Rear Compartment Heating/Air Conditioning (FKA)

Rear Compartment Heating/Air Conditioning (FKA)

Rear automatic heater / air conditioner FKA
The FKA is offered as an optional extra for the IHKA. The FKA regulates the rear air conditioning/heating.

Brief description of components
The following components are described for the rear automatic heater / air conditioner FKA:
- FKA control unit with integrated operating unit
- Blower with blower output stage
- Electric auxiliary heater
- Sensors

Control unit with integrated operating unit
The control unit of the rear automatic heater / air conditioner FKA is fitted in the back of the centre console. The FKA is operated using the operating elements shown in graphic 1. The nominal temperature is set at the operating unit. The control unit regulates the rear air conditioning/heating to the desired temperature. In doing so, the control unit picks up the sensor signals and continuously adapts the adjusting values for delivery temperature and blower output. The seat heating (if fitted) is also operated using buttons in the operating unit.







Blower with blower output stage
The blower creates the necessary air-mass flow. The blower is built into the air duct. The blower output stage is mounted directly at the housing of the blower motor. The blower output stage is activated by the FKA control unit with a pulse-width modulated signal.







Electric auxiliary heater
The electrical auxiliary heater based on the PTC principle (resistor with positive temperature coefficient) is fitted in the air duct. The electrical auxiliary heater directly heats up the air to adjust the temperature of the vehicle interior.
Electrical auxiliary heaters are fitted on the right and left. The heating elements in the electrical auxiliary heater are cold conductors. These heating elements consist of individual ceramic semiconductor resistors.







Sensors
The following sensors are fitted in the rear automatic heater / air conditioner to measure the rear compartment temperatures:
- Rear internal temperature sensor: integrated in the control unit, with internal temperature sensor fan.
- Footwell temperature sensor: 1 sensor fitted at each of the footwell air outlets on the right and left.
- Ventilation temperature sensor: 1 sensor fitted at the ventilation air vents on right and on the left.

System functions
The following system functions are described for the rear automatic heater / air conditioner (FKA):
- Air distribution and air flow control
- Temperature control
- Seat heating
- AUTO function

Air distribution and air flow control
An overview of the FKA is provided in the following graphic.







The air distribution is controlled automatically by the FKA control unit. Here, it is important that all the air vents are opened. A manual setting at the operating unit is not possible. The following graphic provides an overview of the air distribution flaps for the IHKA HIGH with rear automatic heater / air conditioner (4 zones).







As shown above, the air distribution flaps for the rear automatic heater / air conditioner are fitted directly in the IHKA HIGH. The rear centre air vent is shown in the following illustration.







The blower speed can be set using 2 buttons on the operating unit. The set blower speed is shown in the display in a bar gauge. In the automatic mode, the blower speed is set by the control unit.

Rear temperature control
The nominal temperature is set individually for the right-hand and left-hand side using the temperature selection dials. The set nominal temperatures are shown in the display. Depending on the set nominal temperature as well as the measured sensor temperatures, the temperature mixing flaps are adjusted. If required, the electrical auxiliary heater is also switched on.
The temperature is regulated using a master controller. The control operation is based on the set nominal value and the calculated actual value. The actual value is determined from measured values of the internal temperature sensor, the ventilation temperature sensor as well as the footwell temperature sensor. The setpoint value is corrected in the control unit as a function of the outside temperature. In this way, e.g. when it is cold, the nominal value is raised in order achieve a pleasant climate despite the nominal value not having been adjusted.

Seat heating
The seat heating is offered as an optional extra. The seat heating is switched on and off using 2 buttons on the FKA operating unit. Seat heating is fitted on the right-hand and left-hand seats. The seat heating is switched in 3 stages. The status is indicated via function LEDs in the buttons.

AUTO function
The AUTO function is activated by pressing the AUTO button in the left-hand temperature dial. When the AUTO function is activated, the function LED in the button lights up green. Depending on the set nominal temperatures, the control unit automatically regulates the rear air conditioning/heating. If required, the electrical auxiliary heaters are switched on. The blower speed is set automatically.

Notes for Service department

Diagnosis instructions
The flap motors are processed within the framework of diagnosis of the IHKA. More detailed information on this can be found in the functional description for the IHKA.
For diagnosis of the electrical auxiliary heater, refer to the individual functional descriptions of the electrical auxiliary heaters.
No liability can be accepted for printing or other errors. Subject to changes of a technical nature.