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Heated front seat

Heated front seat


The heated seats can either be controlled automatically by the ACC or manually with pushbuttons.

Note

Cars from the factory are manually programmed.


The preconditions are that the engine is running and fan speed OFF is not selected. See User programming .

The heating pad consists of two circuits, one circuit is the heating element and the other circuit provides information on current temperature. The temperature sensor consists of an NTC resistor whose resistance varies with the temperature in the heating pad.

Description of operation

The ACC unit supplies the heating element with a B+ voltage from K2 pin 1 which is then grounded via G 14. The heating element in the passenger seat is supplied with a B+ voltage from K2 pin 6 and is grounded via G 24.

The temperature sensor (254) is grounded internally in the ACC unit via K1 pin 23 for both driver's seat and passenger seat

The driver's seat sensor is supplied from K2 pin 4 with 5V via a 6.8 KΩ resistor built into the ACC unit. The characteristics of the NTC resistor mean that resistance decreases afterwards as the temperature increases. This means that the voltage measured over the resistor will at the same time decrease during rising temperatures.

The passenger seat sensor is supplied from K2 pin 5 with 5V via a 6.8 KΩ resistor built into the ACC unit. The characteristics of the NTC resistor mean that resistance decreases afterwards as the temperature increases. This means that the voltage measured over the resistor will at the same time decrease during rising temperatures.

The voltage value from the sensor is A/D converted and is then available in digital form. The digital voltage value is then converted to temperature and is added to the ACC unit's operating memory. Voltage values close to 0 or 5V are interpreted as faults in the circuit.

Manual control

Repeatedly pressing the button produces the desired temperature level. The selected temperature is started and confirmed by the LED in the button. See table.

Level
Indication in button
Temperature
1

1 LED

31°C

2

2 LEDs

40°C



Automatic control

The ACC control module decides whether seat heating is required. At an outside temperature of <15°C, and with a seat cushion temperature of <20°C, the heated seat will be activated. Irrespective of the temperature selected by the ACC only one LED will come on. When the ACC unit does not detect any requirement for seat heating then the heating is switched off and the LED goes out.

If the passenger seat is unoccupied then the LED for the seat will be off, but the seat heating is activated until the seat temperature is reached.

Level
Indication in button
Temperature
1

1 LED

AUTO

2

2 LEDs

AUTO



Adjustment

Adjusting heated seat offset temperature with the diagnostic tool.

Diagnostics

If the indication for a heated front seat goes out 5 seconds after a manual selection then this indicates that a fault has arisen in the heating pad. The diagnostic trouble code can be read in the diagnostic tool.