E50 Electric Auxiliary Heater, Rear Right

Electric Auxiliary Heater, Rear Right

The rear right electric auxiliary heater is a component of the rear automatic climate control (FKA) with left-right separation. The electric auxiliary heater additionally heats the air flow in the right footwell ventilation duct and supplies it to the vehicle interior. In principle, the electric auxiliary heater works like an electric radiator.

In total, 2 electric auxiliary heaters are installed in the rear (rear left electric auxiliary heater and rear right electric auxiliary heater).

Functional description
The rear- right electric auxiliary heater based on the PTC principle (positive temperature coefficient) consists of an electrically heated heating register with heating elements. The warmer the heating elements become, the higher their resistance. When the electric auxiliary heater is switched on, a high level of current flows. With increasing heat, this level of current falls.

Depending on different signals (e.g. temperature signal from the rear footwell temperature sensor, signals from energy management), a percentage power request for the electric auxiliary heater is generated in the FKA control module and sent by means of a pulse-width modulated signal (PWM signal).







Structure and inner circuit
The heating elements in the electric auxiliary heater are cold conductors. The heating elements are made up of individual ceramic semiconductor resistors.

The electric auxiliary heater is connected to the vehicle network by 2 plug connections.







Assignment of pins at the connection of the power supply for the heating elements







Assignment of pins at the connection of the power supply for the electronic module







Characteristic curve and target values
The resistance of the heating element has a positive temperature coefficient as of a certain temperature T(Rmin), In other words, as the temperature rises, the electrical resistance of the heating element also increases. This limits the maximum current consumption. In the operating range "AB", the heating elements have an almost linear curve. This electrical characteristic of the heating element resistance allows a maximum temperature of approx. 120 °C in the heating cell, which does not constitute a problem for the climate control. This "physical" over-temperature protection is ensured even in the event of a blower failure.

The temperature of the heating element rises quickly when current is applied (maximum operating temperature approx. 120 °C). From about 80 °C, the heating element begins to reduce its current consumption. The heat from the heating element is fed into the heating cell. The airflow generated by the blower flows across the heating cell. This heats up the air-mass flow and feeds it to the vehicle interior.







Observe the following target values for the rear right electric auxiliary heater:







Diagnosis instructions

Failure of the component:
If the rear right electric auxiliary heater fails, the following behaviour is to be expected:
- Fault memory entry in the FKA control module
- Electric auxiliary heater is not activated

No liability can be accepted for printing or other faults. Subject to changes of a technical nature.