Analogue input of shared sensor

I'm trying to use an arduino uno to read the voltage of a sensor on an engine.
The temperature sensor on the engine is connected to the ECU which has an inbuilt 1k pullup resistor. I find that if I connect the sensor to the arduino, it changes the value that the ECU see.

I didn't think that the arduino would have an effect of the voltage of the sensor, but it's obviously is. Is there anyway around this?

This is because the Arduino is only a 5V system and your car's ECU is not. It sounds like the Arduino input is clamping the sensor signal.

Try connecting the sensor to the Arduino through a potential divider circuit so that the Arduino never sees a voltage greater than 5V, that will stop the sensor voltage from being pulled down.

You could use a op amp buffer so you read the ECU signal without loading it. Then voltage divider into the Arduino.

Weedpharma

Yes but the input impedance of a digital input is very high so it is not actually loading the sensor is it.

Grumpy_Mike:
Yes but the input impedance of a digital input is very high so it is not actually loading the sensor is it.

If the voltage is being clamped it is not high impedance.

Maybe I should have said buffer the voltage and do a level change.

Weedpharma

Thanks for the feedback
The sensor on the engine is 5v, with a 1k pull up resistor from the ECU. I didn't think the arduino would have had this much affect on the voltage being read. What it the resistance of the analogue inputs?

What it the resistance of the analogue inputs?

Well in excess of 10M, it is not having any loading effect on the sensor, your problem is something else.