System Description

The heater/ A/C unit is located in the front end of the vehicle. The heater/ A/C unit is used to mix fresh air, cold air (for vehicles fitted with M 573) and hot air in one left and right-hand blower each and to distribute it across air ducts. The use of one left-hand and one right- hand blower is dictated by the horizontally opposed engine design with heat exchangers on the left and right vehicle sides. The hot air is supplied towards the front to the blowers in the heater/ A/C unit across one left and one right-hand heater duct.

The blowers are located on the intake side. The fresh-air inlet of the heater/ A/C unit is fitted with one speed-dependent fresh-air mixing flap and one fresh-air flap controlled by the blower stage setting. Both the left and the right heater ducts in the footwell are fitted with one temperature mixing flap each. With the exception of the bypass air flap, all flaps are controlled by electrical servo motors and linkages.

Hot air treatment
The air is aspirated by the engine cooling fan across the air inlet grille at the engine compartment lid and is then supplied to the heat exchangers by a downstream (electrical) auxiliary blower. The air that is preheated when it passes the cylinder surface is heated further inside the heat exchangers.

To prevent excessive heat buildup in the sill area in the summer period, the hot air is evacuated to the outside across heater flap boxes that are controlled by differential pressure.

To avoid drawing in exhaust fumes when driving in reverse, the temperature mixing flaps are moved to the"max. cold" position when reverse is engaged.

Heater / A/C regulator
The entire electronic regulating system is built into the heater/ A/C regulator. Temperature sensors detect the temperatures of the exiting air flow and of the interior and compare them to the settings. Modifying the position of the temperature mixing flaps allows the interior temperature to be regulated.

In addition, the heater/ A/C regulator houses the following components:
- Slide potentiometer to control the defroster flap and the footwell flaps
- Rotary potentiometer to control the blower speed
- AC switch for switching on the A/C system (M 573 only)
- Max-AC switch (M 573 only)
- Recirculating air switch
- Defroster switch


NOTE: The A/C compressor is not activated until 10 seconds after the A/C system has been switched on.


Function of Max-AC switch
- Switching on the A/C system
- Defroster flap closed
- Footwell flaps closed
- Center nozzle open
- Temperature specification set to max. cold setting
- Blower stage 4

If both AC switches are pressed simultaneously, the Max-AC switch takes priority.

Conditions for switching on the auxiliary blower
With the ignition switched on, the auxiliary blower is switched on and off depending on the setting of the temperature mixing flaps. In the heating mode, blower control is dependent on the blower potentiometer:
1st stage - 0,3 to <2
2nd stage - 2,0 to 4

When the temperature mixing flaps are closed, the system is controlled in accordance with the air temperature at the temperature sensor:
1st stage on: 45 °C
off: 40 °C

2nd stage on: 62 °C
off: 57 °C

When the ignition is off, control is dependent on the air temperature at the temperature sensor.
1st stage on: 75 °C
off: 70 °C