Seat heater, function - GF91.30-P-0003FLA

MODEL 212 with CODE 872 (Electrically heated left and right rear seats) 

Fig 1: Seat Heater Function Block Diagram - Model 212 With Code 872

Courtesy of MERCEDES-BENZ USA

Function requirements, general 

• Circuit 15R ON

The electronic ignition lock control unit sends the status of circuit 15 via interior CAN to the rear seat heater control unit.

Seat heater (SIH), general 

Using the SIH, it is possible to heat the seat cushions and the backrests of the rear seats in three heating stages.

The SIH function includes the following subfunctions:

• Function sequence for manually switching on/off 
• Function sequence for automatic switch off 

Function sequence for manually switching on/off 

The rear seat heater is operated through the rear seat heater buttons. Pressing the rear seat heater button switches the rear seat heater on and off and also selects the heating stage. The status of each rear seat heater button is read in directly by the corresponding rear door analog/digital converter. The respective rear door analog/digital converter sends this via rear door LIN, rear door control unit and interior CAN to the rear seat heater control unit.

The rear seat heater control unit actuates the following heater pads directly depending on requirements:

• Left rear seat sitting surface heated cushion
• Left rear backrest heated cushion
• Right rear seat sitting surface heated cushion
• Right rear backrest heater pad

The heat output is dependent on the selected heating stage. The respective active heating level is indicated via red LEDs in the corresponding rear seat heater button.

The rear seat heater control unit sends the status of the seat heater via interior CAN, rear door control unit and the rear door LIN to the corresponding rear door analog/digital converter. The respective rear door analog/digital converter actuates the LEDs in the corresponding rear seat heater button directly.

Pressing once: Heating stage 3 (100 % heat output, three LEDs light up)

Pressing twice: Heating stage 2 (65 % heat output, two LEDs light up) Pressing three times: Heating stage 1 (35 % heat output, one LED lights up)

Pressing four times: Seat heater is switched off (no LEDs light up)

If the seat heater is selected again with the same circuit status, heating stage 3 is provided with only 90 % of the heat output.

Function sequence for automatic switch off 

The timer control occurs by means of the rear seat heater control unit.

Heating stage 3 is active for t = 5 min. Then the system automatically switches to heating stage 2.

Heating stage 2 is active for t = 10 min. Then the system automatically switches to heating stage 1.

Heating stage 1 is active for t = 20 min. The seat heater is then automatically switched off.

The time settings for the three heating stages can be configured using the diagnostics tester.

Other shutoff criteria:

• Deactivation after circuit 15R OFF:

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

• Deactivation with undervoltage or overvoltage:

  With undervoltage or overvoltage, the rear seat heater control unit switches off the seat heater. The LEDs in the respective front seat heater button remain initially switched on. If the on-board electrical system voltage is stabilized again within a period of t = 3 min, the seat heater starts again automatically. If deactivation lasts longer than t = 3 min, however, the LEDs in the respective front seat heater button are also switched off. If the seat heater is requested again despite shutoff, the LEDs in the respective front seat heater button light up for t = 1.5 s and then going out again.

• Deactivation in event of a hardware defect:

  If actuation of the heater pads without a current request via a front seat heater button is detected, the rear seat heater control unit switches off the seat heater.

• Deactivation at engine start:

  When the engine is started, the rear seat heater control unit switches off the seat heater. The LEDs in the respective front seat heater button remain switched on. Undervoltage is not recognized while starting the engine and no faults are stored.

• Shutoff for overtemperature of PCB in rear seat heater control unit: Heating stage 3 can no longer be activated if the PCB temperature is T > 90 °C.

  Heating stage 3 and heating stage 2 can no longer be activated if the PCB temperature is T > 100 °C.

  The seat heater can no longer be activated if the PCB temperature is T > 110 °C.

If the PCB temperature reaches T > 110°C, a fault message is stored in the fault memory. Manually switching back on requires cooling of T = 2 °C below the corresponding temperature threshold.