Regulation of Automatic air conditioning, function

Function requirements, general

 Circuit 15 ON

 Automatic air conditioning switched on

 Engine running (for cooling operation)

 Refrigerant compressor (A9) (without code (B09) Refrigerant compressor with magnetic clutch) or refrigerant compressor with magnetic clutch (A9/7) (with code (B09) Refrigerant compressor with magnetic clutch) switched on (for cooling operation)

 The status of circuit 15 is transmitted by the electronic ignition lock control unit (N73) via the interior CAN (CAN B).

The "Engine running" signal is provided by the CDI control unit (N3/9) (with diesel engine) or the ME-SFI [ME] control unit (N3/10) (with gasoline engine) via the chassis CAN (CAN E) and forwarded by the front SAM control unit with fuse and relay module (N10/1) via the interior CAN to the automatic air conditioning control and operating unit (N22/7).

Automatic air conditioning regulation, general

Electronic control of the temperature and air distribution in the interior is performed depending on the selected settings at the automatic air conditioning control and operating unit and the variables recorded by various sensors (e.g. interior temperature, outside temperature, coolant temperature, vehicle speed).

The AAC control and operating unit is networked with the vehicle electronics system over the interior CAN, the climate control LIN (LIN B8) and direct lines. This in turn makes it possible to incorporate relevant parameters in order to improve regulation quality and to lower power consumption.

 If settings are altered on the automatic air conditioning control and operating unit that affect the temperature, the air distribution or the air flow, then the relevant adjustment values are shown briefly in the audio/COMAND display (A40/8).

The automatic air conditioning control function encompasses the following subfunctions:

 Function sequence for recording and evaluating variables

 Regulation function sequence

 Function sequence for AC component actuation

 Function sequence for evaporator drying process

 Function sequence for defroster function

 Function sequence for maximum cooling output (with code (494) USA version)

 Function sequence for shutoff of the air conditioning in the event of a crash (as of 1.6.13)

Function sequence for recording and evaluating variables

The following variables are recorded and evaluated:

 Ambient temperature

 Interior temperature

 Coolant temperature

 Solar radiation

 Vehicle speed

 Temperature on evaporator

 Refrigerant pressure

 Engine speed

 Air quality (with code (581) Comfort automatic air conditioning)

 Dew point temperature (with code (581) Comfort automatic air conditioning, up to 31.5.10)

 Temperature and moisture on windshield (as of 1.12.09)

 Door status

 Side window position

 Sliding roof position (in model 204.0/2 with code (414) Power glass tilting/sliding roof) or panoramic sunroof position (with code (413) Panoramic glass sunroof with top sliding sunroof)

 Settings on rear automatic air conditioning operating unit (N22/4) (with code (581) Comfort automatic air conditioning)

 Status of voice control system (VCS) (with code (511) Audio 50 APS with DVD changer or code (512) COMAND APS with DVD changer or code (527) COMAND APS with single DVD drive (with navigation))

Ambient temperature

The outside temperature is detected by the outside temperature sensor (B14). The outside temperature sensor signals are read in directly by the front SAM control unit. The front SAM control unit sends the temperature value via the interior CAN to the AAC control and operating unit. The automatic air conditioning control and operating unit requires this value to reach the desired interior temperature by adjusting the blend air flaps accordingly. If the vehicle is stationary, the measured temperature can differ considerably from the outside temperature. The outside temperature is therefore only included in the control if the vehicle speed is v  20 km/h. The previously measured temperature is used at speeds of v < 20 km/h and when the ignition is being switched on and off.

Interior temperature

Head area:

 In model 204.0/2 with code (414) Power glass tilting/sliding sunroof, the interior temperature in the head area is recorded by the interior temperature sensor with integrated fan (N70b1). The value is read in by the overhead control panel control unit (N70) and sent via the interior CAN to the AAC control and operating unit.

 In model 204.0/2 without code (414) Power glass tilting/sliding roof and in model 204.9, the interior temperature is recorded by the interior temperature sensor with integrated fan (N70/3b1) in the electronic overhead control panel (N70/3). The value is read in directly by the automatic air conditioning control and operating unit.

Instrument panel:

 The interior temperature in the vicinity of the instrument panel is recorded by the interior temperature sensor (B10/4). The signals of the interior temperature sensor are read in directly by the automatic air conditioning control and operating unit.

