Sensor for air quality -G238-, function
The sensor for air quality -G238- recognizes pollutants in the recirculating air (mostly gasoline and/or diesel exhaust) and relies on A/C control head -E87- to switch to the recirculating air mode.
The A/C control head receives a signal from the sensor for air quality -G238- from which the type and amount of air pollution is determined. Depending on the outside temperature and the amount of air pollution, the A/C control head then decides
- At an outside temperature greater than approx. +2°C (36°F) the recirculating air mode is switched on when there is only a slight increase in pollutants.
- At an outside temperature of approx. +2°C (36°F) and - 8°C (-18°F) only after a stronger increase of pollutant concentration is there a changeover to the recirculating air mode (compressor simultaneously switches on).
- At an outside temperature less than -8°C (-18°F) a changeover occurs after a severe increase of pollutant concentration, and only for 15 seconds (the compressor is not shut off). If the concentration drops, the A/C switches over to the fresh air mode.
The function "Automatic air recirculation" can be switched off at any time. If the function is active, the compressor switches on when "Automatic air recirculation" is requested and also with an outside temperature less than +2°C (36°F). At a temperature of less than -8°C (-18°F), compressor operation is no longer possible.
To ensure that the A/C does not operate constantly in the recirculating air mode in areas with a consistently higher pollution, the sensor is self-learning (it adjusts its sensitivity to the environmental conditions).
If the pollutant load of the outside air remains relatively high over a longer period of time, the sensor starts a learn program to assimilate to the changed environmental conditions, so that a recirculating air requirement occurs within less than 12 minutes. If numerous load peaks occur in sequence, the A/C can work for a longer time in recirculating air mode.
Note. A specific amount of time is needed for the A/C flaps to change-over. In the event that a sudden increase in pollutants is encountered (e.g. when driving through a diesel exhaust cloud) gas forming pollutants can be taken in with the fresh air into the passenger compartment until the flap can be closed. For this reason, a dust and pollen filter is installed with the charcoal layer. A filter that has become saturated with pollutants is no longer effective and needs to be replaced. See Dust and pollen filters, removing and installing . In order to avoid quick shifting from the recirculating/fresh air flap, a slight increase in pollutants entering with recirculating air will not allow for immediate shifting (the sensor does not send a signal to the sensor for air quality - E87-). The filtering of the charcoal insert in the dust and pollen filter will be sufficient. See DUST AND POLLEN FILTERS, REMOVING AND INSTALLING . In order to avoid quick shifting from the recirculating air/fresh air flap, the requirement from the sensor for "Automatic air recirculation" mode is at least 35 seconds (minimum duration period), especially if the pollutant concentration has been reduced considerably so that recirculating air mode is no longer necessary. If the compressor is shut off (e.g. in "ECON" operating mode) so that the windows do not fog up, the maximum duration of the "Automatic air recirculation" mode from A/C control head -E87- is limited to approx. 15 seconds. In order to defrost windows as quick as possible, recirculating air mode is not possible with A/C control head -E87- in the "defrost" operating mode. Sensor for air quality -G238- requires approx. 30 seconds to be operational after switching the ignition on (warm-up time). During this time there is no requirement from the sensor to A/C control head -E87- for "Automatic air recirculation" mode. Sensor for air quality -G238- contains a highly sensitive electronic component which can be damaged if it comes in direct contact with solvents, fuels or chemicals. Therefore, do not install sensors which have come in contact with such substances.