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 only 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 width) between 5 and 95 % thus controlling various heating power stages.
The idle speed is increased to 930 rpm when the auxiliary heater is activated.
The auxiliary heater is switched off under the following operating conditions:
Ambient temperature [°C] |
Coolant temperature threshold [°C] |
-40.0 |
86.0 |
-5.0 |
86.0 |
10.0 |
50.0 |
20.0 |
17.0 |
30.0 |
17.0 |
Important: A fixed coolant temperature threshold of 86 °C applies if no outside temperature sensor is fitted.
The outside temperature sensor is fitted in the following E46 models:
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.
Fault code 1B "electric auxiliary heater" is stored if a defect occurs.
Consequence: Poor heating capacity at low temperatures
If the alternator indicates a load signal of 0 %, fault code 75 "Alternator load signal" is stored.
Consequence: Auxiliary heater is switched off