 The interior temperature sensors are each equipped with a fan that draws the air in from the vehicle interior for the temperature measurement. Because the interior air is constantly circulated by the fan, a precise temperature measurement is ensured. This enables a temperature control to be achieved that is mainly independent of the interior airflow conditions.

A mean value is calculated from the measurements from the interior temperature sensors. Together with the value of the outside temperature sensor, the mean value is compared to a characteristics map stored in the control and operating unit and used to control the interior temperature.

The mean value of the interior temperature is maintained in the following situations by the automatic air conditioning control and operating unit:

 Side windows open

 Door open

This ensures that the air conditioning system immediately continues operation at the set specified temperature.

Coolant temperature

The coolant temperature is recorded by the coolant temperature sensor (B11/4). The temperature value is read in by the CDI control unit or the ME-SFI [ME] control unit and transmitted over the chassis CAN, the front SAM control unit and the interior CAN to the AAC control and operating unit. The value is used to control the blend air flaps and the coolant circulation pump (M13/5) (with code (581) Comfort automatic air conditioning or code (B03) ECO start/stop function).

The value is also used to control the PTC heater booster (R22/3) (with engine 642.9 up to 31.5.13, 646.8, 651.9 or with engine 274 with code (824) Additional parts for cold countries).

Solar radiation

The angle of incidence and the intensity of the solar radiation on the vehicle are recorded by the sun sensor (B32/3). The signals of the sun sensor are read in directly by the AAC control and operating unit. The interior temperature and blower speed are adjusted according to the measurements. In the event of one-sided solar radiation, an adequate temperature difference is set between the left and the right regulated specified temperatures.

Vehicle speed

The vehicle speed is transmitted by the instrument cluster (A1) to the AAC control and operating unit over the interior CAN. The vehicle speed is used to control the ram air function and for internal calculation of the outside temperature.

Temperature on evaporator

To prevent any icing-up of the evaporator, the temperature there is recorded by the evaporator temperature sensor (B10/6) and read in directly by the AAC control and operating unit. If the evaporator temperature drops to approx. T = 2°C, the refrigerant compressor is switched off.

Refrigerant pressure

The refrigerant pressure is recorded by the refrigerant pressure sensor (B12). The value of the refrigerant pressure sensor is read in directly by the front SAM control unit. The front SAM control unit sends the value read in via the interior CAN to the AAC control and operating unit. The pressure is compared with a stored temperature/pressure curve. At a refrigerant pressure p < 1.7 bar and p > 30 bar, the refrigerant compressor is switched off.

Engine speed

The engine speed is recorded by the CDI control unit or the ME-SFI [ME] control unit and transmitted via the chassis CAN, the front SAM control unit and the interior CAN to the AAC control and operating unit. The engine speed is used among others to regulate the refrigerant compressor.

Air quality (with code (581) Comfort automatic air conditioning)

The pollutant sensor (B31) records the following environmental influences:

 Carbon monoxide (CO)

 Nitrogen oxides (NOx)

 Outside dew point temperature

 Relative humidity

 Ambient temperature

The values are read in directly by the AAC control and operating unit via the climate control LIN. The AAC control and operating unit controls the automatic recirculation in accordance with the read-in values.

Dew point temperature (with code (581) Comfort automatic air conditioning, up to 31.5.10)

The dew point sensor (B11/12) uses the ambient temperature and the air humidity to calculate the dew point and transmits it to the AAC control and operating unit over the climate control LIN. When the dew point is reached, the windows and the outside mirrors start to fog up. To prevent this, the air conditioning, the heated outside mirror and the heated rear window are activated.

Temperature and humidity at the windshield (as of 1.12.09 and with code (B03) ECO start/stop function)

The interior humidity/temperature sensor (B82) measures the relative humidity and the temperature directly at the inside of the windshield and uses this to determine the dew point temperature. The interior humidity/temperature sensor signals are read in by the AAC control and operating unit over the climate control LIN. If the windows threaten to fog up, appropriate countermeasures are taken such as switching on the A/C compressor or starting the engine (with code (B03) ECO start/stop function).

