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:
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