How are variable 12 V resistors used?

Read 12V sensor with Arduino

Hey Combie, greetings to you!

I also you.

Nice to meet you here too =)

I'm not sure about this. "Bullshit" rings in my ears.

Point 1: The 6V to 24V specification is for stupid auto screwdrivers, so that they do not implant the sensor into a 48V system. NTC have no electronics in their stomach! They don't care at all about the voltage, as long as there is no flashover due to excessive voltage.

Point 2: The VDO sensor is operated with a constant current in the typical VDO circuit. Hence the 3 minutes until the sensor has finished heating itself.

You don't have any of that here. Your currents are much lower than in the VDO circuit. Of course you could recreate them if you want. Search tip: "Constant current with 7805" Then you can also make absolute measurements against the internal reference of the Mega.

So I did something really wrong

I agree!

be it just that I'm trying to run the sensor with 5V from the Arduino.

I do not agree with it.

You should also see your measuring range expanded a bit. The full deflection of the VDO instruments is 120 ° C, which is why this value can also be found in the data sheet. Of course, that's not enough for your air-cooled 58L or 4BB. More can be expected in extreme situations.