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Thread: Air Temp Sensor Transfer Functions

  1. #1
    Senior Tuner CCS86's Avatar
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    Air Temp Sensor Transfer Functions

    On my supercharged 2012, I run the MCT switch enabled and read IAT at the MAF and the manifold.

    Another common approach is to reroute the new sensor into the MAF harness, replacing the integral MAF temp sensor signal, with the new manifold temp sensor signal. But I have wondered, since the sensors themselves are quite different, how different are the transfer functions.

    Not all strategies have both transfer functions visible yet in HPT, but I pulled a fair amount of data to compare:

    IAT transfer functions.jpg

    What we see is two distinct "types" of transfer function: S-curve, and linear.

    My 2012 FPDX has the same linear MCT function as the 2015 GT shown in this graph, and mine does seem to indicate accurate temps. But I have never tested it for accuracy. I guess in the "normal" operational range, the difference wouldn't be blatant.

    All the other calibrations have the S-curve data for the secondary temp sensor.

    So the question I am left with, is which one is correct? Is the linear data just "filler" data for an unused table, or does it really match some available sensor?

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    S curve is the accurate data for a resistor type temperature sensor, even coolant temp and head temp sensors, use the appropriate values for the sensor you are using.

    The linear data is HPT populating an unused table.

    Have you checked any Ecoboost MCT and diesel CAC temp functions? Some are absolutely not correct when populated by HPT. Best to find the transfer data that is for the sensor and use that.

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    Senior Tuner CCS86's Avatar
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    Quote Originally Posted by murfie View Post
    The linear data is HPT populating an unused table.


    Are you sure about that?

    In my personal FPDX tune, that linear data is in the MCT Transfer function table, and I have the MCT switch on, with seemingly valid data on IAT1 and IAT2 channels.

    I haven't checked any Ecoboost tunes.

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    Senior Tuner CCS86's Avatar
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    Can anyone confirm the transfer function for the Ford 9C1Z-12A697-B?

    It should be the stock GT500 MCT sensor, but all the calibrations I have checked have that linear transfer function that murphie says is bogus.

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    Senior Tuner veeefour's Avatar
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    Get yourself a good universal temp sensor with known transfer - the are either 1/4 or 1/8 NPT. Why bother?

    https://www.aemelectronics.com/produ...ure-ait-sensor

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    Senior Tuner CCS86's Avatar
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    Quote Originally Posted by veeefour View Post
    Get yourself a good universal temp sensor with known transfer - the are either 1/4 or 1/8 NPT. Why bother?

    https://www.aemelectronics.com/produ...ure-ait-sensor



    Because these sensors are being supplied with blower kits and then wired directly into the PCM harness, or spliced in at the MAF. When somebody wants a tune, telling them to buy a new sensor isn't what they want to hear.

    I'm not sure why a Motorcraft sensor used in GT500's, F250s, Excursions, etc; wouldn't qualify as well known.

    HP Tuners just doesn't seem to have that table mapped in the tunes I have on hand, for vehicles that use this sensor.

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    Senior Tuner veeefour's Avatar
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    But you said "On my supercharged 2012"? Whatever...

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    Senior Tuner CCS86's Avatar
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    Quote Originally Posted by veeefour View Post
    But you said "On my supercharged 2012"? Whatever...


    Does it upset you that I referenced my personal experience, when trying to make sense of a more general situation?

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    When I get home I'll post the transfer function. I don't have it on me now but I do have it.

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    Senior Tuner CCS86's Avatar
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    Quote Originally Posted by momotunes View Post
    When I get home I'll post the transfer function. I don't have it on me now but I do have it.

    Much appreciated!

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    Where are y'all seeing the MCT transfer data in the Strategy?
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    Quote Originally Posted by CCS86 View Post
    Much appreciated!
    No problem and sorry about the wait. Long day.

    Okay here is what I have. At one time I had them labeled to part numbers. I don't know if I accidently deleted them in my spreadsheet or what though since the labels are gone Anyways I'm almost 99 percent sure it's the middle one. If you test it out please let me know so I can relabel my sheet.

    Motorcraft Sensor.PNG

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    Senior Tuner CCS86's Avatar
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    Quote Originally Posted by blackbolt22 View Post
    Where are y'all seeing the MCT transfer data in the Strategy?


    If HPT has it mapped in your strategy, it will be under [Engine Diag] > [General], alongside the MCT switch.

    At my request, HPT added the MCT switch to all the "stock" Mustang strategies which have been updated. They are still slowly working on rolling that update out. But in my testing, the stock strategies don't have the back-end code to support flipping the MCT switch on. Maybe in the future it could be added with a code patch. What sucks at the moment, is you have no way of knowing whether it works or not before you try.

    Even the updated strategies don't show the primary IAT transfer function table, but Eric says he will get that added soon.

