Rear Heated Seats Function - GF91.30-P-0005FR

Model 257, 290 up to Model year 2021 with code 872 (Left and right rear seat heating) 

Courtesy of MERCEDES-BENZ USA

Function requirements, general 

• Circuit 15R ON or circuit 15 ON

The electronic ignition lock control unit sends the status of circuit 15 over the interior CAN, rear SAM control unit and the heater LIN to the rear seat heater control unit.

Rear seat heater, general 

The seat heater enables the outer seat cushions and backrests to be heated in three heating levels. The master control unit for the seat heating function on the rear seats is the rear seat heater control unit.

The heater pads are energized stronger than in a normal case depending on the interior temperature. This enables faster heating to be achieved. The lower the interior temperature, the faster the heater pads are heated up. The climate control control unit sends the interior temperature value over the interior CAN, rear SAM control unit and the heater LIN to the rear seat heater control unit.

The seat heating for the rear seats includes the following subfunctions:

• Switch seat heating on and off 
• Seat heating for left rear seat 
• Seat heating for right rear seat 
• Switching off automatically 

Switch seat heating on and off 

The rear seat heater switch in the respective rear seat climate control switch group switches the seat heater on and off as well as regulating the heat output (seat heater deactivated in initial condition, all LEDs off):

• Press once: 3 LEDs light up, heating level 3 of the seat heating is activated (100 % heating output).
• Press twice: 2 LEDs light up, heating level 2 of the seat heating is activated (65 % heating output).
• Press three times: 1 LED lights up, heating level 1 of the seat heating is activated (35 % heating output).
• Press four times: All LEDs off, seat heater switched off

If the seat heating is selected again at the same circuit status, only 90 % heating output will be available for heating level 3. The heat output of the respective heating levels can be adjusted ex works using a diagnostic tester.

Function note 

The seat heater is switched off automatically at a set time. The timer control is realized by the rear seat heater control unit.

• Heating level 3 is active for 8 mins. Then the system automatically switches to heating stage 2.
• Heating level 2 is active for 10 mins. Then the system automatically switches to heating level 1.
• Heating level 1 is active for 20 mins. The seat heater is then automatically switched off.

The timer control of the three heating levels can be adjusted ex works using a diagnostic tester.

Seat heating for left rear seat 

The left rear door control unit reads in the status of the left rear door seat heating switch over the left rear door LIN, and sends a corresponding request over the interior CAN, rear SAM control unit and the heater LIN to the rear seat heater control unit.

The rear seat heater control unit then actuates the following heater pads directly:

• Left rear seat sitting surface heated cushion
• Left rear backrest heated cushion

The rear seat heater control unit continuously checks the function of all the components involved in the seat heater and sends the corresponding status over the heater LIN to the rear SAM control unit.

Function feedback 

In accordance with the frequency with which the left rear seat heater switch is operated, the rear SAM control unit sends the status of the seat heater over the interior CAN, left rear door control unit and the left rear door LIN to the left rear seat climate control switch group, which then actuates the LEDs in the left rear seat heater switch accordingly.

Seat heating for right rear seat 

The right rear door control unit reads in the status of the right rear door seat heating switch over the right rear door LIN, and sends a corresponding request over the interior CAN, rear SAM control unit and the heater LIN to the rear seat heater control unit.

The rear seat heater control unit then actuates the following heater pads directly:

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

The rear seat heater control unit continuously checks the function of all the components involved in the seat heater and sends the corresponding status over the heater LIN to the rear SAM control unit.

Function feedback 

In accordance with the frequency with which the right rear seat heater switch is operated, the rear SAM control unit sends the status of the seat heater over the interior CAN, right rear door control unit and the right rear door LIN to the right rear seat climate control switch group, which then actuates the LEDs in the right rear seat heater switch accordingly.

Switching off automatically 

The seat heater is automatically switched off when one of the following events occurs:

• Shut-off in the event of a hardware defect 

  The seat heating is switched off in the event of an active actuation without a corresponding request.

• Shut-off at overtemperature of PCBs in rear seat heater control unit 
  • Heating stage 3 can no longer be activated if the PCB temperature is greater than 70°C.
  • Heating stage 3 and heating stage 2 can no longer be activated if the PCB temperature is greater than 80°C.
  • At a printed circuit board temperature of more than 90°C, the seat heater can no longer be activated.

  If no circuit board temperature value is present, the interior temperature value is used instead.

  If the PCB temperature reaches more than 90°C, a fault message is stored in the fault memory. Manual reactivation requires the printed circuit board temperature to drop to T = 2°C below the corresponding temperature threshold.

• Shutoff for heater pad overtemperature 

  The heater pad temperature is continuously monitored. In the event of an imminent overheating, the power supply for this heater pad is switched off.

• Shutoff in the event of an error (short circuit, overload or open circuit (line break)) 

  In the event of a short circuit, overload or a line break, the power supply of the identified heater pad is switched off. The exact source of the fault can be diagnosed using the diagnostic tester.

• Shutoff when the engine starts 

  When the engine is started (cold start) the seat heater is switched off for 20 seconds. In the event of a warm start the seat heating is switched off for 3 to 5 s. The timer control continues to run, and the LEDs in the respective seat heater switch remain switched on. No undervoltage assessment takes place during startup.

• Switching off by means of energy management 

  To prevent a battery from discharging, the state of charge is continuously evaluated. Depending on the situation, the seat heater is also switched off, if necessary.

  If the seat heating is switched off, the LEDs in the respective seat heating switch remain initially switched on. If the on-board electrical system reaches the specified voltage again within the following 3 minutes, the seat heating also starts again automatically. If the shutoff takes longer than 3 minutes, the LEDs are also switched off. If the shutoff continues, and the seat heating is requested again, the LEDs light up for 1.5 s and then they go off.

• Deactivation for voltage breakdown 

  The seat heating is switched off in the event of a voltage breakdown.

• Shutoff after circuit 15R OFF 

  For circuit 15R OFF and circuit 15C ON, the timer control runs on for another 5 minutes. If circuit 15R is switched on again, the seat heater is also switched on again.

• Shutoff for timeout error 

  If the electronic ignition lock control unit detects a timeout error, the seat heating is switched off.

• Shutoff at end of diagnosis 

  The seat heating is switched off after diagnosis has been completed.

	Electrical function schematic, seat heater, rear seats		PE91.30-P-2055-97XBA
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