Throttle Position Sensor, A-to-D Conversion

Discussion in 'General Electronics Chat' started by mossman, Mar 20, 2015.

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  1. mossman

    Thread Starter Member

    Aug 26, 2010
    The wiper connection on the throttle position sensor on my dirt bike is reading about 240 ohms higher than it should (900 ohms as opposed to 660) and I'm trying to determine a simple way to offset the value so that the ignition control module receives the proper voltage signal vs. throttle position. The full resistance reading across the + and - terminals is 4.4kΩ, which results in a reference voltage of about 4.070V. The resistance on the wiper is supposed to be no more than 15% at idle (660 ohms / 0.611 V) . Right now, the minimum value I can set the TPS to is 900 ohms (because the wiper is dirty and the mounting screw will not allow me to rotate the sensor body any further to get a lower reading). This results in a minimum voltage of 0.833V fed to the TPS input on the ICM, which is about 20% of the reference voltage. This causes the ECM to use the incorrect timing number from the ignition map. I'm hoping to correct this error by inserting a 330 ohm resistor in series with the wiper connection. I believe it works out mathematically, but I'm not positive. It's a little difficult since I don't know what is inside the ICM. I'm also uncertain if adding the 330 ohm resistor will affect something else. For instance, will the actual TPS voltage be different then my calculation because of the A-to-D circuitry inside the ICM? Will the 330 ohm resistor change the total TPS resistance to some value other than 4.4kΩ as the throttle position is changed? If so, the reference voltage will drift, which wouldn't be a good thing (would it)? I realize it may be difficult to give a definitive answer without knowing the internals of the ICM, but I'm hoping someone can provide some insight as to how a typical A-to-D functions and whether or not adding the 330 ohm resistor affects anything. I suppose if no current is flowing through R2, there will be no drop and it will not work. I thought I had a solution earlier by instead putting an R2 of 1.5kΩ in series between Vref and the TPS, but it only worked at one value, because as the wiper is changed, the ratio changes. BTW, the TPS full reading can be anywhere from 4kΩ to 6kΩ according to the manufacturer spec, so I have some wiggle room, and Vtps at idle needs to be between 13% and 15%. Here is the proposed schematic:

    Last edited: Mar 20, 2015
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    I can't see the 330R resistor having any effect at all. I presume that the ECU TPS input impedance is going to be quite high relative to R2, since the TPS is 4k4.
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    I'd suspect its simply starting to wear on you.. How often do you intend to band-aid it before just buying a new one?
    Next month it could be 300 ohms higher.. then 400,etc...
    bwilliams60 and PeterCoxSmith like this.
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    isnt it adjustable? should be a clamp to hold it in position, loosen the clamp and adjust it.
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Clean it?
    I share Mcgyvr's suspicions; the track is worn.
  6. mossman

    Thread Starter Member

    Aug 26, 2010
    I cannot adjust it any further (I said that) and it's an enclosed unit so I'd likely destroy it by opening it. It's really just a puzzle I'm trying to solve to see if I can make it work. Sure I could fork out $70 for a new one, but where's the fun in that? I'd like to fix it with a few resistors if possible.
  7. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    I can't think of any way you can compensate for the missing track by simply adding resistance.
  8. mossman

    Thread Starter Member

    Aug 26, 2010
    It's not missing a track, the track to wiper connection is dirty and/or the track is worn. I'm beginning to realize there may be no simple solution.
  9. wayneh


    Sep 9, 2010
    You could put an op-amp in between to re-calibrate the zero and span. But at that point you might as well just replace the TPS.

    I assume you need the existing, upper range of the TPS? I mean you could add another resistor to ground that might correct the low end but this would also pull down the high end.
  10. BR-549

    Well-Known Member

    Sep 22, 2013
    If this isn't an auto mod......I don't know what is!
    Replacement is your only option!
  11. WBahn


    Mar 31, 2012
    The owners of All About Circuits has elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

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