Steering wheel heater (LRH), function - GF46.11-P-0001FLM

MODEL 212 as of model year 2014 with CODE 443 (Steering wheel heater) 

Fig 1: Steering Wheel Heater (LRH) Communication Diagram

Courtesy of MERCEDES-BENZ USA

Function requirements, general 

• Circuit 15R ON
• Interior temperature T < 30 °C

The electronic ignition lock control unit sends the status of circuit 15 via chassis CAN 1, the steering column tube module control unit and steering LIN to the steering wheel electronics.

The automatic air conditioning control and operating unit sends the interior temperature via the interior CAN, front SAM control unit, chassis CAN 1, steering column tube module control unit and steering LIN to the steering wheel electronics.

Steering wheel heater, general 

It is possible to heat the steering wheel rim using the steering wheel heater. The steering wheel is heated by an energized heat mat under the surface of the steering wheel rim.

The steering wheel heater function includes the following subfunctions:

• Manual on/off function sequence 
• Function sequence for automatic switch off 

Manual on/off function sequence 

Operation is performed via the steering wheel heater switch in the steering column adjustment and steering wheel heater switch. The steering wheel heater is switched on and off by turning the heated steering wheel heater switch. The steering column tube module control unit reads in the status of the steering wheel heater switch directly and actuates the steering wheel heater heating element directly. The steering column tube module control unit then actuates the steering wheel heater indicator lamp in the steering wheel heater switch directly. The surface temperature of the steering wheel rim is detected by the heating element temperature sensor. The steering wheel electronics reads in the resistance value of the heating element temperature sensor directly, calculates the surface temperature and sends it via the steering LIN to the steering column tube module control unit. The steering column tube module control unit regulates the surface temperature of the steering wheel rim to T = 35 °C.

The specified surface temperature of the steering wheel rim can be adjusted using the diagnostic tester.

Function sequence for automatic switch off 

The steering column tube module control unit switches off the steering wheel heater automatically after t = 15 min has elapsed. The steering column tube module control unit is responsible for timer control.

The steering wheel heater timer control can be parameterized through the diagnosis tester.

Other shutoff criteria:

• Shutoff after circuit 15R OFF:

  With circuit 15R OFF and circuit 15C ON, timer control runs on for t = 5 min. If circuit 15R is switched on again, the steering wheel heater is also switched on again.

• Shutting off in the event of undervoltage:
• In the event of undervoltage of U < 8.5 V, the steering column tube module control unit switches off the steering wheel heater. The steering wheel heater indicator lamp remains switched on initially. If the on-board electrical system voltage becomes stable again within t = 3 min, the steering wheel heater also starts up again automatically. If, however, the shutoff lasts longer than t = 3 min, the steering wheel heater indicator lamp is also switched off. If the steering wheel heater is requested again during the shutoff period, the steering wheel heater indicator lamp lights up for t = 1.5 s and then goes out again.
• Cutting out in the event of overvoltage:

  In the event of overvoltage of U > 16.5 V, the steering column tube module control unit switches off the steering wheel heater. Timer control continues running and the steering wheel heater indicator lamp remains switched on. When the on-board electrical system voltage has stabilized again, the steering wheel heater can be switched on again. The switch-off threshold can be parameterized using the diagnostic tester.

• Switch off during overtemperature.

  At an interior temperature of T > 30 °C, the steering column tube module control unit switches off the steering wheel heater. The steering wheel heater indicator lamp remains switched on. If the interior temperature drops below the switch-off threshold of T = 30 °C, the steering wheel heater is switched on again automatically. The switch-off threshold can be parameterized using the diagnostic tester. If no plausible interior temperature value is present, the outside temperature value is used instead.

• Shutoff at end of diagnosis:

  After diagnosis has finished, the steering column tube module control unit switches off the steering wheel heater.