The electric auxiliary heater heats the coolant with electric heating elements to compensate for the slight heat loss at high degrees of engine efficiency. Depending on the vehicle equipment, the heating request is sent either from the air conditioning system (IHKA) or from the heating control to the DDE control unit.
The heating elements only cut in when there are sufficient electrical power reserves. To determine the available power reserves, the alternator sends a pulse duty factor signal corresponding to the current alternator load to the DDE control unit. Depending on the permitted additional alternator load, the DDE control unit activates the auxiliary heater with a square-wave signal with pulse duty factors (= variable pulse widths) between 5 and 95 %, thus controlling various heating power stages.
The auxiliary heater is also switched off when one of the following fault codes is stored:
The activation function is monitored for short-circuit and open circuit.
Consequence of failure of the auxiliary heater: poor heating capacity at low temperatures