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Taken from another forum..
Electric Auxiliary Heater DDE 4.0 (E46 only)
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 idle speed is increased to 850 rpm when the auxiliary heater is activated.
Cutout conditions
The auxiliary heater is switched off under the following operating conditions:
•Battery voltage < 12 V (ON hysteresis 12.8 V)
•Engine speed < 710 rpm (ON hysteresis 730 rpm)
•Time after engine start < 10 s
•Coolant temperature
The coolant temperature threshold that must be exceeded for the auxiliary heater to cut out is determined with the measured ambient temperature from a characteristic curve. Characteristic curve data:
Ambient temperature [°C]
Coolant temperature threshold [°C]
-10.0
86.0
-5.0
86.0
10.0
50.0
20.0
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:
•On all models with 6-cylinder engines
•On models with 4-cylinder engines only with on-board computer or air conditioning.
The auxiliary heater is also switched off when one of the following fault codes is stored:
•0115, ”Coolant temperature sensor”
•1A04, ”Electric auxiliary heater”
•1A0E, ”Alternator load signal”
Troubleshooting
The activation function is monitored for short-circuit and open circuit.
As from DDE software version B1 (phase-in 07/2000) the auxiliary heater will be monitored for overheating.
Fault code 1A04 ”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 1A0E ”Alternator load signal” is stored.
Consequence: Auxiliary heater is switched off
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