Air Conditioning Control

Operation
The A/C compressor, evaporator control and the [1][2]auxiliary fan stage 1 are switched on when the air conditioning or recirculated air button is pressed. As of version 002, the A/C compressor is no longer activated automatically when the defrost button is depressed.

Evaporator sensor
The magnetic clutch of the A/C compressor is disengaged by the A/C relay when the evaporator sensor reaches a temperature of < 2°C (36°F). This prevents the evaporator from icing up. At an evaporator sensor temperature of > 3°C (36°F), the control unit switches on the A/C relay once again and the magnetic clutch engages.

Pressure switch
In order to improve the cooling capacity, the medium pressure switch switches on the [1][2]auxiliary fan stage 2 when the coolant pressure increases to above 18 bar. Auxiliary fan stage 2 is switched off again when the pressure drops below 15 bar. The activation circuit of the A/C compressor clutch leads from the output of the A/C relay (working circuit) to the coolant high pressure switch and via the low pressure switch to the A/C compressor clutch. When one of the pressure switches cuts out because there is too little or too much pressure in the coolant circuit (no or too little coolant or too much coolant), the A/C compressor clutch disengages to avoid damage to the compressor.

A/C Control Panel:

A/C Control Panel

Push Buttons
The push buttons are arranged in 4 blocks:

^ Air distribution buttons, driver's side
^ Air distribution buttons, passenger's side
^ Higher-ranking buttons: Defrost, recirculated air, air conditioning
^ Rear window button

The air distribution buttons are mutually interlocked by mechanical means. The AUTO button is set by the control unit when no button is pressed. The defrost program button has priority over the other buttons.

The function LEDs belonging to the relevant buttons are switched on by switching on the IHKA and pressing the relevant button. The function LEDs have a common power supply from the control unit.

Rear window button
In vehicles as of model year 1988, the rear window defogger can only be activated with the engine running (terminal 61).

When the IHKA system is switched on, the rear window button switches on the rear window defogger for 10 minutes (defrost stage). The function LED lights up.

After the defrost time has elapsed, the function LED goes out and the rear window defogger is switched off for 80 seconds and switched on for 40 seconds.

The system is switched over to defrost if the rear window button is depressed during the modulation period. The rear window defogger is switched off by pressing the button once again.

As of version 003, the rear window defogger is switched off automatically after a modulation time of 20 minutes.