Auxiliary Heater, Checking Activation
The components involved with the activation of the auxiliary heater:
The air is supplied with heat energy via the Auxiliary Heater Heating Element -Z35- -2- after it has left the heater core of the heater -1-. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function. The requirement for this is a request from the Climatronic Control Module -J255- to the Engine Control Module -J623-
The activation of the Auxiliary Heater Heating Element -Z35- is switched off when the Stop function is active to protect the Battery -A- on vehicles with the Start/Stop System. As soon as the engine is started again via the Start function, the Auxiliary Heater Heating Element -Z35- is reactivated if there is a request. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function.
Switch-On Criteria for Auxiliary Heater Activation
- The switch-on conditions must be met in the Climatronic Control Module -J255- and in the Engine Control Module -J623-.
The Climatronic Control Module -J255- sends a request to switch on the auxiliary heater to the Engine Control Module -J623- when the following requirements are met:
- The "automatic auxiliary heater" function is not deselected (shown in the instrument cluster center display -2-, selected via the Multimedia System Control Head -E380- -1-). Refer to the Owner's Manual.
- Engine has been running for at least eight seconds and engine speed is greater than 500 RPM.
- Engine temperature is lower than 75 °C (167 °F).
- The calculated exterior temperature is lower than 6 °C (43 °F).
- The Stop function of the Start/Stop System is not active.
- The electrical system voltage is greater than 12.2 volts and no request that forbids the switch-on is sent by Vehicle Electrical System Control Module -J519-.
- There is no error stored in the Climatronic Control Module -J255- -1- that would prevent the Auxiliary Heater Heating Element -Z35- from switching on.
- No error is stored in the Engine Control Module -J623- and the Generator -C- load is less than 30 to 77% (depending on the engine control module version). Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function.
- Based on the settings on the Front A/C Display Control Head 2 -E775- -3-, the Front A/C Display Control Head 3 -E776- -4- and the measured temperatures, the Climatronic Control Module -J255- has calculated that additional heating output is necessary to reach the specified vehicle interior temperature.
- More than 90% of the air (position of the temperature control doors) is directed through the heater core for the heater for the left or right side.
The Engine Control Module -J623- actuates the Auxiliary Heater Heating Element -Z35- via the Low Heat Output Relay -J359- and the High Heat Output Relay -J360- so that the Generator -C- load does not exceed 95%. The next level is only switched on if the generator load is less than 30 to 77%, depending on the engine speed. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function. Refer to the appropriate Wiring Diagram.
Switch-Off Criteria for Auxiliary Heater Activation
The activation is switched off as soon as the "automatic auxiliary heater" function is deselected (shown in the instrument cluster center display -2-), a switch-on requirement in the Climatronic Control Module -J255- or the Engine Control Module -J623- is no longer met, or one of the switch-off criteria is detected.
The Climatronic Control Module -J255- stops the request to switch on the electric auxiliary heater under the following conditions:
- One of the switch-on requirements is no longer met.
- The calculated exterior temperature is higher than approximately 8 °C (46 °F).
- The Generator -C- load is greater than 95%.
- Less than 60% of the air is directed through the heater core for the heater for the left and right side (position of the temperature control doors).
- The Stop function of the Start/Stop System is active.