Electric Auxiliary Heater

Electric auxiliary heater

The electrical auxiliary heater based on the PTC principle is fitted in the heater/air conditioner. The electrical auxiliary heater directly heats up the air to adjust the temperature of the vehicle interior.

Brief description of components

Heating element
The heating elements in the electrical auxiliary heater are cold conductors. The heating elements consist of individual semiconductor resistors. The diagram below shows their installation location in the vehicle. A number of heating elements form a heating section. The electrical auxiliary heater is fitted directly at the heater/air conditioner.







Index Explanation
1 Heater/air conditioner
2 Electric auxiliary heater

The following graphic shows an example of the characteristic curve of a heating element.







Index Explanation
Down Operating range with linear positive temperature coefficient, PTC
t Temperature
lg R Resistance (logarithmic representation)
T N Room temperature
R min Minimum resistance
T Ref Temperature at start of the linear operating range
R N Resistance at room temperature
T Rmin Temperature at minimum resistance
R Ref Resistance at start of the linear operating range


The resistance of the heating element has a positive temperature coefficient as of a certain temperature TRmin, i.e. with rising temperature, the electrical resistance of the heating element also rises. This limits the maximum current consumption. The operating range features a virtually linear course. These electrical characteristics of the resistors enable a temperature of approx. 120 �C in the area of the heating elements that presents no problem for the heater/air conditioning system. This "physical" over-temperature protection is ensured even in the event of a blower failure. The temperature of the heating element rises rapidly when current is applied. The heating elements can reach a maximum temperature of approx. 180 �C. As of approx. 80 �C, the heating element already starts to lower its current consumption. The heat of the heating element is passed to the heating section. The air-mass flow generated by the blower floes passed the heating section. This heats up the air-mass flow. The warm air is then fed into the vehicle interior.

System functions

Activation of the electrical auxiliary heater
The electrical auxiliary heater is activated by the heater/air conditioning system. The electrical auxiliary heater is supplied with voltage via the junction box. The DDE (Digital Diesel Electronics) controls the electrical auxiliary heater by means of a pulse-modulated signal. The PWM signal from the DDE notifies the electronics of the electrical auxiliary heater regarding the maximum available electrical power output (depending on the resources of the vehicle electrical network). As a general principle, only the surplus power output of the alternator is available to the electrical auxiliary heater. The maximum heat output that can be added is 900 Watts. At maximum heat output, the current consumption is limited to 74 Ampere (�10 %). The electronics of the electrical auxiliary heater automatically regulate to the minimum in each case (request of the IHKA and limitation by the DDE). The 3 heating sections are switched on and off with delayed switching times. This prevents greater current fluctuations in the vehicle electrical network.

Notes for Service department
Observe the following information for service:

General information
The following general data is provided for servicing:

Replacement of the electrical auxiliary heater
The electrical auxiliary heater can be replaced in service (the heater/air conditioner must be completely removed).