VW Golf manuals

Volkswagen Golf Service & Repair Manual: Function of air quality sensor -G238-

The air quality sensor detects noxious substances in the ambient air (primarily petrol and/or diesel fumes).
The Climatronic control unit -J255- evaluates the signal from the air quality sensor -G238-. The air conditioning system is activated depending on the degree and manner of ambient air pollution.
At an ambient temperature higher than approx. +2 °C, air recirculation is activated even when there is a minimal increase in noxious substances in the ambient air.
At an ambient temperature between approx. +2 °C and approx. –5 °C, air recirculation is not activated until there is a strong increase in noxious substances in the ambient air. The air conditioner compressor is also activated at the same time.
At an ambient temperature less than approx. –5 °C, air recirculation is not activated until there is a strong increase in noxious substances in the outside air, but only for approx. 15 seconds, and the air conditioner compressor is not activated. When the concentration decreases, the air conditioning system is switched back to fresh air mode.
The “automatic recirculation mode” can be switched off at any time. If the function is active, the air conditioner compressor will be switched on even at an ambient temperature below +2 °C when “automatic recirculation mode” is required. However, even with this function, it is not possible to operate the air conditioner compressor at temperatures below approx. –5 °C.
On vehicles with “automatic recirculation mode”, the air conditioner compressor can also be switched on at temperatures down to approx. –5 °C even if the air recirculation mode has been activated manually.
So that the air conditioning system does not operate continually in air recirculation mode in areas with consistently high levels of pollutants, the "intelligent" sensor adapts its sensitivity to the prevailing environmental pollution.
If the level of noxious substances in the ambient air remains relatively high over a long period of time, the intelligent sensor starts to adapt to the change in environmental conditions so that, generally, the demand for recirculated air lasts less than 12 minutes in areas where the ambient air exhibits a constant level of pollution. If a series of pollution peaks occur, the air conditioning system may operate over a longer period of time in air recirculation mode.
A certain amount of time is required for repositioning of the air conditioning system flaps. To prevent noxious substances from entering the passenger compartment while the flaps are closing (e.g. when driving through a cloud of diesel smoke), a dust and pollen filter with an activated charcoal layer is installed. A saturated filter cannot perform this task and should be renewed.
To prevent too frequent operation of the recirculation/fresh air flap, the flap is not actuated immediately if there is a minimal increase in noxious substances in the ambient air (the sensor does not send a request to the Climatronic control unit -J255-). The effect of the activated charcoal filter in the dust and pollen filter is adequate for this.
To prevent the air recirculation/fresh air flap from switching too frequently, the sensor's request for “automatic recirculation” continues for at least 25 seconds (minimum waiting period) even if the air pollutants decrease to a level that no longer requires air recirculation.
If the air conditioner compressor is switched off (AC button is deactivated), the maximum time assigned to the “automatic recirculation mode” is limited to approx. 15 seconds by the Climatronic control unit -J255- so that condensation does not develop on the windows.
To clear condensation from windows as quickly as possible, the Climatronic control unit -J255- does not permit air recirculation when the “defrost function” is activated.
The air quality sensor -G238- requires approx. 30 seconds to become operational once the ignition has been switched on (warm-up time). During this time, the sensor can send no request for “automatic recirculation mode” to the Climatronic control unit -J255-.
The air quality sensor -G238- is a highly sensitive electronic component which could be destroyed by direct contact with solvents, fuels and certain chemical compounds. For this reason, do not install sensors that may have come into contact with these substances.
   
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