The fuel-fired booster heater provides extra heat to the passenger compartment if the heat given off by the engine is not sufficient (boost heater function).
By burning fuel, the fuel-fired booster heater generates heat which is conveyed via the coolant circuit to the engine and the passenger compartment.
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Heater core with blower motor
Boost heat mode
When the engine is running, the fuel-fired booster heater assists the engine in heating up the passenger compartment at low ambient temperatures, if the vehicle has an engine that gives off only a small amount of heat.
When the coolant temperature reaches 30 °C, the control unit transmits a switch-on signal for the passenger compartment blower via the CAN bus. When the coolant temperature drops, the blower remains on until the temperature reaches 20 °C whereupon it is deactivated.
The fuel tank must be filled to at least 14% for the system to be switched on. If the fuel level drops below 8% then the system is switched off.
In boost mode, the fuel-fired booster heater is only switched on if all of the following criteria are met:
- Engine speed above 500 rev/min. The fuel-fired booster heater is not allowed to start up while the engine is being cranked; this prevents a shut-down due to low voltage if the battery charge is low.
- Ambient temperature below 5 °C
- Fuel level above 14% of total capacity.
- Function is active on the trip computer menu.
Booster heater with additional parking heater function
The booster heater with additional parking heater function has two modes of operation: immediate start-up and programmed start-up.
The heater status is displayed on the instrument cluster display. The parking heater mode is controlled via a menu on the driver information system. The fuel-fired booster heater can be activated and deactivated via the driver information system. (If set to 'Auto' the system is activated, if set to 'Off' the system is completely deactivated.)
Immediate start-up of the booster heater
This function enables the fuel-fired booster heater to be switched on manually when the engine is not running. This function is activated via the menu on the driver information system.
The ignition key must be in the II" position before this menu can be accessed. The timer function of the booster heater remains active when the ignition key is in position "0".
After an immediate start-up of the booster heater it is switched off again after 30 minutes (or if the fuel level in the fuel tank drops below 8%). The booster heater stops within 2 minutes of the engine starting. This leaves enough time to check whether the switch-on conditions for boost heat mode have been met, thus preventing the booster heater from having to switch off and switch back on again. The heater can be switched off manually at any time from the menu.
Programmed start-up of the booster heater
The driver can use a menu to adjust the time at which the vehicle is to be pre-heated. The following options are available:
- Time setting. One or two times can be programmed for each day of the week. It is possible to program days either individually or together in groups (Mon-Sun/Mon-Sat/Mon-Fri).
- Time and data setting
With the first option, the fuel-fired booster heater will start repeatedly without needing to be reprogrammed. However, if the engine hasn't been run since the last programmed start, the fuel-fired booster heater will not start up the second time so as to prevent the battery from being discharged.
The length of time required to pre-heat the vehicle is calculated in the control unit of the fuel-fired booster heater and is based on two temperature values:
- Ambient air temperature: this message is taken from the Generic Electronic Module (GEM) via the CAN bus.
- Coolant temperature: this is determined via an internal sensor in the fuel-fired booster heater.
The maximum heating time is 60 minutes at an outside air temperature of -10 °C or lower. The heating time decreases proportionally with increasing ambient temperature until the ambient temperature is between +15 °C and +20 °C. Then the minimum heating time is 10 minutes. The parking heater is deactivated at temperatures above +20 °C.
After a programmed start-up of the booster heater it is switched off again after the heating time has elapsed (or if the fuel level in the fuel tank drops below 8%). The booster heater stops within 2 minutes of the engine starting. This leaves enough time to check whether the switch-on conditions for boost heat mode have been met, thus preventing the booster heater from having to switch off and switch back on again. The heater can be switched off manually at any time from the menu.
Whilst the fuel-fired booster heater is in additional heating mode and/or parking heating mode, the instrument cluster receives a fuel consumption signal; this is used to re-calculate the vehicle's remaining range and fuel consumption data.
Emergency shutoff
In the event of an accident in which the airbags are deployed, the control unit of the fuel-fired booster heater receives a message on the CAN bus from the restraints control module (RCM). When this message is received, the booster heater system switches off immediately.
The booster heater control module deactivates the system and does not respond to further messages on the CAN bus. The booster heater control module needs to be activated with WDS.
Initial operation
The fuel-fired booster heater needs to be filled before being taken into operation for the first time. The activation of the fuel pump of the fuel-fired booster heater is controlled by the WDS.
