Prius stops breaking momentarily when coming to a stop.

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by AFindlay, Jul 26, 2018.

  1. AFindlay

    AFindlay New Member

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    My Prius 2007 breaks fine most of the time, however when coming to a full stop, fairly often it will automatically reduce the breaking for a fraction of a second and then apply the breaking again. This occurs when I am traveling slowly and almost stopped. The sensation is similar to sliding on a small patch of ice in the winter.

    No warning lights appear on the dash. My local mechanic drove it with me and noticed it as well. He connected my car up to a computer, but could not see any codes to indicate what the problem was. He couldn't give me any suggestions on what it might be.

    Have other people ever had this issue? Does anyone have a sense of what might be causing it? Thanks.
     
  2. edthefox5

    edthefox5 Senior Member

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    Its brakes not breaks.
     
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  3. ChapmanF

    ChapmanF Senior Member

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    This is completely normal for every Prius if you happen to have hit some sort of one-wheel obstacle (pothole, rock, manhole cover, etc.) while coming to a stop; it is very frequently asked about on PriusChat and the reasons behind it are very frequently explained, including again just sometime in the last week or so, so a forum search should be very productive.

    Now, if it is happening for you during plain smooth deceleration without any wheel having taken a knock, that would be a weird story we haven't heard before.

    -Chap
     
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  4. klevitsky

    klevitsky Junior Member

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    Is this the point at which the car would switch from regenerative braking using the electric motor and the physical braking using the pads/drums? Shouldn’t be a lack of braking though, but maybe if the pads are so worn that there is no grip when it engages the physical brakes?
     
  5. bisco

    bisco cookie crumbler

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    welcome!
    yes.
     
  6. fuzzy1

    fuzzy1 Senior Member

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    And do note that this particular action should always happen at the same speed: 7 mph in a Gen3, I don't know that Gen2 is any different.

    I chalk it up to the regeneration-to-just-friction-braking transition not being fully seamless. I feel it only part of the time. But since it likely relates to mismatch between the two systems, the feel is likely to vary significantly from car to car.

    If this is what is happening, we have heard no indication that it is a hazard. It would happen only during light braking, not serious braking that includes friction pads from the very start.
     
  7. JC91006

    JC91006 Senior Member

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    I think you should clean your brakes and lube your caliper glide pins. The braking should be smooth.

    Did you change your brake pads recently?
     
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  8. wjtracy

    wjtracy Senior Member

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    We only get it if we hit some bumps during the braking
     
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  9. ChapmanF

    ChapmanF Senior Member

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    Yes.

    No. We're not talking about the normal transition at the lower limit of regen speed, we're talking about the forced abandonment of regen to bring in ABS, which can happen at various speeds.

    Exactly. The bumps (usually things like potholes or rocks that change one wheel's speed relative to the rest) are detected, thanks to the wheel speed sensors, as a possible loss of traction. (In most of these cases, you probably haven't lost traction at all, one wheel is just covering a longer or shorter distance so its speed is different, but the ECU can't be sure of that.)

    What's happening up to that point: you are slowing moderately, and the car is using only regen, which only works through the drive train and on the two front wheels. Compared to slowing at the same moderate rate but braking with all four wheels, this places twice the traction demand on the two front tire contact patches, which are the only two slowing the car.

    So, if there's any possibility those two patches are at the edge of losing traction, it is in the car's interest to switch to the four-wheel, conventional hydraulic ABS system. It might not even need to do fancy ABS stuff, just switching from braking on two contact patches to braking on four will probably be enough, if (as the ECU assumes) traction might be just at the edge for the two.

    But here's the thing: the hydraulic brakes haven't been in the game up to this point. It only takes a split second to bring them in, but meanwhile, what about the regen braking of these two front tires, which the ECU thinks might be on the edge of slipping?

    It doesn't want to risk those slipping; they are the front tires, your directional control as well as braking. So it lets those roll right away, simultaneously with bringing the brakes into play, which takes the split second that it takes.