Door status

The left front door rotary tumbler switch (A85s1) in the left front door lock (A85), the right door rotary tumbler switch (A85/1s1) in the right front door lock (A85/1), the left rear door rotary tumbler switch (A85/3s1) in the left rear door lock (A85/3) (model 204.0/2/9) and the right rear door rotary tumbler switch (A85/4s1) in the right rear door lock (A85/4) (model 204.0/2/9) record the states of open or closed at each of the doors. The status of the left front door rotary tumbler switch is read in by the left front door control unit (N69/1) and the status of the right front door rotary tumbler switch is read in by the right front door control unit (N69/2). The status of the rear door rotary tumbler switches (model 204.0/2/9) is read in by the rear SAM control unit with fuse and relay module (N10/2). The status of each door is sent via the interior CAN to the AAC control and operating unit. The AAC control and operating unit retains the current blower setting when the door is open in order to prevent unnecessary readjustment in the event of any high temperature differences between the inside and outside air.

Side window position

Each side window position is read in by the left front door control unit, the right front door control unit, the left rear door control unit (N69/3) (model 204.0/2/9) or the right rear door control unit (N69/4) (model 204.0/2/9) and transmitted over the interior CAN to the AAC control and operating unit.

Sliding roof position (in model 204.0/2 with code (414) Power glass tilting/sliding roof) or panoramic sunroof position (with code (413) Panoramic glass sunroof with top sliding sunroof)

In model 204.0/2 with code (414) Electric glass tilting/sliding roof, the position of the tilting/sliding roof is read in by the overhead control panel control unit and transmitted over the interior CAN to the AAC control and operating unit. With code (413) Panoramic glass sunroof with top sliding sunroof, the position of the panoramic glass sunroof is read in by the panoramic sliding sunroof control unit (A98) and sent over the interior CAN to the AAC control and operating unit.

Settings on rear automatic air conditioning operating unit (with code (581) Comfort automatic air conditioning)

The rear climate control functions can be set via the rear automatic air conditioning control unit.

Operating the rockers for temperature and blower setting in the rear automatic air conditioning operating unit is read in and processed over the climate control LIN by the AAC control and operating unit. This data is used to actuate or regulate the air volume of the rear vents through the rear blower motor (M2/1) and the rear temperature by the rear blend air flap actuator motor (M2/35).

Status of voice control system (VCS) (with code (511) Audio 50 APS with DVD changer or code (512) COMAND APS with DVD changer or code (527) COMAND APS with single DVD drive (with navigation))

With VCS [SBS] active, the blower output is limited. To this end, the radio with auto pilot system (A2/56) (with code (511) Audio 50 APS with DVD changer) or the COMAND controller unit (A40/3) (with code (512) COMAND APS with DVD changer or with code (527) COMAND APS with single DVD drive (with navigation)) sends the VCS status over the interior CAN to the AAC control and operating unit. The AAC control and operating unit forwards the request directly to the blower regulator (A32n1), which then actuates the output limitation of the blower motor (A32m1).

Regulation function sequence

The temperature, air distribution and air volume are controlled depending on the selected settings at the AAC control and operating unit and with code (581) Comfort automatic air conditioning at the rear automatic air conditioning operating unit and through influential parameters recorded by various sensors, e.g. interior temperature, outside temperature, coolant temperature, vehicle speed).

Function sequence for AC component actuation

The function sequence for actuating the A/C components encompasses the following subfunctions:

 Function sequence for actuation of refrigerant compressor

 Function sequence for actuation of blower motor

 Function sequence for actuation of PTC heater booster (with engine 642.9 up to 31.5.13, 646.8, 651.9 or with engine 274 with code (824) Additional parts for cold countries)

 Function sequence for actuation of the air ducting flaps

 Function sequence for actuation of combustion engine fan motor and air conditioning system with integrated control (M4/7)

 Function sequence for actuation of coolant circulation pump (with code (581) Comfort automatic air conditioning or code (B03) ECO start/stop function)

 Function sequence for actuation of rear blower motor (with code (581) Comfort automatic air conditioning)

Function sequence for actuation of refrigerant compressor

The refrigerant compressor is responsible for induction and compression of the refrigerant. The refrigerant compressor output is regulated continuously from 0 to 100 % by the automatic air conditioning control and operating unit through the refrigerant compressor control valve. The AAC control and operating unit sends the actuation request over the interior CAN to the front SAM control unit. This actuates the A/C compressor regulation valve.

