The rear air conditioning (FKA) with integrated cooler box is special equipment additional to the High versionof 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:
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.
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:
The rear air conditioning unit is designed for the following functions:
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.
Four flaps driven by step motors ensure the air distribution and stratification in the rear air conditioning unit:
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.
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: 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).
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.
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: 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.
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.
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.
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:
MAX AC operation: In the case of MAX AC operation, the flap positions are:
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°C ... 25°C (22°C)
Potentiometer setting range: 180 degree setting angle with 18 positions of 10 degrees
Increment: 22°C: 18 positions, approx. 1.22°C per angle
Equalization: 0.2 seconds
Substitute value: 15°C
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°C ... 25°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.
Fan output correction:
The right/left separation corrects the fan output depending on the following factors:
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:
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°C and 3°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.
Communication between control module and step motors: The two ventilation flaps and the two stratification flaps are adjusted by means of four bipolar step motors. In order to ensure the required torques at the flaps, the step motors are linked to the flap mechanism by reduction gears.
All step motors are connected in parallel to a three-wire flat cable connection. The three wires carry the voltage supply, ground and serial data information. The control commands are sent to the step motors across a serial data flow from the control module of the rear air conditioning.
To make each step motor unique, they are given their own addresses. Each programmed address can only be written in once and then no longer deleted or changed. This means that the step motors are not interchangeable. Each step motor operates as a ”slave”. The step motor ”listens” to all the data on the bus, but only accepts and executes a command when its own address has been detected. Moreover, the telegram must have been transferred without errors. When the step motor executes a command, it generates a status message and returns this as a reply to the control module.
Positioning accuracy: Changes in the vehicle voltage result in different torques for the MUX4 motors. Whereas the motors are activated in lower-noise sinus operation with voltages U BKL30 > 11 V, if the voltages are lower they must be switched to full-step operation.
Changing the activation mode:
U BKL30 < 9.0 V motor stops
9.0 V < U UBKL30 < 10.0 V motor runs with 140 Hz and full-step operation
10.0 V < U BKL30 < 11.0 V motor runs with rated frequency and full-step operation
11.0 V < U BKL30 < 16.0 V motor runs with rated frequency and sinus operation
16.0 V < U BKL30 motor stops (due to power loss at activation IC)
These conditions also apply to the reference run!
Reference run: As none of the step motors is provided with actual position detection, they always move relative to one of the flap end positions (reference points: 0 / 100%). If a control module is replaced or the power supply is interrupted during normal operation, the flaps are forced into an end position. The end position is selected in such a way that the subsequent set point position can be reached by the shortest route possible (route optimization).
Inclusion of the nominal position here achieves optimization of the motor running time: If the flap is to remain open after the reference run, the reference run is also carried out in the ”open” direction. There is route optimization according to the following criteria:
Route optimization during reference run:
The reference run can also be initiated by set values of the diagnostic tester.
Position run: When the car is brought to a stop (with dropping terminal 15), the step motors move the flaps into certain positions. Here, the ventilation flaps are closed and the stratification flaps are set to their middle position.
Setting angle, number of steps and adjusting time:
The rear air conditioning can be operated from the left-hand and right-hand control panels and the button for the rear cooler box (ON / OFF). Partial operation is also possible from the IHKA.
Operation using buttons:
Functional relationships: The functional relationships for terminal 15 ON are:
Activated function |
Button pressed |
New function |
|---|---|---|
MAX AC |
OFF |
OFF |
MAN |
OFF |
OFF |
OFF |
OFF |
MAX AC (if MAX AC was before OFF, deleted by VA) |
OFF |
OFF |
MAN (if MAN was before OFF) |
MAN |
MAX AC |
MAX AC |
OFF |
MAX AC |
MAX AC |
MAX AC |
MAX AC |
MAN |
Operation via rotary actuator:
Influence on OFF and MAX AC functions:
NOTE: ”Manual mode active” is a coding option. The function is currently not coded.
Operation via IHKA: Switching on MAX AC at the control panel of the IHKA switches both control panels of the FKA into the MAX AC mode.
If the MAX AC mode is switched off again at the control panel of the IHKA, the status that was set before MAX AC was switched on is recreated on both sides of the rear air conditioning. However, this sequence only applies if no adjustment was made to the FKA during the MAX AC function. Subsequently, the rear air conditioning can be operated in the normal manner.
Operation via Controller (BM front): The Control Display communicates via the K-CAN SYSTEM data bus with the rear air conditioning and can influence its settings. The following settings are possible using the Controller and displays on the Control Display:
Function LEDs: All button operations are indicated by the activation of function LEDs and symbol and locating light LEDs in the buttons:
So that the LEDs reflect the status of the program as true feedback, all function LEDs are activated by the control module of the FKA.