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    Senior Tuner CCS86's Avatar
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    I must be thinking about the IAT signal circuit wrong. Can someone please point out my error?


    • PCM sends a 5V reference to the sensor
    • IAT sensor has a varying resistance, depending on temp
    • The higher the resistance, the more voltage drop, the lower the signal voltage coming back to the PCM
    • Open circuit (sensor unplugged) = maximum resistance = 0v signal = maximum reported temp
    • Short circuit = minimum resistance = 5v signal = minimum reported temp



    However, on this 2016 Mustang I am working on, the behavior seems reversed. When the sensor is unplugged he is getting a 5v signal, and with a jumper wire a 0v signal.

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    Senior Tuner CCS86's Avatar
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    Quote Originally Posted by momotunes View Post
    No problem and sorry about the wait. Long day.

    Okay here is what I have. At one time I had them labeled to part numbers. I don't know if I accidently deleted them in my spreadsheet or what though since the labels are gone Anyways I'm almost 99 percent sure it's the middle one. If you test it out please let me know so I can relabel my sheet.

    Motorcraft Sensor.PNG



    Thanks for that data.

    Looking at all 3, I have some observations:


    • The "Left" and "Right" tables are essentially the same, besides a questionable data point on the left table @ 4.452v and the right tables @ 0v
    • All three converge above 120*F, but you could probably test the difference in some ice water



    Sensor-check.jpg

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    I think the left one is a CHT sensor and the right one is a different type of IAT. I should have included the three part numbers in my post just didn't know which one went to each function.

    Yes, unplugged/open circuit = max volts
    No resistance = 0volts.

    I believe it works by total voltage drop across the circuit. So unplugged is a drop of 5 volts.

  17. #17
    These sensors use a simple circuit called a voltage divider. It uses two resistors and an applied voltage. There is an internal resistor in the PCM or harness that "divides" the voltage by a ratio of that resistor and the resistance of the sensor. Basically, momotunes almost got it right in that it measures the voltage dropped across the sensor, but unplugged is no drop, not a drop of 5V. If you unplug the sensor, there is no longer a resistance to drop across, so the measured voltage stays at 5V. If the sensor is very hot, its resistance gets very low, which pulls the voltage down to a low value. You can think of it like the sensor is starting to short out the voltage to ground (0V).

    CCS86, your error above was that the higher the resistance, the less voltage drop it creates, so the voltage the PCM sees is higher, not lower. Unplugged should be 5V, and the temp should rail to -40
    Last edited by Ruiner46; 02-06-2020 at 12:10 AM.

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    Senior Tuner CCS86's Avatar
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    Quote Originally Posted by Ruiner46 View Post
    These sensors use a simple circuit called a voltage divider. It uses two resistors and an applied voltage. There is an internal resistor in the PCM or harness that "divides" the voltage by a ratio of that resistor and the resistance of the sensor. Basically, momotunes almost got it right in that it measures the voltage dropped across the sensor, but unplugged is no drop, not a drop of 5V. If you unplug the sensor, there is no longer a resistance to drop across, so the measured voltage stays at 5V. If the sensor is very hot, its resistance gets very low, which pulls the voltage down to a low value. You can think of it like the sensor is starting to short out the voltage to ground (0V).

    CCS86, your error above was that the higher the resistance, the less voltage drop it creates, so the voltage the PCM sees is higher, not lower. Unplugged should be 5V, and the temp should rail to -40


    Thanks Ruiner, that makes much more sense. I wasn't thinking about it as a voltage divider, with the V_out "upstream" of the IAT sensor.

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    Senior Tuner CCS86's Avatar
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    This 2016 is still giving us fits trying to splice in the Motorcraft IAT sensor in place of the MAF IAT.

    What originally seemed like a wiring issue, is now looking more like sensor differences. Here's what we have going on:

    • Factory MAF wiring gives us a good IAT signal
    • Splicing the Motorcraft sensor in (blue & violet/grey) gives us a -40*F reading (~4.7v)
    • At first this seemed like an incomplete circuit, but that has been ruled out
    • Checking resistance of the new sensor against the MAF sensor sheds some light. New sensor is ~61 kohm, while the MAF is 3.8 kohm
    • On my 2012 GT, I get matching resistance between the IAT2 sensor and the MAF sensor, for a given temp (50+ kohm)
    • Ford does list a different part number for the 2016 MAF sensor, but are they really this different in the expected temp sensor resistance?
    • The transfer functions are very close between the 2016 GT stock IAT transfer function and the Motorcraft sensor, which implies the sensors shouldn't be so different.
    • The only way I could see them having such similar transfer functions and such different resistances is if the R1 in the voltage divider circuit was also quite different (with the IAT sensor being R2). But I imagine that resistor is integrated in the PCM and out of our control



    Any ideas? Am I missing something? Do we just need a completely different IAT sensor with far lower resistance?