With code (B09) Refrigerant compressor with magnetic clutch, a magnetic clutch is also installed. This can be used to completely disconnect the A/C compressor from the belt drive power flow. This in turn enables additional fuel savings to be made. The automatic air conditioning control and operating unit transmits the request via the interior CAN to the front SAM control unit, which directly actuates the A/C compressor with magnetic clutch.

The AAC control and operating unit also sends the request for an idle speed increase over the interior CAN, the SAM control unit and the chassis CAN to the ME-SFI [ME] control unit or the CDI control unit, which increases the idle speed in accordance with the additional load when the refrigerant compressor was switched on.

 In vehicles as of 1.6.13 the refrigerant compressor is shut off over the refrigerant compressor regulation valve or over the refrigerant compressor magnetic clutch (with code (B09) Refrigerant compressor with magnetic clutch) after an accident.

Function sequence for actuation of blower motor

Ventilation is achieved with the aid of the blower motor. The AAC control and operating unit forwards the request directly to the blower regulator, which then actuates the blower motor. With code (581) Comfort automatic air conditioning, the ventilation of the rear passenger compartment is also supported by the rear blower motor. It is actuated via climate control LIN by the automatic air conditioning control and operating unit.

 With VCS [SBS] active, the blower output is limited.

 The blower output is limited by the AAC control and operating unit on vehicles with code (B03) ECO start/stop function when the engine stops. To this end, the CDI control unit or the ME-SFI [ME] control unit sends the engine stopped status over the chassis CAN, front SAM control unit and the interior CAN to the automatic air conditioning control and operating unit.

 In vehicles as of 1.6.13, the blower motor is shut off after an accident.

Function sequence for actuation of PTC heater booster (with engine 642.9 up to 31.5.13, 646.8, 651.9 or with engine 274 with code (824) Additional parts for cold countries)

If necessary (e.g. in the event of a cold start) the PTC heater booster is activated over the climate control LIN by the AAC control and operating unit.

 In vehicles as of 1.6.13 the PTC heater booster is switched off after an accident.

Function sequence for actuation of the air ducting flaps

The air distribution in the vehicle interior is controlled by corresponding adjustment of the air duct flaps in the air conditioning housing.

The automatic air conditioning control and operating unit actuates the following components over the climate control LIN:

Vehicles without code (581) Comfort automatic air conditioning:

 Fresh air/recirculated air flap via the fresh air/recirculated air flap actuator motor (M2/5) (fresh air/recirculated air control)

 Left blend air flap via left blend air flap actuator motor (M2/6) (temperature control)

 Right blend air flap via right blend air flap actuator motor (M2/7) (temperature control)

 Defroster flap via the defroster flap actuator motor (M2/16) (ventilation control)

 Air distribution flap via the air distribution flap actuator motor (M16/22) (ventilation control)

Vehicles with code (581) Comfort automatic air conditioning:

 Fresh air/recirculated air flap via the fresh air/recirculated air flap actuator motor (fresh air/recirculated air control)

 Left blend air flap via left blend air flap actuator motor (temperature control)

 Right blend air flap via the right blend air flap actuator motor (temperature control)

 Rear blend air flap over the rear blend air flap actuator motor (temperature control)

 Left defroster flap via the left defroster flap actuator motor (M2/8) (ventilation control)

 Right defroster flap via the right defroster flap actuator motor (M2/9) (ventilation control)

 Left footwell flap via the left footwell flap actuator motor (M2/10) (ventilation control)

 Right footwell flap via the right footwell flap actuator motor (M2/11) (ventilation control)

 Left center air vent flap via the left center air vent flap actuator motor (M2/12) (ventilation control)

 Right center air vent flap via the right center air vent flap actuator motor (M2/13) (ventilation control)

 Diffuser flap via the diffusor flap actuator motor (M2/21) (ventilation control)

 Ram air flap (up to 31.5.10) via the ram air flap actuator motor (M2/34) (ram air control)

 Fresh air/recirculated air flap (as of 1.6.10) via the fresh air/recirculated air flap actuator motor (ram air control)

 In vehicles as of 1.6.13 the fresh air/recirculated air flap is closed after an accident.

