E25 Electric Auxiliary Heater, Front
Electric Auxiliary Heater
The electric auxiliary heater is a separate component and is installed in the climate control. The electric auxiliary heater consists of 4 heating cells. The heating registers are the heat-exchanger elements in the electric auxiliary heater. Each heating register contains 1 heating element.
The air is heated directly by these heating elements and is then fed through the air vents into the vehicle interior.
In principle, the electric auxiliary heater works like an electric radiator.
The electric auxiliary heater accelerates the heating of the vehicle interior, especially at low ambient temperatures and during the cold-start phase.
Functional description
The electric auxiliary heater is activated by the control module of the integrated automatic heating and air-conditioning system (IHKA control module). 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 IHKA control module and sent on the LIN bus
The maximum electrical power output of the electric auxiliary heater depends on the resources of the vehicle network. As a general principle, the only power that is available to the electric auxiliary heater is the excess power from the alternator.
The maximum heat output available is 1250 watts. At maximum heat output, the current consumption is limited to 100 Amperes. The electronic circuitry in the electric auxiliary heater automatically adjusts to the respective minimum.
The heating elements are switched on and off with delayed switching times. This prevents larger current fluctuations in the vehicle electrical system.
The electric auxiliary heater based on the PTC principle (positive temperature coefficient) consists of electrically heated 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.
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 3 plug connections.
Assignment of pins at the connection of the power supply for the heating elements
Assignment of pins at the connection of the load ground for the heating elements
Assignment of pins at the connection of the LIN bus 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. These electrical characteristics of the resistors enable a maximum temperature of approx. 120 °C in the area of the heating elements that presents no problem for the climate control. This "physical" overtemperature 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. 170 °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 electric auxiliary heater:
Diagnosis instructions
Failure of the component:
If the electric auxiliary heater fails, the following behaviour is to be expected:
- Fault memory entry in the IHKA 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.