    The common experience of this is when braking gently or moderately and hitting some bump. An often-voiced concern is "what if the same thing happens when I am panic stopping?" and it doesn't. At least, a Gen 3 has a pedal angle sensor on the brake pedal, which it uses to detect the rate at which you're pushing the pedal down, and if you are doing so in a hurry, it classifies the stop as urgent and goes straight to hydraulic ABS, skipping the regen.

    This has been explained countless times on PriusChat. When I bought my Gen 1 ten years ago, somebody here explained it to me. Lots of threads about it. This seems like the second or third time I've written this explanation this month. It might be worthwhile for the mods to choose one of the threads about it over the years, one that explains it well, and make it a sticky ....

    -Chap
     
  10. Starship16

    Starship16 Senior Member

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    Chap just wrote the best explanation I've ever seen. Thanks very much. That should go into a permanent "Prius Brakes" thread.

    Every time I feel that skip, its kind of like an "oh sh*t" moment. What was THAT??! And in my Gen4, I have felt it when braking at any speeds... under the circumstances such as Chap describes.
     
  11. tpenny67

    tpenny67 Active Member

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    I've often wondered why the car seems to "give up" on regen after hitting a bump, this explains the logic perfectly!!

    I assume I'm not the only Prius driver who has learned to steer around bumps and/or ease off on the brake pedal to avoid losing regen :)
     
  12. jerrymildred

    jerrymildred Senior Member

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    Traffics lights here do that to me all the time. 60 mph and just at the last moment, it turns yellow and forces not a panic stop, but one that's hurried enough to not get any regen out of it. Grrrr...

    Good idea!
     
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  13. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    When I was in Russia a few weeks ago, I noticed that the traffic signals all lacked a yellow aspect. Instead, they'd replaced the simple yellow light with a two-digit timer that counted down the remaining time for either red or green aspect- whichever was active at the time.

    It struck me as a really smart way of upgrading traffic control.
     
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  14. ChapmanF

    ChapmanF Senior Member

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    Something else I've noticed (Gen 3 anyway) is that if you have cruise control set, and you do any braking at all (which cancels the cruise setting), that is always treated as 'urgent' and skips regen, no matter how slowly or gently you go on the pedal. Just canceling cruise by pulling on the stalk an instant before braking solves that.

    -Chap
     
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  15. fuzzy1

    fuzzy1 Senior Member

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    While you are not talking about it, I am (and guessed that Klevitsky was too) in order to be more complete. OP hasn't yet provided sufficient detail to distinguish between these scenarios, which are different.

    Your low speed bump scenario is the more commonly disconcerting situation with the greater feel of momentary lost braking. But the 7 mph issue I describe, while smaller and less disconcerting, also happens in some units, and appears even on perfectly smooth surfaces. Both my Prii, past and present, do it, though I don't notice it all the time.
     
  16. jerrymildred

    jerrymildred Senior Member

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    It's extremely rare that I use brakes to turn off CC, so I've never noticed that. I'll check it out. I typically allow enough distance that I have time to use the stalk to turn it off. Also, in traffic using CC, I keep my fingers right next to the stalk so they get there quicker than getting my foot from gas to brake.
     
  17. SRQ

    SRQ Member

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    I don't understand how people confuse "brake" with "break", despite reading the word "brake" all the time inside the vehicle, on digital displays, warning signs, the vehicle manual, online, auto shop signs, etc.
     
  18. BurkPhoto@aol.com

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    Some people use the words interchangeably... just to pi$$ us off! It’s distracting, but decipherable.


    — Bill Burkholder
     
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  19. padroo

    padroo Senior Member

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    I have noticed that Gen 4 brakes are different than my Gen 2. I don't remember getting a light at low speed over manhole covers and loose rocks like I did on my old Prius.
     
  20. kenoarto

    kenoarto Senior Member

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    Gimme a brick

    Posted via the PriusChat mobile app.
     
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