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Converting from stock IAT sensor to 1.8T/2.7T sensor

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  • Converting from stock IAT sensor to 1.8T/2.7T sensor

    Recently my IAT sensor gave up the ghost leaving me with bad readings in the negative numbers area. Borrowed one off a friends engine to verify the problem wasnt elsewhere and my sensor was working fine and this confirmed it was the sensor because data logging showed normal readings with the borrowed sensor. The PTC sensor used on these cars is not very easy to come by here in the states plus they are expensive if found so im looking for alternatives like using the far more common NTC IAT sensor from the 1.8T or and or 2.7T engines.

    To start i know that the scales are opposite from PTC to NTC. so if 0 ohms of resistance on the stock PTC sensor means -50 degrees F i will be the exact opposite on the NTC sensor meaning its maxed out at 150 degrees F. so this is pretty obvious the ECU is not directly compatible with the NTC sensor. When the NTC sensor is plugged into the stock motronic i get 120 degrees F constantly with no fluctuation in temperature at all. What i am wondering is if anyone has successfully converted from PTC to NTC? would you care to share how you did it? does anyone know the address of the IAT sensor scalar or if there is one in the motor and boost chip BIN files? or over all is this just simply too complicated to convert and i should just bite the bullet, search for a stock IAT sensor and pay out of my nose for it?

    I think it would be nice to have the ability to swap to a more readily common sensor once the original one fails.
    "The really good drivers got the bugs on the side windows." Walter Röhrl

  • #2
    i thing best way is bypass ic s640 (look on pin 45 NTC and pin 44 PTC on ecu int. sch. ) and then play with software...

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    • #3
      Originally posted by hcman View Post
      i thing best way is bypass ic s640 (look on pin 45 NTC and pin 44 PTC on ecu int. sch. ) and then play with software...
      I was poking around in the damos and there is no scalar for it directly. im guessing this would have to be a hardware mod since whatever code to the sensor would have to be chased in both motor and boost chips. i've been looking at R640 and R641, i think these 2 play a role in being a voltage divider because the back of R641 is attached to the VDD pin of the diode limiter chip. the divider circuit is not the same as the MAF. another thing i have been looking at is other DME style motronics because they are practically the same in how the sensor input works. the M1.3 from the 944 turbo circuit is damn near identical less the fact that R640 and R641 in that ECU have different resistance values than the AAN ECU. yes the hardware is even labeled the same in the 944 ECU lol. i think im going to do some testing with variable resistors and ambient temperature to see if changing the resistance of R640 and R641 can tip the scales the other direction.
      "The really good drivers got the bugs on the side windows." Walter Röhrl

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      • #4
        make mod like i draw, for IAT pullup just copy value from ECT pullup. LPF you can leave stock... normaly NTC IAT and ECT have nearly same ohm values....

        look in ida how is an3 and an2 used...

        btw also i want to install map/iat combo sensor in my car...

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        • #5
          Originally posted by hcman View Post
          make mod like i draw, for IAT pullup just copy value from ECT pullup. LPF you can leave stock... normaly NTC IAT and ECT have nearly same ohm values....

          look in ida how is an3 and an2 used...

          btw also i want to install map/iat combo sensor in my car...
          what you drew is exactly what is in a E30 BMW ECU. i was wrong above. it wasnt the 944 ECU i was looking at, its an E30 BMW ECU. i spoke with a well known BMW tuner about the E30 and he confirmed its an NTC sensor in those cars because its built into the MAF and when they go bad they replace them with a GM NTC sensor in the intake pipe. i will mod my test dummy ECU like your diagram and see what the result is.

          There is a MAP/IAT combo sensor?
          "The really good drivers got the bugs on the side windows." Walter Röhrl

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          • #6
            Originally posted by vwnut8392 View Post

            I was poking around in the damos and there is no scalar for it directly.
            Yes there is. There is for each sensor, look at the code.

            http://tuner.ee - http://www.facebook.com/tuner.ee

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            • #7
              Originally posted by vwnut8392 View Post
              There is a MAP/IAT combo sensor?
              example: 03K 906 051

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              • #8
                Originally posted by hcman View Post

                example: 03K 906 051
                its like whats used on 1.8T for MAP signal. i didnt know they had IAT on board too. neat!
                "The really good drivers got the bugs on the side windows." Walter Röhrl

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