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What are the foreseeable problems, if any, with wiring a variable resistor into the accelerator pedal position sensor circuit?

The goal is to have a variable power selector using a dial, such that as the dial is turned down, an equal change in pedal position will result in a progressively lesser change in throttle position. For example, with the dial halfway through its travel, pushing the pedal to the floor would result in a half-open throttle.

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  • You state on the connection to the TPS, but from your description it sounds like this would only work on a drive-by-wire setup. if so, you wouldn't have a TPS, because the car would use pedal input to place the throttle in position to match. If you did apply this to a TPS vehicle, it would mean the computer would think the throttle plate would be half open and wouldn't be giving the engine the right amount of fuel (without completely reworking the fuel map, which would defeat the purpose of what you seem to want to do here.) – Pᴀᴜʟsᴛᴇʀ2 Nov 15 '17 at 20:22
  • I don't think that's true, in all drive-by-wire systems I've seen there's a sensor in the (throttle/pedal, whichever) that feeds to the ECU, then the ECU controls the actual throttle butterfly. I'm proposing hijacking the control -> ECU wires. – Ceshion Nov 15 '17 at 20:24
  • Correct, but that's not a TPS. It's an accelerator pedal position sensor ... yes, semantics, but I wanted to ensure I knew what you were asking. – Pᴀᴜʟsᴛᴇʀ2 Nov 15 '17 at 20:28
  • Oh! My mistake, I'll correct the question. – Ceshion Nov 15 '17 at 20:28
  • It sounds like you want to put some sort of switch in place which would limit the power of the vehicle when you don't want it to have full power? As in, some sort of "valet key" for your car? – Pᴀᴜʟsᴛᴇʀ2 Nov 15 '17 at 20:35
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The main problem that I would expect is that you'd probably get disagreements with the computers readings, which would start messing with the EFI mapping.

First problem is - does the TPS you're looking at actually use resistance on the signal to measure or does it not (although I believe the VAST majority of TPS modules do work that way). After that, is the question of, does your TPS increase or decrease resistance as it's moved to WOT?

Example would be that the signal is sending at 5vdc and returning at 3vdc at closed throttle. Then, at WOT, it returns 5vdc. In this case resistance is reduced with throttle travel therefore adding a variable resistor would result in a backward affect.

The next issue is that (assuming resistance is increased with WOT) adding a variable resistor set to 25k Ohm, you'll have this resistance at all points of throttle position. Meaning that if the TPS supplies 45k Ohm, and you add another 25k Ohm, that the signal will fall outside the ECU's expected parameters when the throttle's closed.

As a final note, the computers can have multiple sensors for throttle action. So if the TPS is reading something, the pedal position sensor is reading something else, this will create a conflict for the computer. That's why I say that you'd minimally get EFI issues I think.


I wrote all that as you guys were discussing in the comments. So to update it to match that information, it still depends on which way the sensor works, and you'll still face the problem as noted above with italics.


ADDED NOTE: These are what I'd call the "foreseeable" problems. There are other possible issues that you'd run into, and there are ways of building much more complex circuits to mitigate those issues as well.

Ultimately, it seems like what you're looking to handle would require some pretty advanced communication with a few different subsystems. At least with regard to fuel consumption and winter driving, that's what the various implementations of traction control and EFI are for. Allows the computer to handle data MUCH faster than a person can, and adapt accordingly.

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  • I would think most vehicles with a drive-by-wire system will not have a TPS, but rather a motor which drives the throttle plate which the ECU manages. It doesn't really need one, because in order for the throttle motor to work correctly, it would give feedback to the ECU as a known position. Kind of opposite of how a TPS works. I'm no expert on the subject ... this is just my understanding. – Pᴀᴜʟsᴛᴇʀ2 Nov 15 '17 at 21:19
  • @Pᴀᴜʟsᴛᴇʀ2 I believe that’s mostly correct (I’m also no expert in that part) but from what I was reading, some vehicles do implement (seemingly) redundant sensors. I suppose every manufacturer is looking for a patent-able way to achieve the same thing, so you’ll get oddities like that. But you’re correct, if the ECU drove a stepper motor, that would handle both functions. – kyle_engineer Nov 15 '17 at 21:32
  • Just remember that not all vehicles have a throttle plate... – Solar Mike Nov 15 '17 at 21:57
  • @SolarMike Also true. The wider the scope of vehicles, the harder to give an accurate answer. :) – kyle_engineer Nov 15 '17 at 22:14
  • @SolarMike - Even these days, diesels have throttle plates ... go figure. – Pᴀᴜʟsᴛᴇʀ2 Nov 15 '17 at 22:15

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