Car ecu thermistor resistance

Using Arduino Project Guidance
1

Hello, everyone!
I am a high school student.
I'm doing an experiment on the ecu of the car
It is an experiment that reads the resistance of the thermistor (thermo unit) connected to the ecu in Arduino.

If I connect Arduino's 5v to the thermistor, I think it will affect the existing circuits in the ecu. So I didn't use these methods.

So I'm going to connect it in parallel with the thermistor and then I'm going to connect one resistor to measure the voltage. I'm going to use a large resistor to make the current flow small.

I have a problem here and I don't know the value of the resistance R1 between the ecu and the thermistor. R1 is required to read the resistance of the thermistor
I want to find out the value of R1 through experiment, but I can't experiment at the moment.

Is there a way or circuit configuration to know the value of the thermistor even though I don't know the value of R1?

I'm sorry that I'm not good at English, and I hope you understand what I mean.
And thank you for your help.

2

Draw a wiring diagram.

3

KakaoTalk_20240627_131429089
KakaoTalk_20240627_1314290891006×1440 73.8 KB

Hi, everyone here.
I am a high school student.
Recently, I am conducting an experiment on the thermistor(thermo unit) of the car ecu
I would like to measure the resistance of thermistor in ecu by arduino.
IF 5v of arduino is applied to the themistor, it will affect the existing ECU circuit.
I made one resistance in parallel with the thermistor and connected it to arduino.
The value of the resistance in parallel is large and the current flow less

The problem is the resistance between the ecu and the thermistor. I need to know this resistance value to know the resistance of the thermistor.
Problem occurred because this resistance value is unknown.

After measuring the resistance of the thermistor, i would like to know the value of R1, but I am currently unable to do it because there is no connection between the ecu and the electronic control unit.

Is there a way to figure out R1 in this situation or find out the resistance value of thermistor even though I don't know R1?

I'm sorry that I'm not good at English
Thank you for your response

4

Please do not start a new topic about the same subject, it's called cross-posting; @xfpd did not ask for a new topic :wink: Topics merged.

Cross-posting wastes people's time.

5

No idea where your idea for any parallel resistance comes fron but....if you need to know the instantaneous value of the thermistor( changes with temperature) measure voltage across the thermistor and the current through it....called ohms law

6

If the thermistor is already being used by the car's ecu then you can't power it from an Arduino without upsetting it's original function.

Powering it from your circuit is likely to be detrimental to the operation of your car.

7

image

uint16_t ThermResist(byte aPin){ // we suppose 5V and 1023 max value
  uint16_t rawADC = analogRead(aPin);
  const uint16_t R1_resistor=1000; //Ohm
  return R1_resistor/(rawADC/1023-1);
}
8

As the ecu is using the thermistor you must not do anything that will change the reading.
You CAN (with care) measure the voltage across it.
However you would need to see if the ECU is measuring it with a constant current (as is most likely) or a pulsed current (to reduce self heating).
In either case you will also need to protect the arduino.
I'd suggest a simple CR filter with series R = 100k and C = 1 uF or so to give Fc=1.6Hz.
That will significantly attenuate any noise frequencies above 40Hz, and protect the analog input from spikes.

1 Like
9

I'm sorry. I made a mistake posting this question. I'll be careful next time to make sure this never happens again

10

How many wires on the thermistor? Can you unplug the thermistor from the car's wiring harness?

11

How do you know there even is an R1?
How do you know that the voltage at A0 will be less than 5V, so that you don't damage the Arduino?