Seats: Description and Operation
Active Seat Ventilation, Rear
The active seat ventilation in the rear enables individual cooling of the seat surfaces and rear seat backrests. The optional equipment (SA454) "Active seat ventilation, rear" is available for the seats behind the driver and the front passenger.
Active seat ventilation for the rear seats is equipped with temperature control. The power and speed of the fans are thereby controlled according to the fan setting and depending on the temperature in the seat cushion and rear seat backrest. The seat surface and rear seat backrest are controlled separately (function logic same as seat heating).
Four fans are installed in the rear seats, two on the driver's side, rear and two on the passenger's side, rear:
- 1 fan for ventilation of the seat surface (fan for the seat cushion on the driver's and/or passenger's side, rear)
- 1 fan for ventilation of the rear seat back (fan for the backrest on the driver's and/or passenger's side, rear)
Brief component description
The following components are described:
SMFAH and SMBFH: Rear seat module on the driver's side and rear seat module on the front passenger side
The seat module activates the fans in the seat surface and backrest. The seat module supplies voltage to the fans. The seat module controls the functional display of the button for the active seat ventilation (3 function LEDs each above the left button and right button in the control panel for the automatic rear air-conditioning system). The seat module is connected to the body CAN (K-CAN).
The rear seat module on the driver's side and the rear seat module on the front passenger side are identical components. The identification of the rear seat on the driver's side or back seat on the front passenger side is implemented via pin coding.
The seat module is connected to the vehicle electrical system via 8 plug connections.
The rear seat module on the driver's side (SMFAH) or rear seat module on the front passenger side (SMBFH) is an independent control unit.
Button for active seat ventilation
The control panel and control unit together form a single component. The automatic rear air-conditioning system (FKA) control panel / control unit is fitted in the rear side of the centre console.
The button for the active seat ventilation is integrated in the control panel / control unit of the automatic rear air-conditioning system (FKA). The control unit of the automatic rear air-conditioning system processes the button signals for the active seat ventilation.
The first push of the button for active seat heating will switches on ventilation stage 3. Repeated pushing of the button will set the next lower ventilation stage or switch off active seat ventilation. When the button for the active seat ventilation is pushed for more than 1.2 seconds, active seat ventilation will be switched off.
The FKA control unit converts the button signals for the active seat ventilation to digital signals. The FKA control will transmit the signal as a message using the K-CAN (body CAN). The seat module on the driver's side in the rear (SMFAH) and seat module on the front passenger side in the rear (SMBFH) are bus users.
The corresponding seat module will activate the function LEDs via bus signals.
Fan for the ventilation of the seat surface and ventilation of the rear seat backrest
Every fan is connected to a climate-control mat. The air that is drawn in is fed via the climate-control mat of the seat surface and rear seat backrest. The climate-control mats have openings in the area of the seat surface and also in the area of the rear seat backrest. The air entrained by the fans flows through these openings. This cools the seat cover.
The active seat ventilation in the rear can be switched on as of terminal 15 ON using the button for active seat ventilation on the control panel for the automatic rear air-conditioning system.
When the seat module detects undervoltage or overvoltage, it will switch off seat rear cushion fan on the driver's and/or front passenger side.
The following graphic shows the installation location of the fans with example F01.
The fans draw the air out of the rear footwell.
The following graphic shows the installation location of the fans with example F07.
The fan in the backrest draws the air out of the space between the backrest and bulkhead.
CAS: Car Access System
The Car Access System is the master control unit for the terminal control. The Car Access System activates terminal 15N and terminal 30B among others. The power distribution box in the rear supplies the two seat modules (rear driver's seat module and rear passenger seat module) with terminal 30B.
Temperature sensor in the rear seat backrest and in the rear seat cushion
The active seat ventilation, rear, is temperature controlled. The values of the temperature sensors for the seat heating are used for the climate control of the seats. Depending on the fan speed that was selected and the surface temperature that was reached, the fan speed and thereby the power for the climate control for the rear seat are controlled.
NOTICE: Temperature controlled operation of the active seat ventilation with activated seat heating is not possible.
Temperature controlled operation of active seat ventilation is not possible when the seat heating and active seat ventilation are operated at the same time. The fans are operated at a fixed speed depending on the fan setting that is used. Temperature controlled operation of the active seat ventilation is not possible until the seat heating has been switched off (seat heating and active seat ventilation use the same temperature sensors).
System overview
The following graphic shows the system network of the active seat ventilation, rear.
System functions
The following system functions are described:
Activating the fan settings
The active seat ventilation can be switched on from terminal 15 ON. The fans have 3 fan settings in addition to the "OFF" condition:
- Stage 3: Cold
- Stage 2: Medium
- Stage 1: slightly cool
Depending on the ventilation stage that was selected, the seat module, driver's side, rear (SMFAH) and/or seat module, front passenger side, rear (SMBFH) activates the 3 speed stages for the fans via pulse-width modulated signal as follows:
The first push of the button for active seat heating will switches on ventilation stage 3. After an operating period of 10 minutes (encodable, e.g. no back-switching), the system will automatically switch back from ventilation stage 3 to ventilation stage 2.
If the ignition is switched off for a short period of time (terminal 15 "OFF" less than 15 minutes), the current setting for the active seat ventilation will be maintained. This means: If the active seat ventilation was switched on before terminal 15 "OFF", it will automatically switch on again at terminal 15 "ON".
After switching on fan setting 3, the active seat ventilation will run unregulated. This means: The fans will run at the highest speed. After a specified period of time (encodable, e.g. 30 seconds), the fans will run at temperature controlled operation with the speed that is assigned to the corresponding fan setting.
If the fans were active at the next higher fan setting for more than 1 minute when switching back from fan setting 3 to 2 or 2 to 1, the fans will be switched off for a specific period of time. The fans will then continue to run with temperature controlled operation.
When the MAX button (for maximum cooling) is pushed while active seat ventilation is switched on, the seat module will switch the fans up to stage 3 (regardless of the currently selected fan setting). When the air conditioning function for maximum cooling is switched off again, the seat module will switch the fans back to the fan settings that were previously set. The MAX button is integrated in the control panel for the automatic rear air-conditioning system.
During temperature controlled operation, certain temperature values are assigned to the fan settings in the seat module. The seat module collects the temperatures on the seat using the temperature sensors in the seat surface and backrest. The seat module activates the fans according to the stored characteristic curves. For example, the fan in the seat cushion may run at stage 1 and the fan in the backrest may run at stage 3. When the specified nominal temperature values are reached, the seat module will switch the fan settings accordingly.
The temperature controlled operation is not displayed. This means: The functional display (3 function LEDs) always displays the fan setting that was selected manually.
Switch-off by energy management
The energy management in the vehicle can briefly switch off the active seat ventilation in the rear depending on the energy balance. However, the functional display remains active.
The energy management ensures that sufficient starter motor current is always available. Energy management monitors the vehicle even when the engine is at a standstill. The energy management system monitors all components in the vehicle which generate, store and consume energy. The data for energy management is distributed to several control units (e.g. Car Access System, junction box electronics and DME or DDE).
Switch-off with undervoltage or overvoltage
The rear seat module on the driver's side (SMFAH) and/or rear seat module on the front passenger side (SMBFH) recognizes undervoltage and overvoltage. The seat module switches the fans for active seat ventilation off.
Conditions for switch-off:
- Undervoltage: Vehicle voltage less than 9 volts
- Overvoltage: Vehicle voltage greater than 16 volts
Notes for Service department
General note
The SA454 "Active seat ventilation, rear" is only available together with SA460 "Comfort seats rear, electrically adjustable".
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.