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2010 Toyota Prius - Click above for high-res image gallery

2009 has not been a good year for Toyota. After getting its first full year loss in seven decades, it faced its biggest safety recall ever over unintended acceleration caused by floor mats and or throttle control software. Now, as the year draws to a close, a new potential problem has sprung up, although it's not nearly on the same scale as the recall. The new problem is related to the braking system on the latest Prius, which debuted early this year.

The problem, which has been reported to National Highway Traffic Safety Administration (NHTSA) by several dozen drivers, relates to a loss of deceleration when transitioning from regenerative braking to friction braking. At this time, NHTSA has not opened a full investigation but it is monitoring the situation.

This is actually quite a complicated issue and it highlights one of the reasons why it takes automakers so long to bring cars to market. Read on after the jump for some discussion on what might be happening.


[Source: The Detroit Bureau]

The problem is that hybrids (and upcoming electric vehicles) actually have two different braking mechanisms. They have the traditional hydraulic friction braking system that has been used for most of the past century. However, they also use regenerative braking to convert kinetic energy back to electrical energy to charge the battery. The problem arises from the fact that there is only one control mechanism, the brake pedal. So electronics and complex hydraulics are used to blend the amount of braking from the two systems to provide a net deceleration in proportion to the amount of pressure the driver applies to the pedal.

This would all be fine if the behavior of both systems was completely predictable. If that were the case, both could be modeled mathematically and the estimated torque from each could be calculated. Unfortunately, friction brakes can exhibit significant variability caused by wear, humidity, temperature and numerous other factors. The controls and calibration engineers spend tens of thousands of man hours testing and developing the systems to attempt to ensure that they behave predictably and consistently in over the life of the car and under different operating conditions.

The problem reportedly being experienced on the new Prius relates to an apparent loss of deceleration when the brakes transition from regen to friction braking. Having worked on the controls for similar brake systems in a previous life as an engineer, this is not an uncommon problem. Typically, it occurs when the friction brakes are producing less torque than the model predicts.

A number of things could cause this. The most basic is the brake linings, which could have less friction than expected. In this case, the control algorithm should be able to learn and compensate for this. The problem might also be related to the pressure sensors in the hydraulic unit that are part of the closed loop control. Sensors can have erroneous signals or drift over time and temperature. Again, there are mechanisms to correct for this, but they are not perfect. Not being familiar with the details of the Prius system, we can only speculate about has changed in the new version.

Based on the information in the reports, there is one more possibility. Apparently, at least some of the owners experienced the perceived loss of deceleration after hitting a bump or pot-hole. When we reviewed the Prius, we noted that its newly revised suspension setup seemed to handle larger road imperfections well, but it was a bit over-damped on small sharp inputs. This could cause the tire to lose adhesion with the road in this case and thus start to exhibit slip. The brake system could be detecting this sudden increase in wheel deceleration (as opposed to vehicle deceleration) and proactively reducing brake torque to try and prevent impending wheel lock and the need for full-blown ABS control. If (and we stress IF) this is being done too aggressively, it could be what drivers are feeling. We never experienced any such behavior when we drove the Prius, but we wouldn't rule it out under the right conditions. If you own a third-generation Prius and have experienced anything like this, let us – and more importantly NHTSA and your Toyota dealer – know. It may not be a real problem, but if it is Toyota, will need as many data points as possible in order to diagnose and fix it.


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    • 1 Second Ago
  • 36 Comments
      • 5 Years Ago
      I have a 3rd gen Prius. I believe that I can duplicate this phenomenon. It is most noticeable for me at a 4-way stop near to were I live. There is a small bump that crosses the entire lane a short distance from the stopping point. As the rear tires hit that bump the car feels like it is surging forward. It is only a reduction is braking force, but because of the close proximity to the stopping point, it is a little unnerving.

      I have complained to my Toyota deal, but they claim to have no knowledge of any other complaints and didn't even give the appearance of being interested.

