Toyota Throttle Position Sensor

Discussion in 'General Electronics Chat' started by MikeML, Feb 3, 2010.

  1. MikeML

    Thread Starter AAC Fanatic!

    Oct 2, 2009
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    Toyota Throttle Position Sensor?

    I own a 2006 Corolla, and have been listening to the coverage of Toyota's accelerator recall. I haven't received a recall notice yet, but I expect to. Something about the mechanical fix doesn't seem right to me, and there are folks who have been saying that it could be an Engine Control Module software problem, so I got to thinking about it. The Throttle Position Sensor is a potentiometer built by CTS

    I pulled out the Corolla's wiring diagram to see how the Throttle Position Sensor is wired to the ECM. I was questioning what would happen if one of the wires between the ECM and the Throttle Position Sensor became disconnected? Could that cause the ECM to think that the pedal was all the way to the metal, and cause a "runaway"?

    [​IMG]

    Here is a fragment of a wiring diagram that spans 13 pages. The Throttle Position Sensor is T1, the light blue rectangle in the lower right. The ECM is the big blue mass at the top. What follows is my speculation as to how it functions: If there are any trained Toyota mechanics that can correct my speculation, please chime in...

    T1 pin 3 is the wiper of the Throttle Position Sensor pot and it goes to ECM pin VTA. T1 pin 2 is the low side of the pot and goes to ECM pin E2 via the junction block and brown wire. T1 pin 1 is the high side of the pot and goes to ECM pin VC via the junction block and yellow wire.

    Interestingly, the pot high side is driven by a pin on the ECM rather than being tied to an external positive source. Similarly, the pot low side is driven from a pin on the ECM rather than being tied to an external ground lug. I'm speculating that this is so the ECM can self-test for an open Pot or bad connection.

    During normal operation, presumably the ECM drives VC high to a regulated voltage, and drives E2 to ground. That allows the ECM to A/D what comes in on the VTA pin to determine the pedal position.

    Presumably, during a Power Up test, the ECM can float (disconnect) VC which would cause VTA to go to the voltage at the E2 pin so that the ECM can check half the the connections. By driving VC high, and floating E2, that would cause VTA to rise to the voltage at VC, so the ECM can check the other connections and the pot wiper integrity.

    I know that this type of scan is done when the key is first turned on, but is it done continuously during driving?
    If the self-test is periodic while driving, say once a second, then if the connection between T1 pin 2 and ECM pin E2 became disconnected, the ECM would know that the High on VTA is caused by a broken wire, not because the pedal was floored. If the self-test only happens once during key-on, then the ECM could think that a broken wire means go fast???

    So, does anybody know how often the self-test is done????

    It was pointed out to me on another forum that if the entire range of the pot is not traversed by the wiper, then if the low connection was lost, the ECM could tell that because the voltage at VTA would be higher than normal.

    Anybody have some real knowledge?
     
  2. beenthere

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    You may be on a valid track. There is some conflicting information in the nature of the recall. The "fix" is represented as a small stiffener and/or a more robust washer to prevent the mechanism from sticking.

    However, the reports of "sudden" and "unexpected" acceleration point to some failure such as you are looking into.

    If I were going to design such an input, I would arrange the open position to be at the high end of the resistance, so it would fail safe - an open would be interpreted as no power called for. A shorted input wire could mess with this arrangement by looking like the accelerator on the floorboard, unless there is also some minimum resistance that has to be seen.
     
  3. t06afre

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    May 11, 2009
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    I have a 89 model car. It was one of first standard production cars with a drive by wire system. I have had this car for a long time and this system have never given me any problem
     
  4. retched

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    Odd. I was helping a person with a Toyota Corolla with a jump start about a year ago, they had two cars, they just needed another pair of cables to extend their reach.

    The guy that hooked the cables up, did so backwards. He attached black to red and red to black on the center where the 2 sets of cables were linked together.

    The guy connected the black to the frame rather than the terminal post(Good practice) and the pos to the + battery terminal. But the cables were backwards in the middle. When connected, the running car reved quite high until the cables were disconnected.

    I do not know what was affected in the short, if it was the accel-pot or the throttle body motor. But it was a first for me.

    Come to find out the dead cars battery wasn't dead, it was his starter. So it had enough juice act on the other vehicle.
     
  5. Von

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    Oct 29, 2008
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    I'm not a mechanic or familiar with the Toyota wiring but the "Throttle position sensor" is likely just that... not the throttle "command" or similar.

    Probably just feedback to adjust other parameters for optimal efficiency or emission control.
     
  6. MikeML

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    No, the sensor in question is mounted inside the passenger compartment right where the gas pedal hinge point is. There is no linkage through the firewall. There is a separate servo on the throttle body, but that is not what we are talking about...
     
  7. Duane P Wetick

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    Apr 23, 2009
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    See my thread on this subject in Off-Topics as they did not want it here.

    Regards, DPW [ Everything has limitations...and I hate limitations.]

    ps: I still think it is a combination of mechanical-electronic-software bugs that have contributed to this problem and the ultimate solution is still in the vat.
     
