Rear Air Conditioning Part 1

Rear Air Conditioning

The rear air conditioning (FKA) with integrated cooler box is special equipment additional to the High version of the air conditioning system (IHKA). The control and connection of the FKA to the IHKA are designed in such a way that independent operation is possible. The air required for rear climate control is drawn in through a particulate filter (recirculating air cleaner) from the luggage compartment. With the engine running, the rear air conditioning supplies an integrated cooler box (not the special equipment 'electrically operated cooler box') with cold air.

The following functions are available with the rear air conditioning:
- Air distribution, rear of car interior
- Fan, rear of car interior
- Control of the delivery temperature
- Operation

The rear air conditioning can be used to adapt the in-car climate in the rear of the car interior to the needs of the passengers. Separate settings are possible for the left-hand and right-hand sides. Information is exchanged between the vehicle components, the IHKA and the rear air conditioning via the K-CAN SYSTEM data bus.

Brief description of components

Rear air conditioning unit
The rear air conditioning unit is fitted in the front area of the luggage compartment.

The rear air conditioning unit is made up of the following components/functional units:
- Flap box with actuator drive and flaps
- Evaporator with expansion valve
- Shut-off valve for refrigerant circuit
- Connection for refrigerant lines
- Rear cooler box
- Fan motor with fan control
- Recirculating air cleaner
- Temperature sensor
- Wiring harness with connectors
- Air supply channels left / right of the roof air vents
- Roof air vents left / right in the control panels

The rear air conditioning unit is designed for the following functions:
- Intake of required air volume via a recirculating air cleaner from the luggage compartment
- Generation of the air stream by means of a steplessly adjustable fan
- Control and blocking of air volume separately on left and right
- Cooling system of drawn-in air volume
- Control of air temperature (stratification) separate on left and right
- Electronic control of evaporator temperature (frost protection)
- Operating an integrated rear cooler box

The rear air conditioning unit is connected to the refrigerant circuit of the IHKA by refrigerant lines (pressure and suction line) via shut-off valves. The shut-off valves are arranged in front of the evaporators in the pressure line.

There is an outside condensate outlet to draw off the condensation water that comes about when the air at the evaporator cools.

NOTE: During assembly, ensure that the condensate outlet is correctly seated. The servomotor for the parking brake is located directly underneath.

Flap drives
Four flaps driven by step motors ensure the air distribution and stratification in the rear air conditioning unit:
- Ventilation flap for air volume, right
- Ventilation flap for air volume, left
- Stratification flap for stratification (temperature), right
- Stratification flap for stratification (temperature), left

Components of the refrigerant circuit of the rear air conditioning

Evaporator: The temperature control in the evaporator is regulated by the control module of the rear air conditioning. The evaporator temperature controller operates independently of the other control loops with established controlled variables.
Expansion valve: The expansion valve is attached to the evaporator. The expansion valve regulates the amount of liquid refrigerant injected into the evaporator. The liquid refrigerant is metered in such a way that only enough refrigerant as can be fully evaporated is admitted into the evaporator.
Shut-off valves: The front shut-off valve blocks the refrigerant lines (pressure and suction line) to the rear air conditioning behind the evaporator of the IHKA. The rear shut-off valve blocks the refrigerant lines in front of the expansion valve at the evaporator of the rear air conditioning unit.

Rear cooler box
The rear cooler box with interior light is fitted behind the center armrest of the rear seats. To open the rear cooler box, the lid can be folded back. The lid contains a button for switching the rear cooler box on and off.

Fan and recirculating air cleaner
Fan: The fan consists of a fan motor with fan rotor and power output stage with regulator.

Recirculating air cleaner: The recirculating air cleaner with filter box is located below the fan. The particulate filter can be replaced (intervals specified in the maintenance manual).

Control panels, rear air conditioning
The control panels of the rear air conditioning (satellites) are fitted on the left-hand and right-hand side in the headlining. The arrangement and function of the control elements is the same on both control panels. Button operations are indicated by LEDs in the buttons. The buttons are equipped with symbol and locating light LEDs. The control module for the rear air conditioning is fitted in the right-hand control panel.

Control module, rear air conditioning
The signals of the control modules of the FKA regulate the climatization process in the rear of the car interior. Information is exchanged between the vehicle components and the FKA control module via the K-CAN SYSTEM data bus. The rear air conditioning control module has diagnostic capability.

Control Display with Controller
Control Display: Apart from a few exceptions, the Control Display incorporates the controls and displays for the body electronics. The Control Display coordinates the function requests from the system and assigns them to the relevant functions. The status of the rear air conditioning is reported to the Control Display and displayed there.

Controller: The Control Display is operated by means of the Controller located on the front center armrest.

LM Light Module
The LM light module provides the rear air conditioning control module with information on the dimming setting and the light status (e.g. low beam activated). The control module controls the brightness of the function and symbol LEDs in the buttons in accordance with this information.

PM Power Module
The power module monitors the battery charge state and the quiescent current consumption of the car. Information is received and sent via the K-CAN PERIPHERALS data bus.

Main functions

Flaps / Flap kinematics / Flap positions

Programs without terminal 15: For the rear air conditioning, there are currently no fixed programs for positioning of the flaps without terminal 15.