      There are several other inconsistencies in the feel of the hybrid power transitions, but this one is a little scary to experience.
        • 5 Years Ago
        yes, I experience the same surge at low speeds when I am breaking at a stop at the bottom of the hill where I live. When the front wheels hit a certain dip in the pavement, its as if the brakes lose their efficiency for a second or so. It happens at 3-5 mph. mostly unnerving.

        I've also noticed that the brakes are extremely touchy when backing out of my garage in the morning... a slight tap on the brake pedal in reverse triggers a sharp braking action.. this seems to go away soon.

        Plus, I've noticed the car hydroplanes easily above 55 mph.
      • 5 Years Ago
      I have experienced this issue. It occurs. when breaking lightly and hitting uneven pavement (wheel slip). Th breaking pressure decreases giving the perception of a surge. I do not believe it is a safety issue, as it does not happen in heavy breaking.
        • 5 Years Ago
        My experience is that it does happen in moderately "heavy" braking. I'm not sure what you call "heavy braking" but it happens to me even when I am braking what I would refer to as "heavy braking".

        Another problem I am experiencing (not in the beginning though) is the brakes almost locking up when I intend to brake lightly. I actually hit the windshield hard with my head once it locked up so tight. This IS an obvious safety hazard and must be caused by a different problem than the one experienced when hitting a bump while braking.
        • 5 Years Ago
        Perhaps now is the time to introduce electric brakes because solenoid and regen braking should be much easier to combine and control via electronic sensing and control.
        • 5 Years Ago
        Totally electric brakes are not fault-tolerant enough. Systems that are reliant on measuring wheel deceleration are possibly also not fault-tolerant enough. Yes, you can use multiple computers and all sorts of redundancy to make it into an aircraft on wheels. Or you can use a simple, as-direct-as-possible, mechanical / hydraulic mechanism that is a whole lot easier to validate - and a whole lot easier to establish after the fact whether a system fault was responsible for a collision. Pedal meet hydraulic cylinder meet wheel cylinder meet brake pads. Still works after a collision? Brake failure in the vehicle wasn't at fault. Period. If there are a ton of computers between the operator and the tires, you don't really know ...

        If anyone ought to be taking notes on fault-tolerance based on recent events, it's Toyota.
        • 5 Years Ago
        Happens to me too with my 2010 Prius. Also occurred with my 2006 Prius. Exit 79 on I-77 approaching Rock Hill, SC. I take that about every other week. Just as has been said, uneven pavement and a little wheel slip or lock (I figured the wheel was locking while "bouncing" for a second in the air. If that is the case, it is a suspension issue.

        No big deal. It is just a sensation. But happens ever time, so it would be a good test facility for Toyota and the NHTSB.


        Bill G.
        Mt. Pleasant, SC
      • 5 Years Ago
      All owners of ABS should be told that ABS in non-skid situations will have longer breaking distances. Most are not told this.

      Our 04 Prius has the ABS break loss regularly and require letting off the pedal and reapply to get full breaking. A problem.

