AD595AQ for Car Exhaust Gas Temperature

That's the correct way to use a grounded thermocouple with that chip. Let's check other things. Have you grounded your circuit to the vehicle chassis? Is your circuit powered from a nine volt battery or from some derivation of the vehicle battery?

My circuit is connected to a L7809CV. L7809CV is connected to car's ignition.

The circuit's GNDs are connected to Arduino GND.

Is the Arduino ground connected to car chassis ground?

I just reread your original post. Is the thermocouple that works at home the same as the one in your exhaust pipe?

Yes, from the same place and grounded type.

M.

Okay, at the car, disconnect the exhaust TC and substitute the one that worked at home. If that works okay then touch it to the car chassis and see what happens. What you're describing sounds like a ground loop problem. Are you sure that the Arduino AND its power supply are grounded directly to the car chassis?

I have substituted them with one another the same thing.

Arduino uses the same L7809CV chip as the AD595AQ, and the L7809CV is connected to the chassis.

By the "same thing" do you mean they both read wrong now or that just the one on the car reads wrong. What happens when you touch the "home" one to the car.

They both read wrong.

Oh, okay. Something happened to your circuit in transit apparently. You need to thoroughly check it for loose, broken wires, short circuits, etc. Anything that is different from when it last worked correctly.

It works fine when I don't connect it to the car, so the probe that's on the car, when removed and pin 1 connected to pin 4 it works fine!

So the chip is fine, isn't it?

Is it better to go and get an ungrounded probe then? And leave the circuit with pin 1 connected to pin 4?

Thanks.

You could try that. Otherwise it's a mystery to me.

By the way, why did you choose to run it off 9V? That part works on 5V.

I needed temperature of more than 300'C so I increased the input voltage.

Ah, of course. Did you consider this part? Thermocouple Amplifier Digital MAX6675 - COM-00307 - SparkFun Electronics

No I haven't, but does that solve my grounding problem?

Thanks.

No I haven't, but does that solve my grounding problem?

No, only in the sense that it might correct the problem that we haven't been able to sleuth out yet. Plus I just like the fact that it has a digital interface so it leaves out some steps in processing the data.

Without looking at your project, I'm at a standstill. Can you post the EXACT schematic diagram and some closeup photos of all sides of your physical circuit?

I got an ungrounded thermocouple and everything is working normally now.

Thanks for all the help.

Good deal!