Glossary

Glossary

A/C clutch
This makes the connection between the pulley and the compressor drive shaft.

A/C Compressor Regulator Valve -N280
With an externally regulated compressor, the regulator valve is actuated by the control module to control the pressure on the low-pressure side and thus the evaporator temperature.

A/C Compressor Speed Sensor
A control module uses the sensor pulses (4 per compressor revolution) and the engine speed to calculate the belt slip. If the belt slip exceeds a specified value, the compressor is switched off by the control module via the A/C clutch.

Boiling point
R134a boils at -26.5 degrees C at normal atmospheric pressure (1 bar).

Charge factor
To avoid overfilling of vessels, regulations governing compressed gases specify the number of kilograms of refrigerant with which a vessel may be filled per liter of internal vessel volume. The product of multiplying this "charge factor" by the internal volume is the permissible capacity. For refrigerant the figure is 1.15 kg/ltr.

Combustibility
Refrigerant is non-flammable. In fact, it has a fire-retardant or extinguishing effect.

Compressor
The compressor is driven via a belt or shaft by the vehicle engine. The compressor draws in refrigerant gas from the evaporator, compresses it and conveys it to the condenser.

Condenser
The condenser dissipates heat from the compressed refrigerant gas to the surrounding air.

Critical temperature/critical pressure
The refrigerant R134a remains chemically stable up to a gas pressure of 39.5 bar (corresponding to a temperature of 101 degrees C). Above this temperature, the refrigerant decomposes.

Evaporator
The liquid refrigerant evaporates in the coiled pipes of the evaporator. The heat required for this is extracted from the air flowing past the evaporator fins.

Expansion valve
The expansion valve atomizes the refrigerant flowing in and controls the flow rate in line with the quantity of heat transferred such that gas does not form until it reaches the evaporator outlet.

Fin comb
This is used to straighten bent condenser fins.

Frigen/Freon
Trade names also applying to refrigerants which are not to be used in vehicles.

High-pressure sensor
The high-pressure sensor generates a square-wave signal or data telegram when voltage is applied. This signal changes with the pressure in the system. The downstream control modules (coolant fan control module, Engine Control Module (ECM), air conditioning system control head etc.) use this signal to calculate the pressure in the refrigerant circuit and to actuate the A/C clutch, A/C Compressor Regulator Valve N280, coolant fans and fan motor accordingly.

Hygroscopic
Highly water-absorbing (example: refrigerant oil)

Inspection port
This is found on air conditioning systems converted from R12 to R134a refrigerant. It is not suitable for R134a, as the refrigerant does not entirely mix with the refrigerant oil.

Leak detector
Minor leaks can only be traced by way of an electronic leak detector or by first rendering them visible in UV light, e.g. using a special additive. Start up leak detector in line with relevant operating instructions.

Nitrogen
Nitrogen is used to flush refrigerant circuits.

O-rings
These rings seal the joints between the individual components of the refrigerant circuit.

Pressure gauge
The high-pressure gauge measures the pressure distributed evenly from the compressor outlet via the condenser to the constriction (restrictor, expansion valve) when the air conditioning system is switched on. The low-pressure gauge measures the pressure distributed evenly from the constriction (restrictor, expansion valve) via the evaporator to the inlet of the compressor when the air conditioning system is switched on.

Pressure gauge set for nitrogen
The function of the pressure gauges in this unit is to reduce the nitrogen vapor pressure. The vapor pressure in the nitrogen cylinder is approx. 50 bar at workshop temperature. The set can also be used to check for leaks in the refrigerant circuit.

Pressure relief valve
The valve opens at approx. 38 bar. The valves of certain compressors are equipped with a disc which breaks off when the valve responds.

Pressure switch
This switches the coolant fan to the appropriate speeds and the compressor "off" in the event of loss of refrigerant or excess pressure.

Receiver
The receiver collects the droplets of liquid and conveys them in a continuous stream to the expansion valve, as otherwise the expansion valve would not be able to fill the evaporator properly. Any moisture penetrating into the refrigerant circuit during assembly is trapped by a filter (desiccant bag) in the receiver. The receiver is installed in conjunction with the expansion valve.

Record sheet
This is used to register the amount of refrigerant used.

Refrigerant
Use is made of a substance with a low boiling point. The refrigerant R134a used is tetrafluoroethane. It boils at -26.5 degrees C at normal atmospheric pressure (1 bar).

Refrigerant pressure/temperature sensor
When voltage is applied, the A/C Pressure/temperature Sensor G395 exchanges information via the air conditioner data bus system ("UN-Bus") with the corresponding control module. The relevant control module uses this information to calculate the pressure and temperature in the refrigerant circuit and any faults detected are signalled to the control module.

Repair notes
When working on the refrigerant circuit, always observe the generally valid safety precautions and the pressure vessel regulations.

Reservoir
The reservoir collects the mixture from the evaporator to ensure that the compressor only draws in dry refrigerant in gas form. Any moisture penetrating into the refrigerant circuit during assembly is trapped by a filter (desiccant bag) in the reservoir. The reservoir is always installed in conjunction with a restrictor.

Restrictor
The restrictor forms a constriction. This constriction restricts the flow rate, thus governing the quantity of refrigerant for the evaporator. The restrictor also divides the refrigerant circuit into high and low-pressure sides.

Retrofit procedure
This refers to the conversion of an air conditioning system from refrigerant R12 to R134a.

Schrader valve
These are valves installed in the connections, e.g. for service work in the refrigerant circuit.

Service station (e.g. V.A.G 1885)
The service station makes it possible to extract and clean the refrigerant, separate the refrigerant oil and re-charge the refrigerant circuit.

Solenoid valve
Cut-in or shutoff of the refrigerant flow in the second evaporator is controlled by the solenoid valve.

Solidification point
-101.6 degrees C for R134a at normal atmospheric pressure (1 bar)

tc
tc stands for critical temperature (also known as critical point). Above the critical temperature there is no longer a boundary between liquid and gas. A substance above its critical temperature is always in the gaseous state. At temperatures below the critical temperature, all types of refrigerant in pressure vessels exhibit both a liquid and a gas phase, i.e. there is a layer of gas above the liquid. As long as gas is present in the vessel as well as liquid, the pressure is governed by the ambient temperature.

Temperature switch for evaporator
The temperature switch determines the temperature between the cooling fins of the evaporator to stop the evaporator icing up.

UV leak detection
Refrigerant circuit leaks result in a loss of refrigerant oil together with the refrigerant. This oil generally remains in the vicinity of the leak location. Adding a small quantity of fluorescent fluid to the refrigerant circuit makes this oil visible under UV (ultraviolet) light. The fluid (PAG oil containing an additive which shows up under UV light) is poured into the refrigerant circuit and becomes distributed with the refrigerant oil when the air conditioner is switched on.

UV leak detection lamp
The UV leak detection lamp emits ultraviolet light of a certain wavelength. A specific additive, which can be introduced into the refrigerant circuit, shows up in this light. In the event of a leak, this additive emerges from the refrigerant circuit together with the refrigerant and refrigerant oil and generally remains in the vicinity of the leak location.

Vapor pressure
Vapor pressure is the pressure acting on the surface of a liquid. It is a function of temperature.

Water content
Water destroys the air conditioner as it combines with other impurities at high pressures and temperatures to form acids.