Does GIII have brake logic override?

Discussion in 'Gen 3 Prius Main Forum' started by maomao, Mar 16, 2010.

  1. maomao

    maomao New Member

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    I heard previously that Toyota does not have "brakes Priority" functions in their cars, but will install it on all new cars and retrofit some of their older models.

    But yesterday's report about the "run-away car" investigation quoted Toyota technician as saying the 08 Prius had some sort of brakes priority function. If you moderately apply brakes while the gas is open, they system cuts the power of the engine (if I remember correctly).

    Could someone her clarify if our GIII's have brakes priority? If not, does Toyota have a plan to retrofit our cars?

    Thanks!:)
     
  2. justlurkin

    justlurkin Señor Member

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    Re: Does GIII have brakes priority function?

    Yes it does. The HSD computer will automatically command the gas engine to power down if both gas and brake pedals are pressed at the same time, in order to prevent the MG1 electric motor in the transaxle from getting burnt out.

    This is true of ANY car that has a Power Split Device transaxle, from the Prius to the Ford Fusion Hybrid.
     
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  3. maomao

    maomao New Member

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    Re: Does GIII have brakes priority function?

    Great to know. Thanks a lot!
     
  4. jdcollins5

    jdcollins5 Senior Member

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    Re: Does GIII have brakes priority function?

    Why not try it on your car to verify it and understand how it works and feels?

    In a safe area, with the right foot on the accelerator pedal at some slow speed, lightly press the brake pedal with your left foot. Be prepared to brake. If you have the HSI screen being displayed, you can see the power bar shoot to the regen area on the left as the engine reduces power and the car immediately begins to slow.
     
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  5. David Beale

    David Beale Senior Member

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    Re: Does GIII have brakes priority function?

    Prius does have throttle cutback but it's complicated. From Hobbit (some verified in my GII):

    If you are already applying the brake and floor the throttle the engine will rev to about 2000 RPM. This is how you force charge the battery.

    If you lightly apply the brake -after- the throttle is floored (or any more than at 50% down as I recall) the HSD will fight the brakes. This is how you overheat the brakes (fronts).

    If you press firmly on the brake pedal -after- the throttle is floored (or any more than at 50% down as I recall) the HSD will throttle back the engine to just over idle. This is how you bang your unbelted passengers head into the dash and launch objects inside the car. BEWARE!

    The sensing medium is NOT the brake lamp switch, it's a hydraulic pressure sensor in the system.
     
  6. john_dough

    john_dough New Member

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    Re: Does GIII have brakes priority function?


    Thanks for this explanation David---I'm wondering, if somehow a faulty computer causes gas to flow (accelerating without depressing the accelerator pedal), will brake override work, or does the circuitry need to sense motion of the accelerator pedal?
     
  7. Politburo

    Politburo Active Member

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    Re: Does GIII have brakes priority function?

    If the computer has a fault, no one can say what might or might not occur.
     
  8. qbee42

    qbee42 My other car is a boat

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    Re: Does GIII have brakes priority function?

    Gas going to the engine will not cause a Prius to move. The control system also has to apply a field current to MG1.

    Some combination of failures that create a runaway situation will always be possible. The question becomes one of how likely. Most modern electronic systems are less likely to run away than old fashioned mechanical linkages.

    Tom
     
  9. justlurkin

    justlurkin Señor Member

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    Re: Does GIII have brakes priority function?

    If the HSD computer is malfunctioning and causing the gasoline engine rev up to full speed and you jam on the front disc brakes, it will cause the MG1 electric motor in the transaxle to overload and burn out. A destroyed transaxle will cause your car to come to a stop. PERMANENTLY. (i.e. the car can't move again without a transaxle replacement.) Can't think of a better brake override than that! :D

    How the Power Split Device transaxle works: Toyota Prius - Power Split Device

    Use the PSD simulator on that page to see what happens when you hold the drive wheels/outer ring gear/MG2 at 0 RPM (disc brakes fully engaged), while revving the ICE to max RPMs (almost 4500rpm). You will see that the MG1 / sun gear will be taking the full brunt of the gas engine's power, revving up to 16,000 RPMs-- That is 6000 RPMs above the 10,000-RPM protective limit for MG1 to keep it from burning out.

    So if the HSD computer is working properly, it will never allow the gas engine to rev up when the brakes are stomped on, so as to protect MG1 from burning out. And if the HSD computer has malfunctioned and is allowing the gas engine to run wild while you stomp on the brakes, the MG1 will sacrifice itself for your safety and self-destruct the transaxle.

    Either way the car comes to a stop.

    This is true in ANY car that has a Power Split Device transaxle, including the Prius (ALL GENERATIONS), Camry Hybrid, Nissan Altima Hybrid, Ford Fusion Hybrid, Ford Escape Hybrid, etc.
     
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  10. HTMLSpinnr

    HTMLSpinnr Super Moderator
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    Re: Does GIII have brakes priority function?

    From the end-user perspective - Both Gen II (2004) and Gen III (2010) do have brake override - you do need to mash the pedal before it kicks in on both cars.
     