Function sequence for actuation of combustion engine fan motor and air conditioning with integrated control

The fan motor is used for cooling the coolant and refrigerant. The AAC control and operating unit calculates the specified fan speed depending on the refrigerant pressure and sends the request over the interior CAN, the front SAM control unit and the chassis CAN to the CDI control unit or the ME-SFI [ME] control unit. The CDI control unit or the ME-SFI [ME] control unit then calculates the target fan speed depending on the engine temperature and compares this with the request from the AAC control and operating unit. The fan motor is then actuated with a pulse width modulation (PWM) signal by the CDI control unit or ME-SFI [ME] control unit.

Function sequence for actuation of coolant circulation pump (with code (581) Comfort automatic air conditioning or code (B03) ECO start/stop function)

The coolant circulation pump is used to support the coolant circuit, e.g. at low engine speeds, when the engine is stopped (with code (B03) ECO start/stop function) or when the residual heat utilization function is used. The automatic air conditioning control and operating unit transmits the request for actuation of the coolant circulation pump via the interior CAN to the front SAM control unit. This then directly actuates the coolant circulation pump.

Function sequence for actuation of rear blower motor (with code (581) Comfort automatic air conditioning)

The rear blower motor is used to provide additional ventilation in the rear compartment in order to achieve faster cooling or heating. The rear blower motor is actuated via the climate control LIN by the AAC control and operating unit.

 In vehicles as of 1.6.13, the rear blower motor is shut off after an accident.

Additional function requirements for evaporator drying process

 Air conditioning was active at least once during the last ignition sequence

 Evaporator temperature T > 3 °C

 Outside temperature T > 20 °C

 Engine Off

 Doors closed

 Kilometer reading > 250 km

Function sequence for evaporator drying process

To counteract evaporator odors in the interior and fogging of windows when starting the air conditioning, an evaporator drying process is performed. A timer is activated in the instrument cluster when the engine is switched off. The "Engine OFF" signal is sent by the CDI control unit or the ME-SFI [ME] control unit via the chassis CAN to the instrument cluster.

After a waiting time of t = 1 h the instrument cluster transmits the request for evaporator drying over the interior CAN to the automatic air conditioning control and operating unit, which then starts evaporator drying. For this purpose, the blower is actuated at the slowest speed for t = 30 min.

The AAC control and operating unit sends the request directly to the blower regulator. The blower regulator actuates the blower motor at the slowest speed.

Function sequence for defroster function

When the "defrost" button is pressed, all air flaps of the air conditioner housing are closed with the exception of the defroster flaps and the fresh air/recirculated air flap. These are fully opened. The defroster flaps are adjusted so that the maximum air flow is directed to the windshield and the front side windows. The blend air flaps are set to full warm air supply. The blower motor is actuated by the automatic air conditioning control and operating unit over the blower regulator to ensure the best possible defrost mode depending on the temperature of the air flowing out.

The refrigerant compressor is activated in order to ensure that the air flowing out is dried. The air is dried and then heated up to the maximum temperature.

 With code (581) Comfort automatic air conditioning, the defroster flaps are adjusted separately for the driver through the left defroster flap actuator motor and for the front passenger through the right defroster flap actuator motor. Without code (581) Comfort automatic air conditioning, the defroster flaps for the driver and front passenger are adjusted by the defroster flap actuator motor.

Function sequence for maximum cooling output (with code (494) USA version)

Pressing the "MAX A/C" button causes the blower output to be raised to 100 % and the temperature set to "maximum cold".

Function sequence for shutoff of the air conditioning in the event of a crash (as of 1.6.13)

To prevent the ingress of noxious gases into the vehicle interior in the event of an accident, the automatic air conditioning control and operating unit starts the following activities:

 The refrigerant compressor regulation valve is switched off without code (B09) Refrigerant compressor with magnetic clutch. The refrigerant compressor no longer circulates the refrigerant.

 The refrigerant compressor magnetic clutch is opened (with code (B09) Refrigerant compressor with magnetic clutch). The refrigerant compressor is no longer driven.

 The fresh air/recirculated air flap is moved to the recirculated air position. Outside air can no longer enter the vehicle interior.

 The blower motors are no longer actuated. Air is not circulated in the vehicle interior.

 The positive temperature coefficient heater booster is switched off.

 The crash signal is sent by the supplemental restraint system control unit (N2/10) over the chassis CAN, front SAM control unit and the interior CAN to the AAC control and operating unit.

 Detailed information on actuation of the individual components in the air conditioning is available in the "Actuating the air conditioning components" subfunction.