The brightness of the function LEDs is kept constant by a voltage regulator. The priority of the buttons is set by a program. For priority display, the function indicator lamp of the lower-ranked button is switched off.
Day / night switching of LEDs: The brightness of the function LED is designed for daylight. In order to avoid dazzle during nighttime operation, the brightness is reduced when the lights are on and terminal 58g is active. For brightness control, the LED supply is clocked with a PWM signal. The value for the minimum brightness of the function LED is stored. The symbol LEDs are also dimmed depending in terminal 58g. The status of terminal 58g is passed on to the control system of the rear air conditioning with the K-CAN message ”Dimming”. The light status is transferred by the light module with the telegram ”Lamp status”.
If one of these two modes is activated, the rear air conditioning switches into the OFF mode. This status is retained as long as one of the modes is active (operation is blocked). After termination of the transport mode, the rear air conditioning remains in the OFF status, but it can be operated once again.
These modes are activated/deactivated by means of standard diagnosis telegrams.
For the rear air conditioning, a filling mode can be activated via the diagnosis interface. During activation, the front shut-off valve on the IHKA and the rear shut-off valve on the FKA are opened.
Manual operation of the rear air conditioning is separate on the left-hand and right-hand control panels. As only the right-hand control panel is ”intelligent”, it must be calibrated to the exact limit values of the potentiometer of the left-hand control panel.
Prerequisite for calibration: terminal R active, terminal 15 not active.
Starting the calibration:
Following the start of the calibration, the LEDs of the OFF button and the MAX button of the left-hand control panel flash.
Default values that lie safely above the minimum values or safely below the maximum values are entered in the memory locations for the limit values of the potentiometer of the left-hand control panel.
Calibrating the potentiometers:
NOTE: The order of steps must be followed without fail. If the rotary actuators are not turned up to the limit position, a malfunction can occur (incremental adjustments are no longer registered).
1. Turn both rotary actuators to the left-hand stop (minimum values)
2. Press the MAX AC button: The MAX AC LED stops flashing as acknowledgment that the left-hand limit stops have been stored.
If the measured values are above the values that were entered in each memory location on starting, these values are not accepted and the MAX AC LED continues to flash.
3. Turn both rotary actuators to the right limit position (maximum values).
4. Press the OFF button: The OFF LED stops flashing as acknowledgment that right-hand limit stops have been stored.
If the measured values are below the values that were entered in each memory location on starting, these values are not accepted and the OFF LED continues to flash.
Reference run of the step motors (MUX motors):
The reference run of the step motors can be triggered using the diagnosis tester.
The diagnostic concept consists of two parts: The self-diagnosis has the task of diagnosing faults in hardware components by means of application tasks and making the information resulting from the diagnosis available to a tester via the fault memory management. The interface between the two diagnosis sections is the data gained from the diagnosis.
Self-diagnosis:
The self-diagnosis is activated when terminal 15 is switched on. The following diagnosis conditions must be met before switching on:
The self-diagnosis is active following a system buildup time of t = > 4 seconds.
During normal operation (up to deactivation of terminal 15), a cyclical diagnosis is performed at the inputs at intervals of 4 seconds. In the case of the outputs, only a limited diagnosis is possible, as with various system outputs only the current operating status is checked for faults.
If a sporadically occurring fault is detected, the corresponding diagnosis defect bit is set in the memory of the control module. A fault is entered in the fault memory of the control module.
Fault messages are only generated if the diagnosis conditions have been met.
In the event of a fault, the defective system input is replaced by the relevant substitute value. System outputs are deactivated in the event of a fault. In the case of drivers for two outputs and a status line, the output must remain activated in certain cases, otherwise a fault is also detected for the intact output.
In the subsequent diagnostic cycle (repair attempt every 20 seconds), the fault status of the system ports is determined once again. In the event of a fault, in general a maximum of 15 repair attempts are made. Thereafter, the output is regarded as defective and is initially no longer activated in this operating cycle. The self-diagnosis is only run and another 15 repair attempts for each defective output are only made when terminal 15 has been switched to OFF and then back to ON.
Within 3 seconds of switching off the ignition (terminal 15), the fault entries are transferred from the fault memory into the memory. Coding data is stored immediately.
Following ”Power on reset” (terminal 30), the memory content is transferred into the fault memory of the control module. In the fault memory, the entries (fault location and type of fault) are supplemented and/or updated according to the current fault situation.
Following the last change to the control panel settings, these settings are secured in that they are saved again:
- with the engine running, every 10 seconds
- with the engine stopped, every second