Additional heaters are used in diesel vehicles, as they do not provide enough residual heat for the heating of the vehicle. At low outside temperatures, these heaters ensure that the passenger compartment can be heated up quickly.
If the interior temperature has been set to HI, or the heater controls have been switched to the highest setting, the two-zone air conditioning system sends an "electric additional heater ON" request signal to the Generic Electronic Module (GEM) via the MS CAN bus. If a manual air conditioning system is installed, the signal is transmitted via a conventional cable connection.
The heating power of the electric additional heater is in proportion to the pulse-width modulated input signal from the GEM. The total power output of the electric additional heater depends on the input signals from the powertrain control module (PCM) and from the outside air temperature sensor.
The GEM adjusts the heating power of the electric additional heater as a function of the following parameters:
- Coolant temperature between 60 °C and 70 °C.
- Ambient temperature between 0 °C and 10 °C.
- Sufficient generator power (via CAN bus).
Once the signal reaches 10% of switch-on time, the heater begins to warm-up. As the signal switch-on time continues to increase, the heater power of the electric additional heater increases proportionally until maximum power output is achieved at a switch-on time of approx. 95%.
