Issue with temp sensor calibration

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Concker
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Joined: Tue Jun 21, 2022 12:56 pm

Issue with temp sensor calibration

Postby Concker » Sat Jul 23, 2022 2:11 pm

Hello.
I am trying to use the OEM temp sensor on my Porsche 944 but i cannot figure out what i am doing wrong when it comes to calibrating it :oops:

Its hooked onto the OEM sensor that goes to the gauge (resistor sensor) to A8/ Adc1.
I get readings in ucomp together the the oem gauge, so far so good.
So now to calibration.

I got the following information:

Skjermbilde 2022-07-23 135745.jpg
Skjermbilde 2022-07-23 135745.jpg (39.75 KiB) Viewed 69 times


A quick measurement with ignition off, between A8 and ground confirms these values to be correct.
At approx 90C (guessed from oem gauge) i get around 2,4V reading in ucomp and physical with ignition on.
So i tried to enter the previous values into the calculator

Skjermbilde 2022-07-23 135631.jpg
Skjermbilde 2022-07-23 135631.jpg (202.03 KiB) Viewed 69 times


as you can see this wont work, as the voltage values are way off..
Am i missing something here?

For "fun" i tried to just use some random value at R25, and at 9000 / beta 3907 i am getting around 90C at the ucomp.
So the sensor works, from cold to warm engine the temp at ucomp increases, oem gauge still works fine.
However it would be nice to find out what i am doing wrong, if it would be possible to calibrate this with the provided values for the oem sensor?

ArT
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Re: Issue with temp sensor calibration

Postby ArT » Mon Jul 25, 2022 1:27 pm

If this is OEM sensor (which is still connected to stock installation) you can not calibrate it using resistance - you will need to measure voltage and use NTC(U) sheet for calibration. The reason is that stock circuit is converting already from resistance to voltage. Please prepare temperature/voltage table and we will calibrate it together. Voltage should be in range 0-5V to make it working.

Concker
Posts: 3
Joined: Tue Jun 21, 2022 12:56 pm

Re: Issue with temp sensor calibration

Postby Concker » Mon Jul 25, 2022 1:50 pm

ArT wrote:If this is OEM sensor (which is still connected to stock installation) you can not calibrate it using resistance - you will need to measure voltage and use NTC(U) sheet for calibration. The reason is that stock circuit is converting already from resistance to voltage. Please prepare temperature/voltage table and we will calibrate it together. Voltage should be in range 0-5V to make it working.


I see.
The same way i did with the fuel level sensor then, Thank you for help on this by the way :)
What i did, is that i hooked up an potmeter to the oem wiring and read out the voltages on full and empty tank.
I guess i could do the same with this sensor, but then i need to rely on the oem gauge for refrence, so it wont be 100% accurate, but its a start. One way would be to have the sensor in a cup of water that i can measure for a control. Problem is the location for the sensor making it very, very! much work to remove.. even the oem wires is hard to get to without removing the whole intake and more..
Measuring in the water tank is an idea perhaps.. not sure how accurate this is either. :?:

ArT
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Re: Issue with temp sensor calibration

Postby ArT » Tue Jul 26, 2022 9:46 am

You can rely only on 2 measurements point if you are sure that you can get them very accurate.
You can also disconnect plug (or instrument cluster if sensor is directly connected to instrumnet cluster), measure resistance on sensor, connect plug and measure voltage. If you have resistance/temperature table then you can get voltage/temperature table with this method.


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