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  11. Tideland Prius

    Tideland Prius Moderator of the North
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  12. Susan4ET

    Susan4ET Member

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    Re: Does GIII have brakes priority function?

    I played with Brake Override the other day and there was nothing hard or mashing about it. It feels pretty much the same as braking when you have cruise control (CC) set. There is nothing dangerous about it. You don't slam the brakes to the floor. Just maintain any speed with the accelerator and hold that speed and then apply the brakes until you feel and/or see on the HSI the engine rpm fall off. This is something that every dealer should have you demonstrate when you test drive the car. That and how to put the car into Neutral while moving [without flipping it. ROTFLMAO.]

    Question... If you pick out a good stretch of straight road with no traffic is it perfectly safe to go say 50 or 60 and slam the brakes on and take the car to a full stop--thus testing Brake Assist? If I understand enough correctly you track straight and there's no screeching tires? If you do it once is it still hard on the brakes?
     
  13. jdcollins5

    jdcollins5 Senior Member

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    Re: Does GIII have brakes priority function?

    Susan,

    I did this on mine at 60 mph on an open highway with no one in front. I pressed the accelerator to the floor to WOT and then hit the brake with my left foot. The HSI went hard left to the regen area and the car came to very fast stop. Straight ahead and no screeching of tires.

    Try it. I cannot see this having any adverse affect on the brakes to do this.

    Dwight
     
  14. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Re: Does GIII have brakes priority function?

    I am betting againist Rainbow Bright finding Jimmy Hoffa's body if the computer has a fault. I live in a world where fantasy has no place, so there are limits to what can happen if the computer has a fault. With computer failure you will still have hydraulic friction brakes that will stop the car (in a longer distance) even with the engine at full power.
     
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  15. Susan4ET

    Susan4ET Member

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    Re: Does GIII have brakes priority function?

    Thanks Dwight...for the previous post. I'll try it sometime.

    I've watch to many videos now of how well the Prius does stopping when everything is working normal--no more. What I'd like to see now is the real thing where Toyota demonstrates in a test cell (for instance) that braking forces of an average built person equivalent could stop a 2010 Prius going around 60 mph using hydraulic brake pressure only--no Brake Assist. I think I've read that in actually doing this the transaxle is trashed. Perhaps Toyota could dig up and share a qualifying demonstration proving that in an emergency the car is stopped this way even if at some point the transmission is destroyed?

    Are the brakes and in particular Brake Assist designed such that if something catastrophic occurred such that all ECUs were dead including the brakes ECU, but you still had the independent power to Brake Assist's electric hydraulic pump keeping its accumulator charged up, you would still have assisted stopping power?

    Everybody knows the car qualifies and here I am asking you to doubt this and find some certifiable qualifying proof eliminating here-say and speculation.
     
  16. qbee42

    qbee42 My other car is a boat

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    Re: Does GIII have brakes priority function?

    Braking with manual hydraulic brakes will not harm the transaxle. Perhaps what you are thinking about is MG1 overspeed. If you stop the ICE, remove power from MG1, and go fast, you can force MG1 to spin too fast. This could wreck MG1, and potentially damage other parts of the transaxle. Note that this in no way is connected to manual or power brakes. The two come together only in that a total power failure could lead to a loss of power assist and power for MG1.

    Stopping a Prius with manual hydraulics shouldn't be hard. I did much of my driving in cars with only manual hydraulic brakes. From looking at the design of the Prius, I don't see anything special that would make it hard to stop.

    The power brakes do have backup power for a limited number of applications. This works much like the vacuum reservoir on normal power brakes, except in the Prius it is a bank of super capacitors.

    Tom
     
  17. usbseawolf2000

    usbseawolf2000 HSD PhD

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    Re: Does GIII have brakes priority function?

    There are more ECUs (computers) than you can count with both hands. If one of them has a fault, an error will be logged. From that, we can say what occurred.

    There is a chance that all ECU can failed at the same time and no errors are logged. There is also a chance that the sun will not come up the next morning.
     
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  18. Susan4ET

    Susan4ET Member

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    Re: Does GIII have brakes priority function?

    I drove a number of cars in the past too that had no power assists BUT I never tried braking while holding my speed with the accelerator going say 60 mph. Was the 'design story' back then that you could always overpower and stop a 60 mph car with a stuck accelerator?

    Also, it seems to me when I have played with bleeding off power brakes and/or steering at slow speed (maybe in a Honda Civic prior to my first Prius) that it took a lot more force to brake and steer then it would have with a car that didn't have any braking or steering assist to begin with (iirc)?
     
  19. qbee42

    qbee42 My other car is a boat

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    Re: Does GIII have brakes priority function?

    On normal cars, yes. I assume some highly powered cars could be hard to stop, as well as low speed vehicles with a lot of gearing. Brakes were normally designed to stop a car and stall the engine, assuming a hard application before brake fade set in.

    Tom
     
  20. Politburo

    Politburo Active Member

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    Just to clarify. My point is that once a computer is in an unhandled error state, as has been asserted by several posters (you know who they are), then you cannot predict what will happen. I fully understand that this possibility is quite remote.
     
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