DDE 5.0 Electric Auxiliary Heater

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. The DDE control unit uses a square-wave signal to control the output of the auxiliary heater according to the permissible additional alternator load. A variable pulse duty factor (= variable pulse width) of between 5 and 95 % is used to modulate the heat output.

The idling speed is increased by 50 rpm when the auxiliary heater is activated.

Cutout conditions

The auxiliary heater is switched off under the following operating conditions:

• Pulling away
  
• Battery voltage < 12.0 V (ON hysteresis 12.8 V)
  
• Engine speed < 780 rpm (ON hysteresis 790 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 by comparing the intake air temperature measured against a characteristic curve. Characteristic curve data:
  

The auxiliary heater is also switched off when one of the following fault codes is stored:

• 3EE0, 3EE1, Coolant temperature sensor
  
• 3F70, 3F71, Intake-air temperature sensor
  
• 40A1, Alternator load signal
  

Troubleshooting

Control of the auxiliary heater and its temperature are monitored, and the following faults are detected:

• 43F0, Activation short circuit to B+
  
• 43F1, Activation short circuit to B-
  
• 43F2 Activation circuit break
  
• 43F3, Excess temperature output stage
  
• 4AB2, excess temperature in the auxiliary heater NB: If this fault is detected, fault 43F1 is also stored as secondary fault in every case. If both faults have occurred at the same time, this fault is to be ignored.
  

Consequence: the auxiliary heater is switched off resulting in poor heat output at low temperatures.

If the alternator transmits a load signal of < 5 % or > 100 %, the fault 40A1 ”Alternator load signal” is stored.

Consequence: the auxiliary heater is switched off.