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

MODEL 212 up to model year 2014 with CODE 443 (Steering wheel heater) 

Function requirements, general 

Fig 1: Steering Wheel Heater (LRH) Communication Diagram

Courtesy of MERCEDES-BENZ USA

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

The electronic ignition lock control unit sends the status of circuit 15 via the interior CAN to the steering wheel heater control unit. The automatic air conditioning control and operating unit sends the interior temperature via the interior CAN to the steering wheel heater control unit.

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 

The steering wheel heater is operated using the steering wheel heater switch. The steering wheel heater is switched on and off by turning the steering wheel heater switch. The steering column tube module control unit reads in the status of the steering wheel heater switch directly and sends the status via the chassis CAN, front SAM control unit and interior CAN to the steering wheel heater control unit. The steering wheel heater control unit then directly actuates the steering wheel heater heating element in the steering wheel rim. The steering wheel heater control unit then sends the status "Steering wheel heater active" via the interior CAN, front SAM control unit and chassis CAN to the steering column tube module control unit. The steering column tube module control unit directly actuates the steering wheel heater indicator lamp as function feedback. 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, steering column tube module control unit, chassis CAN, front SAM control unit and interior CAN to the steering wheel heater control unit. The steering wheel heater 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 wheel heater control unit switches off the steering wheel heater automatically after t = 15 min has elapsed. The steering wheel heater 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 wheel heater 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 wheel heater 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.

• Shutoff in the event of a hardware defect:

  If the steering wheel heater is actuated without a request being issued or without function requirements being fulfilled, the steering wheel heater control unit switches off the steering wheel heater and stores a fault in the fault memory. The steering wheel heater indicator lamp is switched off.

• Switch off during overtemperature.

  At an interior temperature of T > 30 °C, the steering wheel heater 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 wheel heater control unit switches off the steering wheel heater.