susceptible to damage. It is driven directly by a belt and has a 10 HP impact on the engine. The connection mechanism with belt disc and magnetic clutch is rather delicate. When the air conditioner must be drained due to a defective expansion valve, this is an excellent occasion to perform compressor maintenance. This implies at least the installation of a new drive belt as well as a magnetic clutch. Bearings and the performance & tightness of the compressor should be checked as well. Following an expansion valve repair, this ensures a reliable and lasting air conditioner.