How can there be ANY runaway Prius'es if they have brake override?

Discussion in 'Gen 3 Prius Main Forum' started by alokeprasad, Mar 10, 2010.

  1. CharlesJ

    CharlesJ Member

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    You must have an interesting court system then.;)
    The CHP is trained observer as I bet is your RCMP and equivalents.
    The smell lingers for a long time around the brake system so he could very well smell the the hot break pad smell. I smell it frequently on the highways, in my car with windows rolled up. It doesn't take much.
     
  2. mgb4tim

    mgb4tim Noob

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    I pressed my brake and accelerator pedals to the floor at about 30, and the ICE stopped. I guess you need to get the brake pedal to the point as if you were trying to engage the hill assist.
     
  3. CharlesJ

    CharlesJ Member

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    That is what I need to test next, more aggressive baking.:D
     
  4. ScottG10

    ScottG10 Member

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    My brake lockout thingy works as soon as the brake lights come on. After the brake light comes on the eng doesn't pull anymore but it will rev slightly.
     
  5. CharlesJ

    CharlesJ Member

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    Thanks for the reminder:D It will sink in one day:D Certainly different from a lifetime of experience in other cars:D
    Perhaps because the Pri is so different from the other cars, that can be a contributor for most people, certainly not the PC membership with extensive experience in the Pri.Old habits are hard to break.;):D
     
  6. Harold Bien

    Harold Bien Member

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    I would be much more interested in hearing the CHP testimony - did he observe the brake lights on the Prius to be activated as he was approaching the vehicle from behind the entire time? While we will never know as he was traveling whether or not he was actually "standing" on the brakes, we do know that at the end (when he did successfully stop) his brake lights were ON and he decelerated.

    What would be really interesting would be an observation of brake lights ON (and presumably brake over-ride activated unless it's dependent on a separate mechanism) AND acceleration of the car. That _would_ be rather strong evidence to support Sike's case.

    So I wonder - did he "put on a show" when CHP arrived? Or as he was driving up he saw the brake lights remain lit the entire time?
     
  7. hobbit

    hobbit Senior Member

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    Kbeck, you really need to go to autoshop101.com and pull down all the
    hybrid training material, which is based on how the Prius works, and
    read up before forming entire elaborate bodies of fallacious theory
    based on one or two tiny bits of information. You've been doing this
    for weeks, and it's getting old. Engineering theory is fine, but not
    when it's completely orthogonal to what ALREADY EXISTS in the field
    and where you keep making it obvious what you haven't studied.

    The brake light switch is not really a "microswitch", it's a fairly
    substantially-sized can that screw-adjusts in its bracket to set the
    plunger depth. It's a double-pole, double-throw device which closes
    the brake-light switch and opens a second set of contacts that goes
    to an auxiliary input at the hybrid controller. The physical depth
    of this determines where in the brake pedal stroke the brake lights
    actually go on, and from the reports appears to be possibly misadjusted
    at the factory for the 2010s. There are no pressure sensors in this
    switch, just electrical contacts.

    The pressure sensors in question are inside the brake actuator, and
    receive input from the normal hydraulic master cylinder when the pedal
    is pressed. The pot mounted on the pedal pivot is only a RATE sensor,
    not a POSITION sensor. That helps determine how "paniced" a given
    pedal application is, and changes braking algorithms accordingly.

    The "override" functionality comes from hydraulic pressure thresholds,
    not the brake-lights line. That's why the brake pedal needs meaningful
    pressure, i.e. the driver really wants to apply enough brake to begin
    stopping, before the engine override is asserted. A light press to
    just light the brake-lights isn't meaningful enough to bother. This
    is actually useful, when trying to get rid of interlopers on your six.

    All that hydraulic pressure simply goes to a dead-end spring piston
    known as the "stroke simulator" under normal operation, and from
    there on the braking computer fires solenoid valves to send pressure
    out to the wheel cylinders. With feedback from yet more pressure
    sensors. If all that fails for some reason, there's still a direct
    hydraulic path to the front wheels [which take the brunt, by far,
    of any braking event].

    The Prius engine "keeps revving" as you come to a stop in the both-
    pedals-applied scenario because below a certain very low speed, both
    pedals applied means "I want to force-charge the battery". This is
    absolutely by design, and helps simulate the "transmission loading"
    tests that auto techs do at a standstill to determine engine running
    characteristics. Except in the Prius' case the energy can be sent
    usefully to charge the battery [up to a point] instead of wasting
    it all as heat.

    Please try to gain a better understanding of how the car works,
    whether or not you agree with every small detail of the design.
    Study hard enough, and you could walk away with that million-dollar
    "eureka on cause of SUA" prize that Edmund's is offering.

    _H*
     
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  8. tumbleweed

    tumbleweed Senior Member

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    Thanks Hobbit, that explains why I observed what I did during my "brake override test". When holding the brake down at full throttle I did experience override but the engine did not go to idle it throttled itself down to around 1400 RPM as I recall. I didn't think about the force charging function, but that explains what was happening.

    I can also confirm that the pedal switch has absolutely nothing to do with the brake override. I now have my brake light pedal switch readjusted up toward the top of the pedal throw but the override function still requires quite a bit of pedal travel just as it did before the adjustment. Probably not relevant to this discussion but the brake lights on the 2010 are also turned on by the cruise control when it applies the brakes.

    From what I observed while testing the adjustment the brake pedal switch actually does three things: 1. it turns on the brake lights 2. it cancels the cruise control and 3. it is one of the inputs that allows the car to go to "Ready" and allows you to shift into and out of P and R when the car is on.
     
  9. FormerlyOnlyV8s

    FormerlyOnlyV8s New Member

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    My "other" car is 07 400+ HP Corvette Convertible that I drive hard and race regularly (autocross, 1/8, 1/4, standing mile drag strips and Sebring FL Road Course) and if I am to believe the national news, I am in more danger driving our Prius. :D Never would have guessed. Not to pass early judgement but, to say the least, I am a little leery of the latest story of acceleration.

    BTW on the Prius, tried shifting to neutral: worked; tried the off button: worked; tried stopping the car while accelerating: worked.

    Waiting anxiously "for the rest of the story."
     
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  10. Walt999

    Walt999 New Member

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    This was either a "set-up" or he is positively stupid. Furthermore, there isn't a car made that has an engine with enough power to overcome the braking system. Look at it this way.
    Normal brakes (even without power assist) can stop a car traveling 60 mph in about 100 to 200 feet. For an engine to be even that strong, it would have to be able accelerate the car to 60 mph in 100 to 200 feet. When is the last time that you saw a car that could hit 60 in 200 feet?
    Walt
     
  11. autoxic

    autoxic Commuter

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    I believe there is a power override, if you press the brakes hard. You did not try that. I did. Full throttle plus hard brake pedal pressure makes you stop, at least in my 2004 Prius.

    Here is what Sikes _might_ have done: he used the brakes intermittently, pressing lightly or just enough to keep his speed up in the 70-95 MPH range. This moderate brake application over 23 minutes overheated the brakes and caused a fair amount of brake dust buildup.

    If Sikes is telling the truth, I'll feel pretty bad for doubting him, but I'm siding with Toyota so far.
     
  12. masterpanda

    masterpanda Junior Member

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    I have been dreading testing my 2010 as you did. And your result is surprising. However, I have read that to trigger the brake override, you have to depress the brake a lot. Did you?
     
  13. cossie1600

    cossie1600 Active Member

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    50% according to Toyota's press release.
     
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