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    Thread: Heated seats - What's the thermistor do?

    1. Member
      Join Date
      Sep 7th, 2014
      CT, USA
      2001 Audi TT Quattro 225hp coupe
      05-16-2017 09:08 AM #1
      OK, I know what a thermistor IS (resistor that varies resistance based on temperature,) I just don't know what it's function is in the heated seat circuit.

      Does it have any effect on whether the seats come on? Like, do the seats only heat when the thermistor is over a certain temperature? Or under?

      Or is it just for preventing overheating/overpowering the heater element?

      In other words, should the seats heat even if nobody is sitting in them? Does ambient temperature matter? How hot should they get to the touch, and how quickly?

    2. Member
      Join Date
      Sep 7th, 2014
      CT, USA
      2001 Audi TT Quattro 225hp coupe
      05-20-2017 12:02 PM #2
      Answering my own question for the benefit of anybody else searching...

      Yes, the heat will come on even if nobody is sitting in the seat. The thermistor is just a temperature sensor that is used to determine whether or not to allow power to the heater element.

      More info on that...

      The voltage applied to the seat heater element is only on/off. It is not a variable voltage. So the thermistor is the "thermostat" that lets the circuit in the "switch" decide whether to apply power or not, based on the measured temperature.

      This means, if you are measuring voltage on the green connector under the seat (not sure of pin numbers but they are diagonal opposite pins on the outer edge of the connector)...

      With switch on 5, you should read about 12 volts (battery voltage if engine off or charging voltage if running.)

      With switch on 4, if the ambient temperature is WARM, you will probably get 0 volts. If the ambient temperature is COLD, you should read the same voltage as with the switch on 5.

      The same for 1 to 3, the colder it is the lower setting where you will read voltage.

      You can test this by applying an ice pack over the area where the thermistor is. You can also use the ice pack when measuring resistance on the thermistor on two other pins of the green connector. Resistance should go higher the colder it is.

      I'm providing this info so you don't think your switch is defective if it seems to inconsistently allow voltage to the heater element. Just test it on 5 and ignore if you get no voltage on 1-4.

      Also, the driver and passenger side switches are different parts, but will plug in to either side.

      They are actually different internally. They're easy to pop apart with a small screwdriver. The driver's side switch has a totally different circuit board and a heat sink on a chip. It also has a contact switch that closes when you move the setting from 0 to 1 (and stays connected on 2-5.)

      I still don't know why my passenger seat won't work. I've checked the resistance of the back and lower heater elements, and the thermistor. All fine. When I plug the driver side switch into the passenger side connector, I get heat, in both the back and lower elements, so the seat itself seems to be totally fine. (But I know the driver's side switch is the wrong part so I don't want to leave it there in case it causes other problems.)

      For the passenger side switch, I've tried a spare from another car and also two brand new ones. No heat/voltage with any, so I don't think the switch is defective.

      I'm wondering whether VW put in some sort of "safety" where the passenger seat won't heat unless the driver seat is also on. But I've tried that and still no passenger heat. I also have a spare driver side switch and that didn't help either.

      I'm at a loss.

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