Heater circuit function

P20.00-0387-09

Coolant system with auxiliary heater system, shown in engine 668.940

1 Screw cap (one-stage)

2 Coolant expansion reservoir

3 Radiator

4 Filling ventilation (functional only when screw cap is removed)

5 Coolant thermostat (in engine at operating temperature position)

6 Crankcase

7 Coolant pump

8 Oil-water heat exchanger

9 Electric heating water circulation pump

10 Auxiliary heater

11 Bleed screw

12 Heating system heat exchanger

A Coolant return flow

B Coolant infeed

C Short-circuit mode temperature < 87 °C
coolant flow via engine and passenger compartment heating only

D Mixed-mode temperature > 87 °C < 102 °C
thermostat opens. Commencement of flow through radiator

E Cooling mode temperature > 102 °C
thermostat opened, full flow through radiator

L Air flow

M Engine

Function

The coolant pump (7) pumps cold coolant through the engine (M). The coolant absorbs the engine's heat and becomes heated. The heated coolant is then conducted through the heat exchanger (12). The air passing by at the heat exchanger (12) is heated and conducted into the vehicle interior.

Via the coolant return (A), the coolant returns back to the coolant pump (7) again.

A heating water circulation pump (9) is installed in the return.

This pumps coolant to the auxiliary coolant heater (10), which enables the vehicle interior to become heated more rapidly.

By far the greater percentage of the coolant does not flow through the heater, but from the engine (M) to the coolant thermostat (5). Depending on the water temperature, this conducts the water through the radiator (3) or directly to the coolant pump (7).