This group covers automatic heating system component replacement. Refer to the following groups for further information:

• 800 Heating, Ventilation, Air Conditioning-General

• 850 Ventilation

• 870 Air Conditioner

• Some of the cars covered by this manual may be fitted with a manual heating control system and not the optional automatic system (M424 option code). For information on the ducting and ventilation controls for these cars, see 850 Ventilation.

• Automatic heating system troubleshooting is covered in 800 Heating, Ventilation, Air Conditioning-General.

Special tools

Some of the procedures require special tools.

Automatic heating system

The automatic heating system controls the passenger compartment temperature based on the setting of the temperature control knob (arrow). By comparing the passenger compartment temperature to the setting at the temperature control knob, the control module servo motor automatically opens or closes the heater control box valves to maintain the desired temperature. For a description of the automatic heating system, see 800 Heating, Ventilation, Air Conditioning-General.


Heated air is blown through the exhaust system heat exchangers by an engine-mounted electric fan and supplemented by two blower motors in the left and right footwells in the passenger compartment.

Temperature is controlled by automatic regulation of the flaps on the heat exchanger valve housings. Control of the heat exchanger valves is via the servo motor in the control module housing.

Heat exchanger replacement is covered in 260 Exhaust System.

Temperture control module housing

Heat exchanger duct

Footwell blowers

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