The air conditioning system is switched on by pressing the air conditioning button. The function light signals that the air conditioning system is in standby mode.
The compressor only cuts in if the switch-on conditions are fulfilled. The evaporator then produces cold air which is raised to the required temperature in reheat mode with the aid of the heat exchanger.
To protect the evaporator from icing up, the outlet temperature is monitored by means of a sensor (evaporator temperature sensor) and the compressor is controlled by means of an electromagnetic clutch.
If a mechanically controlled compressor is installed, the evaporator cutout temperature is dependent on the outside temperature. If the outside temperature increases above 20 degrees Celsius the cutout temperature drops to 1 degrees Celsius.
Note
The component coding can be checked via diagnosis "3 Service functions" , "2 Read coding data" . This data indicates what type of compressor (regulated/non-regulated) and interface (PWM signal/conventional) are installed and coded.
1. PWM signal
The control unit output (DME_KO) is used to send the preparation signal (DME_AC) and the switch-on signal (DME_KO) of the controlled compressor to the DME. The individual switching stages are transferred by way of pulse width modulation with a frequency of 5 Hz.
Pulse duty factor in % |
Functions |
|---|---|
0 |
Short to U-batt. or line break |
15 |
Air conditioning off |
30 |
Air conditioning standby (DME_AC) ON |
45 |
A/C compressor ON, stage 1 = 5-10 Nm |
60 |
A/C compressor ON, stage 2 = 10-15 Nm |
75 |
A/C compressor ON, stage 3 = 15-20 Nm |
90 |
A/C compressor ON, stage 4 = 20-24 Nm |
100 |
Short to ground |
The DME sends feedback in the form of the DME_KOREL signal (compressor relay) via a second line to the air conditioner for the purpose of switching on the A/C compressor.
2. Conventional signal
The air conditioning function is activated by pressing the A/C button. As a result, the idle speed is increased via the DME_AC signal on a separate line. With the aid of the output DME_KO on a further line, the air conditioner informs the DME of its intention to switch on the compressor.
The DME adjusts the throttle valve in the direction of "more air" and at the same time increases the fuel injection rate. The DME sends feedback in the form of the DME_KOREL signal (compressor relay) via a further line to the A/C control unit instructing it to switch on the A/C compressor. If all conditions for switching on the A/C compressor are fulfilled, the A/C compressor is activated directly by the control unit.
To avoid excessive water discharge from the evaporator, after the air conditioning system has been switched off while driving, the conventional compressor is switched on and off at set intervals for 15 minutes. In this way, the evaporator temperature is slowly brought to the value of the outside temperature. The cyclic mode does not apply to mechanically controlled compressors.
The pressure switch mounted on the drying container is switched by the refrigerant pressure and opens the power circuit to the magnetic clutch when the pressure drops below the specified low pressure value or exceeds the value specified for high pressure.
Low pressure |
Medium pressure |
High pressure |
|---|---|---|
ON < 2.8 +/- 0.2 bar |
ON > 20.0 +/- 1 bar |
ON > 33.0 +/- 1 bar |
OFF > 1.9 +/- 0.2 bar |
OFF < 17.5 +/- 1 bar |
OFF < 23.5 +/- 1 bar |
The evaporator temperature controller is made up of a two-point controller on the basis of a variable positive switching threshold with 1 o K hysteresis. The refrigeration circuit is activated at an evaporator temperature > 3 o C and deactivated at an evaporator temperature < 2 o C. This function prevents the evaporator from icing up.
The instrument cluster electronics (IKE) features a coolant sensor on the radiator circuit. This analog sensor signal is processed in the IKE and read in by the IHKA via the K-bus.
To avoid the engine overheating, the cooling water is monitored by the air conditioning system. At a coolant temperature above 117 o C the compressor clutch cuts in and out at 20 s intervals. If the coolant temperature exceeds 120 o C, the compressor clutch is switched off until the temperature drops below 117 o C. The compressor clutch then cuts in once again at a rate of 100%.
In order to shorten the rev-up time of the engine from idle up to full load, the electromagnetic clutch is switched off for a limited period of time at full throttle. The cutout time is between 7 and 10 s depending on the type of engine.
Full load condition: Speed signal < 14 km/h and acceleration
To ensure smooth operation is not impaired by the compressor load, the control lines AC and KO are provided for activation of the DME by the air conditioning control unit.
When the air conditioning system is switched on, the air conditioning control unit output AC requests an idle increase from the DME. When the signal is active, the DME increases the idle speed by 110 - 200 rpm irrespective of the electromagnetic clutch.
The idle increase function (anti-stall) is additionally possible by means of coding.
Coding 1: After switching on the rear window defogger, the blower in maximum setting, low beam or high beam.
Coding 2: When undervoltage detected. The idle increase function is activated at terminal 30 < 11.4 V and deactivated at terminal 30 > 12.2 V.
The air conditioning system cuts in stage 1 of the auxiliary fan by means of relays when the compressor clutch is active and the outside temperature is greater than 10 o C. The auxiliary fan cuts out when the compressor clutch has switched off or the outside temperature is less than 8 o C.
For safety reasons, a thermostat switch also causes stage 1 to cut in at a coolant temperature of 91 o C irrespective of the instruction given by the air conditioning control unit.
Stage 2 of the auxiliary fan is not cut in by the air conditioning control unit.
If the cooling water exceeds a value above 99 o C or if the medium pressure switch has closed, stage 2 cuts in by means of a relay.