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.
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.
To protect the evaporator from icing up, the outlet temperature is monitored by means of a sensor (evaporator sensor) and the compressor is controlled by means of an electromagnetic clutch.
ON (all conditions must be fulfilled) |
OFF (one of the conditions must be fulfilled) |
Timed mode (switching on and off in 20 s clock cycle) |
|---|---|---|
Terminal 15 ON |
Terminal 15 OFF |
Coolant temperature greater than 117 o C and less than 120 o C |
Air flow control wheeldriver out of zero position |
Air flow control wheeldriver in zero position |
|
Air conditioning standby ON (function light ON) |
Air conditioning standby OFF (function light OFF) |
|
Evaporator temperature > 3 o C |
Evaporator temperature < 2 o C (controlled compressor < 1 o C) |
|
Coolant temperature < 117 o C |
Coolant temperature > 120 o C - 3 o C hysteresis |
|
Air conditioning system signal sent from DME (compressor ON) |
Full throttle signal from DME initiates shut-down with respect to time |
|
Delay time 5 s (on reaching engine speed > 600 rpm) elapsed |
Speed dropped below minimum value (400 rpm) |
|
The triple pressure switch opens the power circuit to the electromagnetic clutch when the refrigerant pressure drops below the value set for low pressure or exceeds the value set 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 refrigerant circuit is activated at an evaporator temperature > 3 o C and deactivated at an evaporator temperature < 2 o C for open-loop and at < 1 o C for controlled A/C compressor. 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 electromagnetic clutch is switched on and off at 20 s intervals. If the coolant temperature exceeds 120 o C, the electromagnetic switch is switched off until the temperature drops below 117 o C. The electromagnetic 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 4 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.
Option 1: After switching on the rear window defogger, the blower in maximum setting, low beam or high beam.
Option 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 function is activated by pressing the A/C button. This is followed by idle increase. By way of the output DME_KO, the air conditioning control unit informs the DME that it intends to switch on the compressor. Consequently, the DME implements an interfering variable circuit, i.e. it moves the throttle in the direction "more air" while at the same time increasing the injection volume.
By way of the DME_KOREL signal (compressor relay), the DME sends a feedback message to the air conditioning 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 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. This switching procedure is not necessary if controlled compressors are installed.
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 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.