Automatic heater control function

General function requirements

 Circuit 15

Automatic heater control in general

The electronic control of the temperature and the air distribution in the interior is performed depending on the selected settings at the automatic heater control and operating unit (N18) and the variables measured by various sensors
(e.g. outside temperature, coolant temperature, vehicle speed).

The determination of these variables is described following the description of the adjustments that are relevant for regulation.

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The control and operating unit is networked via the interior CAN and the AC bus with the vehicle electronics system. This makes it possible to incorporate relevant influences in order to improve the control quality.

The automatic heater control is made up of the following subfunctions:

 Function sequence, adjustments that are relevant for regulation

 Function sequence, determining variables

 AC component actuation function sequence

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Function sequence, adjustments that are relevant for regulation

The function sequence for adjustments that are relevant for regulation comprises the following subfunctions:

 Regulation function sequence

 Function sequence of defroster function

Additional regulation function requirements

 Air distribution and air volume manually changed

Regulation function sequence

The regulation of temperature, air distribution, and air volume takes place depending on the temperature selections made on the control and operating unit and the currently existing basic conditions.

In the process, the control and operating unit takes the following variables into account:

- Ambient temperature

- Temperature downstream of heater heat exchanger

- Vehicle speed

- Coolant temperature

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- Engine speed

- Door status

- Position of side windows

- Vario roof status

Additional function requirements, defroster function

 Defroster button pressed (light emitting diode lights up)

Function sequence of defroster function

When the defroster button is pressed, all air flaps are closed with the exception of the defroster flaps and the fresh air/recirculation flap. These are fully opened.

The air flaps for the defroster and side air vents are set so that the maximum air flow is directed toward the windshield and the front side windows.

The blower motor (A32m1) is actuated by the control and operating unit via the blower regulator (A32n1) to ensure the best possible defrost mode dependent on the temperature of the air flowing out.

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Function sequence, determining variables

The function sequence for determining variables comprises the following subfunctions:

 Function sequence, determining the outside temperature

 Function sequence for determining temperature downstream of the heater heat exchanger

 Function sequence, determining the coolant temperature

 Function sequence, determining the vehicle speed

 Function sequence, determining the engine speed

 Function sequence for determining door status

 Function sequence for determining position of side windows

 Function sequence for determining Vario roof status

Function sequence, determining the outside temperature

The outside temperature indicator temperature sensor (B14) measures the outside temperature. The temperature value is read in by the driver SAM control unit with fuse and relay module (N10/1) and transmitted via the interior CAN to the control and operating unit.

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Function sequence for determining temperature downstream of the heater heat exchanger

The left heat exchanger temperature sensor (B10/2) and right heat exchanger temperature sensor (B10/3) record the actual temperature of the air flow downstream of the heater heat exchanger.

The heat exchanger temperature sensors are read in by the control and operating unit.

A comparison takes place here between the measured temperatures (actual value) and preselected specified temperatures (specified value). The duo valve (Y21) opening time is calculated using the comparison of actual and specified value and is then actuated accordingly.

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Function sequence, determining the coolant temperature

The temperature of the coolant is measured by the coolant temperature sensor (B11/4). The temperature value is read in by the ME-SFI [ME] control unit (N3/10) and transmitted via the engine compartment CAN to the central gateway control unit (N93). It transmits the value to the control and operating unit via interior CAN.
This value is used to control the blend air flaps and to actuate the heater circulation pump (M13).

Function sequence, determining the vehicle speed

The vehicle speed is transmitted by the instrument cluster (A1) to the control and operating unit via interior CAN. The vehicle speed is used for the internal calculation of the outside temperature, e.g. during operation while standing.

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Function sequence, determining the engine speed

The engine speed is recorded by the ME-SFI [ME] control unit and transmitted to the central gateway control unit via engine compartment CAN. It transmits the value to the control and operating unit via interior CAN.

Function sequence for determining door status

The open and closed state is recorded at the respective door by the left door contact switch (S17/3) and right door contact switch (S17/4). The status of the door contact switch is read in by the rear SAM control unit with fuse and relay module (N10/2) and transmitted via the interior CAN to the control and operating unit.

The control and operating unit retains the current blower output when the door is open in order to prevent unnecessary readjustment in the event of high temperature differences between the inside and outside air.

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Function sequence for determining position of side windows

The window position is read in by the left door control unit (N69/1) and the right door control unit (N69/2) and sent via the interior CAN to the control and operating unit.

Function sequence for determining Vario roof status

The soft top mechanism control unit (N52) transmits the Vario roof status via the interior CAN to the control and operating unit.

AC component actuation function sequence

The AC component actuation function sequence comprises the following subfunctions:

 Function sequence for actuation of the blower motor

 Function sequence, actuation of the air ducting flaps

Function sequence for actuation of the blower motor

Ventilation is achieved with the aid of the blower motor. The control and operating unit transmits a signal from 0 to 100 % via the AC bus to the blower regulator. Depending on the control voltage, the blower regulator regulates the operating voltage for the blower motor from U = 0 V up to U = 12 V and thus the rotational speed of the blower motor from 0 up to 100 %.

Air ducting flaps actuation function sequence

The control and operating unit actuates the following components for adjustment of the air ducting flaps via the AC bus:

- For fresh air/recirculated air control:

- Fresh air/air recirculation flap actuator motor (M2/5)

- for ventilation control:

- Left and right footwell flaps actuator motor (M2/15)

- Left and right defroster flaps actuator motor (M2/16)