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Brake Grunting

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by CR94, Mar 11, 2019.

  1. CR94

    CR94 Senior Member

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    I've read of other strange brake sounds, but haven't found a thread about a mysterious noise I'm hearing lately.

    Clues:
    1) It used to occur only occasionally, but recently has become more (but not very) consistent.
    2) It occurs as I release the brake pedal after either hard or light braking.
    3) It lasts maybe half a second.
    4) It sounds like sort of a grunt or moan.
    5) Seems to come from under the hood (the hyper-expensive brake controller?).
    6) Brakes function normally (so far!).
    7) Volume is moderate, loud enough to be obvious at low speed or while stopped, but not necessarily loud enough to hear at highway speeds, if it happens then.
    8) Car is a 2011 with almost 97k miles now. Zero mechanical or electrical issues so far.
    9) Brake disks are always cool when I feel them after parking.
    10) I had the dealer change the fluid a couple of years ago.
    11) Fluid level hasn't moved noticeably.

    Who knows what this could be, and whether it's something to worry about?
     
    #1 CR94, Mar 11, 2019
    Last edited: Mar 11, 2019
  2. Mendel Leisk

    Mendel Leisk Senior Member

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    I'm noticing similar sound, and again: just intermittent. Brake fluid's been changed a couple times, it's around 80K kms.

    I noticed it last when stopped at red light on a steep up hill, and I pushed the brake extra hard to activate the hill-hold.
     
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  3. bisco

    bisco cookie crumbler

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    Very beginnings of actuator failing, but it may never fail
    You don’t think it’s the same as the honking threads?
     
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  4. CR94

    CR94 Senior Member

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    Thanks! This occurs as the pedal is being released after light, moderate or hard braking, maybe every third time.
     
  5. CR94

    CR94 Senior Member

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    Are you trying to scare me (and Mendel)?! I don't remember "honking threads," but do remember there have been complaints of actuator failures. This noise isn't loud---so far. Therefore, I hear it only at low speeds or while stopped.

    Thanks.
     
  6. bisco

    bisco cookie crumbler

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    not at all, that's why i said it may never fail. relax and drive it. if things get worse, come back, but until and if, no worries.
     
  7. ChapmanF

    ChapmanF Senior Member

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    I am pretty sure that honk, bark, quack, squawk, grunt, and moan are nothing but different people's way of describing the same sound that's described in T-SB-0363-10, which I suspect to be fluid returning under pressure (perhaps of some trapped gas bubble) through a partially open solenoid valve, honking it like a reed.

    The version of the brake firmware mentioned in the TSB (F152647108 if you have the 15 inch wheels, F152647128 if you have the 17 inch). is updated to cut the honking (by, I suspect, modulating the valves differently, but that's a guess). When mine started quacking noticeably, around a year ago, I checked my firmware version and it was lower than that, so I had my dealer update it and the quacks went away.

    That's what I would try first, unless you already have that version or later.

    If you get the updated firmware and still have grunts/honks, I would consider going through a bleed. My speculation is that gas bubbles trapped in the system (perhaps escaping slowly over time from the accumulator) play a role in this, and I am also beginning to think that a sufficient amount of gas accumulating (without other symptoms than this) could be connected to the floored-pedal-no-stoppie events that seem to get occasionally reported, as happening out of the blue with little advance warning.

    At this point that's nothing but speculation on my part, but that's the way I would approach it ... check the firmware version, and as a followup, see if you can get any bubbles out of the system.

    The TSB is not a recall, so getting the firmware updated will involve a charge for shop time (getting any extra stuff out of the back may save time, because they attach a power supply to the battery before starting the reflash). But it wasn't much, and I like my quack-free brakes.
     
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  8. CR94

    CR94 Senior Member

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    Thanks, ChapmanF, for the informative reply!

    I'd think any significant loss of gas from the accumulator might spell trouble, even aside from the noise, and even if a "floored-pedal-no-stoppie event" never happens.
     
  9. ChapmanF

    ChapmanF Senior Member

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    Well, the less gas there is inside the accumulator, the more the pressure changes for a given volume of fluid in or out, and the more often the pump has to run, until it's eventually cycling on a period of seconds, just like the well pump in your house if you've got a pressure tank with too little air.

    That's a failure mode that gets reported on PriusChat pretty commonly; it develops gradually over many years and miles, and when it finally gets annoying enough (or annoys the car enough to set codes) you grit your teeth, grab your wallet and buy an accumulator. It doesn't really come out of the blue as a surprise.

    And generally the worst case, even if the pump fails, or all the pressure's lost from the accumulator, is that you end up with loss of pressure assist to the brakes. The pedal is high and won't press very far, and you have to stand on it hard to slow the car much, but aside from the lack of assist, you've still got brakes.

    There are way fewer floored-pedal-no-stoppie events reported on PriusChat, but there are just enough to think that the drivers probably weren't all imagining it, and it's a much more, um, undesirable thing to have happen. So far, I don't think anyone has offered a definitive explanation, and what I'm thinking is only speculation at this point. I don't have any experimental results to say if I'm on the right or wrong track.

    But what I'm thinking has to do with the small amounts of nitrogen that may escape from the accumulator over the years, because where does it escape to? It doesn't leave the system, just mixes with the fluid in the lines.

    In your granddad's car, you'd fill the system with fluid and bleed the bubbles out, and it would stay bubble-free forever unless you had to open a line to replace a part or something. But in this system, even if it's never opened to the air, there's a built-in source of nitrogen bubbles over time, if there's some slow escape from the accumulator.

    Also, in granddad's car, you could always tell if there were bubbles in the lines. If they were small bubbles, the pedal would be slightly mushy, and go closer to the floor before slowing the car. If they were medium-sized, the pedal would be even mushier and you'd have to push it even farther to brake the car. If the bubbles were big, your pedal went to the floor and you couldn't stop the car.

    But the system we've got is very good at masking the early stages of that. The braking is electronically controlled, there's a big accumulator filled with fluid under pressure, and valves let it into the brake lines until the sensors read the target pressure that the ECU has chosen.

    If there are small bubbles, the ECU has to hold the valves open just an instant longer to build the target pressure. You don't notice anything different as the driver. If there are medium bubbles, it takes just an instant longer than that. Still nothing feels any different to the driver. Maybe you hear a little bit more of a squirty sound than you were expecting.

    Where I'm wondering if there might be trouble is if that problem slowly, and unnoticeably, develops through small bubbles and medium bubbles, and one day reaches a point where the ECU has to hold the valves open so long to reach target pressure that it says "oh my heavens, something is wrong here, I'm going fail-safe."

    ... which for almost any other reason would be the reasonable thing you would want it to do, except in this case going fail-safe more or less turns the brakes back into your granddad's, where you have nothing but your pedal stroke to make pressure and too much gas in the lines for it to do anything, and it goes to the floor and the car doesn't stop.

    Again, I can only say I'm speculating, but that's the story that makes most sense to me.
     
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  10. CR94

    CR94 Senior Member

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    Thanks, ChapmanF ! Your plausible "story" makes makes more sense than anything a dealer is likely to tell me. If proven true, it should perhaps be a sticky. Permeation of N2 out of the accumulator should occur at a similar rate in all examples, I'd suppose.

    My car just passed 8 years and 97k.