Rear cooler box operation: During rear cooler box operation, the left-hand and right-hand side of the rear air conditioning are OFF. The flap positions are then:
- Ventilation flaps: 0%
- Stratification flaps: 50%

MAX AC operation: In the case of MAX AC operation, the flap positions are:
- Ventilation flaps: 100%
- Stratification flaps: cold

Manual settings of the flaps:
Ventilation flaps: The setting of the ventilation flap depends on the fan ratio of the left-hand and right-hand side. The side with the greatest fan share is opened to 100%. The choking of the ventilation flap on the side with the smaller fan share is determined according to a special characteristic curve. This theoretically determined flap opening still has to be corrected by a unit-dependent flap characteristic curve. This means that the actual flap opening that corresponds to the desired air volume is determined on an individual basis for each side.

Stratification flaps: The opening of the stratification flaps is determined by the control operation to the nominal value on the left and right:
Temperature setting range: 3 degrees C ... 25 degrees C (22 degrees C)
Potentiometer setting range:180 degrees setting angle with 18 positions of 10 degrees
Increment:22 degrees C: 18 positions, approx. 1.22 degrees C per angle
Equalization:0.2 seconds
Substitute value:15 degrees C

Control of the air temperature at the roof air vents
The air temperature is controlled separately on the right-hand and left-hand side. The rotary knob on the control panel can be used to set a nominal value in the range 3 degrees C... 25 degrees C for the air temperature at the roof air vent. The set nominal temperature lies as a proportional voltage value at a summing point in front of a PI regulator to avoid remaining control differences. At the summing point, there is also a proportional voltage value from a temperature sensor arranged in the air stream. A differential value (Y) resulting from the proportional voltage values is corrected by the flap characteristic curve. The ventilation flap and stratification flap are controlled with the corrected differential value. The cold air from the evaporator and the warm air coming from the luggage compartment are distributed according to the position of the ventilation flap. The position of the stratification flap mixes the air. A change in the air temperature caused by the change in flap position is again determined by the temperature sensor arranged in the air stream. The temperature change returns to the summing point as a proportional voltage value and thus influences the control loop.

Air-flow control

Fan output correction:
The right/left separation corrects the fan output depending on the following factors:
- Each fan setting
- Maximum value of both fan settings
- Correction factor

Rear cooler box operation: With the engine running, the rear cooler box is supplied with cooled air by the rear air conditioning. The cold air intake for the rear cooler box is arranged before the ventilation and stratification flaps. The escaping air can thus not be influenced by the temperature control. In order to maintain cool air operation when the rear air conditioning is switched off (left-hand and right-hand side OFF), the ventilation flaps are closed. The cooling air is fed exclusively to the rear cooler box. The fan is operated with a stored setting.

MAX AC function: Switching on the MAX AC function on one side sets the corresponding stratification flap to maximum cooling. At the same time, the fan is set to 100% power. The setting of the ventilation flaps distributes the air stream proportionally on both sides.

Flap-adjustment-dependent fan ramp: Depending on the adjusting time of the flap with the larger range of adjustment, the adjustment speed of the fan is fixed (stored value).

Manual setting: In the case of manual setting, the fan can be set directly using the fan adjuster between minimum and defined maximum values. The application of the potentiometer values of 0 - 100 % to the fan proportions takes place according to a stored characteristic curve. The maximum setting in manual operation does not correspond to the maximum fan output.

Fan reduction dependent on vehicle electrical system voltage: If necessary, priority stages for reducing the fan output are transmitted by consumer deactivation of the power module via the K-CAN bus. For priority stages 4 and 1: fan maximum = 50% of possible fan output

Influence of terminal 50:
- Start sequence: In order to relieve the load on the car battery during the starting sequence, the fan voltage is switched to zero as long as terminal 50 is "logical on".
- Fault handling, terminal 50: A defect in the bus transmission of "terminal 50" could stop the fan. To prevent this, the time of the active status is evaluated.
The status "terminal 50 physically on" is inverted if:
- it is active for longer than 30 seconds
- the engine speed is ≥ 500 rpm,
- the K bus telegram "Engine running" is received.

The previous status is then regarded as invalid. A fault is entered in the fault memory. However, the function is "logically off". The fan deactivation remains inactive until terminal 15 drops. In the case of terminal 15 new start, the fan deactivation becomes active again.

Refrigerant circuit
The refrigerant circuit of the rear air conditioning is connected to the refrigerant circuit of the IHKA. Both refrigerant circuits are supplied by the refrigerant compressor of the IHKA. The refrigerant circuits are separated by two shut-off valves. Both shut-off valves are activated by the rear air conditioning and are de-energized CLOSED.

NOTE: The shut-off valves are only fitted in the case of special equipment 'rear air conditioning'. If an FKA is fitted, the IHKA must be coded to "FKA is fitted". Without coding, the shut-off valves are without function and remain CLOSED. In this case, damage can occur.

The refrigerant temperature of the rear air conditioning is kept constant between 2 degrees C and 3 degrees C by means of a two-point control with alternate clocking of the front and rear shut-off valves. An additional expansion valve is fitted at the evaporator of the rear air conditioning; this has the same function as that at the evaporator of the IHKA.

NOTE: The expansion valves must not be confused or replaced.

The expansion valve on the FKA has a greater refrigerant throughput. This higher refrigerant throughput ensures higher oil circulation. This prevents oil deposits in the refrigerant circuit of the FKA and thus damage to the refrigerant compressor.