      Our Prius otherwise is a great car, 50 mpg doing 70mph, AC on, 4 people, full of camping gear! Why cant Detroit get it by now. Toyota is eating their lunch and they are getting billions of tax dollars for better cup holder designs in their "tanks".
      • 5 Years Ago
      Toyota Prius 2010- car surged while braking to a halt. It felt as though i had suddenly lost braking power for a few moments, like i had hit an ice patch. But the road surface was dry. This was the second time it happened to me, the first about a week or 10 days ago. Car was
      purchased on 1/4, delivered on 1/8/2010. 500 miles on the car.
      Both times the car was moving at relatively low speed (10
      mph or less). Approximate Incident Date: 1/16/2010. Will report to dealer on Tuesday.
      • 5 Years Ago
      I have this happen on my '05 Prius (2nd gen). The ABS explanation makes sense. I lose braking power when I hit a bump, or if I run over a wet sewer lid. It's disconcerting, but nothing tragic has happened.. yet.
      • 5 Years Ago
      Sudden application of brakes, especially in a turn could cause a skid or spin. Fun for some in a giant parking lot, not so much on the road. Seeya in court!
      • 5 Years Ago
      Yeah, my 2010 does the same thing. I don't imagine it's a huge safety concern as it has only happened at low speeds, but I'm fairly paranoid of rear-ending someone or having some other low-speed collision in a parking lot or someplace similar. It's REALLY disconcerting to have the brakes stop working suddenly, even if only for an instant. I never had this problem in previous cars (G35 and Impreza) with ABS and traction control.
      • 5 Years Ago
      To Dan:
      I believe the reason why your Prius has trouble climbing over hills doesnt have anything to do with tires or the traction control, it is probably the size of your engine. Having driven a Prius just a couple of days ago, I don't know how anyone could drive that car with any sense of confidence. There is literally no acceration in that car! It isnt even zippy! Why anyone would buy a Prius over a Golf Diesel is beyond me, or better yet just buy a Civic.. atleast you'll get a proper car. And to all the environmentalist, lets not forget how dangerous the batteries are for the environment.
        • 5 Years Ago
        Define "no acceleration". Toyota Prius 0-60 9.8 seconds, Golf 2.5 TDI 0-60: 8.6 seconds (manufacturers' numbers). It's slower but not that slow; meanwhile getting 51/48 MPG instead of 30/42 is a huge difference. Some people don't like the Prius because you have to floor the pedal before the car leaps forward, but that's their preconception about driving feel. I've no doubt the Golf is much more fun to drive than a Toyota appliancemobile, but did you drive the 2010 Prius or the previous model?

        "And to all the environmentalist, lets not forget how dangerous the batteries are for the environment."
        No, let us ignore people who spout blatant untruths. NiMH batteries aren't any worse for the environment than the other metals in a car. The materials are valuable and are and will be recycled. Lead-acid batteries in regular cars are far more dangerous, the lead is toxic and makes you stupid and the acid is... acid.
      • 5 Years Ago
      I have experienced it also, usually in parking lots around 5 or 10 Mph when you hit a bump while braking. You feel less deceleration for a fraction of a second. I dont think it is realy dangerous. It doesn't happen in the snow, maybe because the ABS is already in function...
        • 5 Years Ago
        Have you also experienced the brakes being way to aggressive at times? I have 13,000 miles on my 2010 Prius and have noticed lately sometimes when braking lightly it will act like I just stood up on the brake. I was actually thrown into the windshield once when I had forgotten to fasten my seat belt. don't know what could be causing this problem.

        I also have the "bump" problem and assume the regenerative braking is tied to how much deceleration is sensed at the wheels so that when the bump is hit and the load is lifted momentarily from the wheel it stops (slides) and the sensors sense this and releases pressure on the brake which causes what feels like a "surge" which is actually just a momentary releasing of the braking.
        • 5 Years Ago
        The SAME thing happens in a regular car when you hit a bump while braking! I bet this is more to do with suspension/tires/ABS than regen (unless the bump causes you to move your foot on the brake pedal).

        The future of regen isn't blended anyway, it's more like the Volt or other EVs: regen braking happens when you let off the accelerator, friction braking happens when you hit the brake pedal, no blending. Some people don't like this idea, but it's not hard to get used to, it's no different than the feel of driving around in a low gear, and "coasting" just happens with steady accelerator pedal when no power is needed to maintain the current speed (it's not more efficient to coast only while your foot is off the accelerator).
        • 5 Years Ago
        No didn't hahe this problem yet, mine only have 6.000 Km.
      • 5 Years Ago
      I can tell you the complaints listed are most definitely legitimate. I bought a 2010 Prius in July of this year. I noticed the braking problems not long after I drove it home. I took it back to the service department – they sent out a guy who drove it around and came back saying it didn’t do anything when he drove it.