  8. BillB3857

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    Feb 28, 2009
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    I drive a 2004 Chevy 6.6L diesel. It is a drive by wire but has THREE potentiometers ganged together for the accelerator. Two increase voltage output to the ECM on acceleration and the third Decreases voltage to the ECM during acceleration. (May be the other way around) but the ECM tracks all three and if they don't match the proper slopes, it sets a trouble code and lights the Check Engine Light. Sure seems less risky than using a single pot.
     
  9. beenthere

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    Some fool suggested that the problem could relate to EMI from a cell phone. Good trick in a spark ignited car.
     
  10. Von

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    Oct 29, 2008
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    Well if Toyota relied on a single pot for this, and failure can cause uncontrolled acceleration, then they deserve all that's coming to them.

    God bless those hurt by this.
     
  11. Duane P Wetick

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    Apr 23, 2009
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    I really think Toyota has dug themselves into a hole that will ultimately
    prove fatal to the company's survival. A mission-critical event such as un-intended acceleration is by any standard un-acceptable behavior in human transportation manufacturing. Their CEO can apologize all he wants, but that does not solve this problem. Direct supervision (not computer) of gas, brakes and steering are mission-critical items that must function under all circumstances, and any failure of either of these three causes the car to come to a controlled stop. This principle has been violated by Toyota.

    Regards, DPW [ Don't buy a Toyota.]
     
  12. chris13409

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    Jan 7, 2010
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    No, it won't. The TPS only senses the position of the throttle plate. It does not control the position. IMHO, the problem could be caused by throttle by wire EMI.
    :D My '89 Toyota has mecanical linkage and no cruise control and no floormats.
     
  13. Duane P Wetick

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    Something as simple as hitting the brake pedal to override the throttle position signal to the computer would have saved lives and would meet the mission critical criteria. However this function did not exist.

    Regards, DPW [ Don't buy a Toyota.]
     
  14. MikeML

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    Duane, I don't want your politics, or your opinions about Toyota's survival. I posted this as a discussion about how the system works, and so that we might learn how to engineer a self-diagnosing, failsafe system.
     
  15. beenthere

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  16. sterlep

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    Toyota Problem: My understanding of how the ECM and TPS interact is as follows: The closed loop control system is made up of the Hall effect sensors in the gas pedal which send a voltage "command" to the Electronic Control Module. The ECM then provides pulses to a stepping motor which rotates the throttle butterfly valve and Throttle Positioning Sensor. The TPS provides a feedback voltage to the ECM and when it is in balance with the “command “ signal the motor stops. If the TPS feedback signal were abnormally low the motor would continue to open the throttle valve attempting to balance, thus increasing speed. Agree with Von that the use of only one TPS is dangerous. Also MikeML enlightened me that a "pot" is used as the TPS, not a Hall effect device. As a design engineer at CTS for 7 years, am well aware of how a potentiometer can fail: crack in resistive element, open wiper or high contact resistance, an intermittent at any terminal.
    The TPS should be a prime suspect, as its failure would open the control loop, resulting in unexpected acceleration. Positive identification of the fault, providing a fix and adequate testing will take quite a while. An interim fix may be to simply add a brake switch contact that disconnects the stepping motor when brake pedal pressed.

    Note: When “cruise control” activated the Vehicle Speed Sensor also provides feedback to the ECM but do not know influence of TPS or which sensor-feedback is dominant. Many times the cruise control is forgotten in “Pause” mode after tapping the brake. Accidental (or spurious) activation of the “resume” switch would cause acceleration to set-point speed; however, unaware if cruise was on or in pause during any of the incidents.
     
    Last edited: Mar 15, 2010
  17. beenthere

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    Activation after the pause mode is only problematic is the brake pedal overriding cancel does not work.
     
  18. Duane P Wetick

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    Apr 23, 2009
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    Now it seems like even the federal investigators have come up short in obtaining the correct diagnosis of un-intended acceleration in Toyotas.
    A new generation of car diagnosis and repair guidelines is required today, as some complex problems are remaining hidden from even the most articulate of sleuths. Its time for the black box memory to have a place in monitoring a car's functions 24/7 to get to the bottom of these mysterious maladies.

    Regards, DPW [ Everything has limitations...and I hate limitations.]
     
  19. Duane P Wetick

    Active Member

    Apr 23, 2009
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    At 60 mph, you travel 1 mile in 1 minute, not 1 second. Or in 1 second, you will have traveled 88 feet, still not a great distance to make a split-second decision. I think all modern cars will require a black box memory eventually.

    Cheers, DPW [ Everything has limitations...and I hate limitations.]
     
  20. retched

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    Vroooooooooooooooommm..

    Oh sorry, I was doing 3600mph.. A mile a second would make the world a much smaller place. Unfortunately no cars have gotten near this speed. Drive by wire or not.

    I have to agree that when your car fails at highway speeds, the anxiety would cause many problems. I would not want to SMASH the breaks on at 60mph, even if you managed to stop, you may be upside down on the side (or middle) of the road. And if the car is accelerating, most people in fright, would try to UN-stick the pedal in a panic thinking I HAVE TO STOP THE PROBLEM rather than stop the car.

    People also dont want there expensive car to be broken. They want the sudden acceleration to stop so they can go about their day so they dont have to deal with a problem.

    we are talking about the PUBLIC here.. 'Stupider' things have happened
     
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