Auxiliary Cabin Heater: Description and Operation

TDI Diesel Auxiliary Heater Components and Operation

Because of their outstanding efficiency, TDI diesel engines develop little waste heat. Under certain circumstances, the heating capacity of the engine may not be adequate to provide heat to the passenger compartment. The auxiliary heater consists of three heating elements at the clutch-side coolant connection of the cylinder head. The three heating elements Q7 extend through this connection housing and into the coolant.

The Intake Air Temperature (IAT) Sensor G72 signals the intake air temperature to the DFI ECM J248. If the temperature is below 5°C (41°F) when the engine is started, the DFI ECM J248 uses relays J359 and J360 to activate these heating elements Q7 in the coolant. The DFI ECM J248 stores the temperature level when the engine is started.

One, two, or all three of these heating elements Q7 are energized, depending on the available capacity of the generator and the state of the battery charge. The generator is fitted with a special terminal (terminal DF) to the DFI ECM J248 for this purpose.





The heating elements Q7 are switched off when the coolant has reached a certain temperature. The cut-off temperature depends on the level that the control module stored when the engine was started. The colder the intake manifold was at the time of starting, the higher the temperature must be to switch the auxiliary heater off.