      I should have driven it with him in the car and showed him what I was talking about. At that time my problem was the problem spoken of in the article that some people experience when hitting a bump in the road when braking.

      It happens when braking quickly and hitting a bump in the road such as you might come upon when turning off into a paved parking lot and crossing uneven pavement, etc. All of a sudden for just a moment you have no brakes at all. This is in the middle of a moderately hard brake mind you. So I think it is a safety issue. Obviously if you are braking moderately hard you need to be decelerating. To be trying to slow quickly and all of a sudden feel the car without brakes entirely for even a moment is an uncomfortable feeling.

      For example, there are some rough spots in the pavement on a side road I live on. Since these bumps are on a steep hill it will do it if I start down the hill to fast and am having to apply the brake more than slightly.

      So far no harm done other than a loss of confidence in Toyota engineering. But in a very tight situation when you need to get stopped quickly, such as someone or an animal stepping in front of you and you hit a bump it could very well be enough of a loss of braking to cause an accident that could have been avoided if the brakes worked as they should be expected to work.

      Another problem which I assume is related to the “regenerative” braking is that if you have to put your brakes on quickly unexpectedly you almost always brake much quicker than expected (well, assuming there is not a bump in the road in which case you’ll decelerate slower than you expect). In fact, I forgot to put my seatbelt on once and was thrown forward and my head actually hit the windshield. I’m not talking of a “stand-on-the-brakes-and-slide-the-tires” braking – just quick braking like you might do to quickly decelerate. It is similar to hydraulic brakes sticking you might see on an older car – however in that case the problem is normally one wheel locking up and sliding so the car continues to move. This problem is on all four wheels so you stop quick! I don’t remember this problem occurring when it was brand new but after it had a few thousand miles on it. And this quick braking problem appears to be getting worse. (I have a little over 13,000 miles on the Prius now.)

      These are definitely problems that needs to be addressed by Toyota.

      I think I’ll print this article out and take it back to the service department. By now they may have received other complaints and will take mine more seriously.
      • 5 Years Ago
      The recall and this are further indication that the Prius is not a safe vehicle.
      Here are some of the obvious engineering / design flaws (including the recall). All of these have been documented in the press, including the NY Times, IIHS and other reliable sources
      - Loss of steering boost and conventional brakes, including lockup, in skids due to computer reboot
      - Floor mat or transmission or throttle related involuntary acceleration
      - Malfunctioning brakes
      - Battery packs bolted under rear passenger seat can break loose and fly across the cabin, causing injury in the event of rear or rear quarter collision
      - EM field emissions caused by battery packs can cause driver to fall asleep
      - Poor acceleration and handling, preventing driver's ability to maneuver out of accidents
      - Roof strength problems in rollovers
      - Potential for battery fires in passenger compartment
      - Emissions from batteries inside passenger compartment
      - Rear passenger side impact rating below par
      Until and unless these problems are resolved, the Prius is not particularly safe, and for all its fuel economy benefits, a risky car to drive, especially in areas prone to inclement weather.
      • 5 Years Ago
      Wincros states: "hybrids are the only solution at this moment"

      Well you are entitled to your ignorance but may I suggest you take a broader view and consider all the other options that we have available.

      From the Sunday Times (UK) 16 Mar 2008: "Oh Mr Prius, it seems you're quite the little gas guzzler"

      "For all my defensive driving, slippery bodywork and hybrid technology, my average fuel consumption was 48.1 MPG (imp). I'd lost to a BMW and I was dissappointed; I had never driven so slowly or carefully for so long in my life" Jason Dawe motoring journalist

      And that's before we rip out the Lithium from Bolivia for LiON batteries or ship Japanese cars all over the globe instead of buying locally or waste agricultural land on ethanol production because Gasoline is getting rarer and more energy intensive to refine......

      Yes, Hybrids have a place but there are more efficient ways to travel.

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