Vehicle climate control via the heater unit is insufficient when the engine is cold.

There is also an optional additional heating system which is activated when the engine is first running, depending on the temperature of the engine coolant.

The system consists of an additional heating device, located inside the heater unit: this device contains two resistors (PTC1 - 250 W and PTC2 - 500 W).

The operation of the resistances is controlled by the climate control system control unit which operates either only one of them or both thereby providing gradual heating with a maximum power of 750 W according to a control logic that depends on:

- the outside temperature (below 20°C)

- the temperature of the engine coolant (about 70°C);

- the engine speed (which should be above 700 rpm);

- the battery voltage (which should not drop below 12.6 V).

For more details

  See descriptions 5040 AIR CONDITIONING CASING AND COMPONENTS 

The power supply for one, two or three resistors is operated by means of two relay switches located on the actual heater unit:

- 1. Level: activated by the first relay switch (250 W);

- 2. Level: activated by the second relay switch (500 W);

- 3. Level: activated by both relay switches (750 W)

The power supply line is protected by two fuses located in a dedicated engine compartment fuse box.

The relay J36 controls the engagement circuit for the first additional heater resistance O25; the power supply for the relay switch arrives from the line for fuse F15 of the fuse box B1.

Relay switch J37 controls the engagement circuit for the other additional heater coils O25; the power supply for the relay switch arrives from the line for fuse F82 of the fuse box B1.

The coil for relay switch J36 is energised by a signal for the engagement of the first coil for O25A - from pin 12 of additional heater control unit M75.

The coil for relay switch J37 is energised by a signal for the engagement of the coils for O25B and C - from pin 6 of additional heater control unit M75.

The control unit M75 receives an ignition-operated power supply (INT) at pin 7 from the line for fuse F51 of M001.

Pin 1 of M75 is earthed.

Supplementary heater control unit M75 is connected – from pins 3 and 2 - via the CAN line, to Body Computer M1 and to the other network nodes.

The Body Computer M001 - connector A - receives a direct power supply from the battery through the line protected by maxi fuse F01 of the engine compartment junction unit B001.

The Body Computer M001 receives an ignition-operated (INT) power supply at pin 11 of connector E, whilst the ignition-operated (INT/A) signal reaches pin 1 of connector E.

Pin 9 of connector E of M001 provides the Body Computer with a reference earth.