• Dec 26th 2009 at 5:37PM
  • 59
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-ever safety recall 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. This time around, the issue 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]

Hybrids and EVs actually have two different braking mechanisms. In addition to the traditional hydraulic friction braking system that's been used most of the past century, they also employ regenerative braking to convert kinetic energy back to electrical energy to charge the battery. A problem arises, however, in that there is only one control mechanism: the brake pedal. As a result, electronics and complex hydraulics are used to blend the amount of braking from the two systems to provide net deceleration proportional 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. In that case both could be modeled mathematically, and the estimated torque from each could be calculated. Unfortunately, the friction brakes in particular can exhibit significant variability caused by wear, humidity, temperature and numerous other factors. Control and calibration engineers spend tens of thousands of man-hours testing and developing the systems in an attempt to ensure that they behave predictably and consistently over the life of the vehicle and under different operating conditions.

The problem reportedly being experienced in 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 braking systems in a previous life as an engineer, I can say from experience that 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. The problem could 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're not perfect. Not being familiar with the specifics of the Prius system, we can only speculate about what has changed in the new version.

There's another possibility based on the information in the reports. Apparently, at least some of the owners experienced the perceived loss of deceleration after hitting a bump or pothole. 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 responding by proactively reducing brake torque in an attempt to 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 explain 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 properly diagnose and fix it.

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    • 1 Second Ago
      • 5 Years Ago
      Oh, what a feeling!
      • 5 Years Ago
      Way to go Crapyota.
      • 5 Years Ago
      I have a 2010 Prius. The first time that the sudden acceleration after hitting a bump happened was kind of scary. I was going down a steep hill, so I attributed to my foot lifting up off the brake because of the bump.
      When this happened again going up hill I started to worry, called the dealer and they gave me several possible reasons of why this happened. None of them were convincing to me. Reading now about other drivers with the same problem is eye-opening. I hope Toyota will rectify this and similar problems, and be more honest about them.
      • 5 Years Ago
      Good Autoblog. You are now the official mudslingers. opposition research against Toyota is now your official job eh? Reporting on something that is not even under investigation just make the list look long. Anyways, you efforts isn't bearing fruit cos Toyota sales have been increasing in the past months instead of decreasing.

      Also, here is a few very recent head lines that you so conviniently omitted: (a)There were reports that Toyota could even turn profit this year despite earlier projects of steep losses,(b) there also were reports that claims (which you guys reported) made against Toyota about hiding crash evidence was dismissed in court. (c)Other Toyota reports includes that they will raising their production by 17% next years. Yet Auto blog just keeps digging mud. I'ld like to see you do the same for the Detroit 3.
        • 5 Years Ago
        It's not so much that Autoblog is biased against Toyota, it's the Autoblog is biased in favour of page-views. Very few things will stir up a ten-page flamefest (and generate an according flurry of ka-ching!) like dirt on Toyota. Remember how they milked every possible news clipping on the new Tundra for every domestic-fanboy-insecurity-exploiting wankfest they could.

        Put it this way, you'd never get those same ten pages if you picked on Suzuki. You wouldn't get them if you picked on Mercedes. You probably wouldn't even get them if you baited GM. Toyota-baiting is a perfect storm of ad revenue, especially in these lean times.

        The joke, truly, is on the people who have no more to contribute to the conversation than "TOY-yoda is teh EPIC FAILZORZ!1!!1". Autoblog and AOL must love you guys, because you bankroll them.

        Now, on that note, this really is a lame story. The editors must be feeling the post-Christmas revenue crunch.
        • 5 Years Ago
        I have been saying the same for years now, Autoblog is clearly D3 biased.

        I once called out of of the editors for their bias and the dolt defended himself by providing a link of Autoblog podcast where they question the management of GM and Chrysler. That fruit cake did not even realize that 99% of the podcast was Wagoner testifying in front of the Congress and begging for money.

        In other words in took the editor begging from GM's management and declaration that they would file for bankruptcy if they did not get money, to realize that the company was not run appropriately.

        I love the blog, but the bias is so clear and so blatant it just crosses every line of journalistic integrity. Is Toyota making bad cars right now, yes, true 100% but it does not change the fact that some of the editors are D3 groupies.
        • 5 Years Ago

        Do they get money even if you don't click the ads?
        • 5 Years Ago

        thats just 3 of the 5 this month

        Autoblog is not big 3 biased, their commenters sure, but they just relay stuff to us that they read elsewhere. It's not their fault Toyota's quality has taken a nose dive.

        If you Toyota cheerleaders don't like it go elsewhere, you won't be missed.
        • 5 Years Ago
        @ Egon, gotta keep 'em honest.
        • 5 Years Ago
        If you frequent this site, you have to be pretty dumb or blind not to not see that Autoblog is pretty obviously anti-Toyota.

        It probably caters to a lot of the readership's sensibilities.
        Whatever, a blog is allowed to have a voice.

        Some of it is well deserved, certain things deserve to be news and be put in front of the public, but this one in particular is completely baseless.

        These are unvalidated complaints on a their site, presented by a no-name site called "Detroit Bureau". Gee, you think they have an agenda?

        There are tens of thousands of these complaints on NHTSA's site for other manufacturers. None of them are news worthy unless we can validate them.
        • 5 Years Ago
        The point you make there is not only valid, but it's absolutely critical to consuming any kind of news. The sad fact is that "news" is dead and has been replaced by infotainmant.

        That said, until recently it's been nigh on impossible to find a "news" outlet that didn't consider Toyota to be faultless and flawless. I'm sure that's also got something to do with everyone piling onto Toyota lately...
        • 5 Years Ago
        @ Sea Urchin

        Couldn't you just swear to never, ever read or post on Autoblog ever again like any good disgruntled reader? Pretty please?
        • 5 Years Ago
        SeaUrchin, I've left you and your moronic posts alone for quite some time now coming to the reality of you being one of the most simple minded nerds yet to post. Just one request. Please don't breed.
      • 5 Years Ago
      "Having worked on the controls for similar braking systems in a previous life as an engineer"

      So thats why I enjoyed this article so much. Thanks for some nice technical explanations!
      • 5 Years Ago
      This is a blatant attack on Toyota. No other road safety organizations around the world has any serious problems with Toyota. The NHTSA are doing this on purpose to discredit Toyota as they Dominate the Automotive industry in America. Is it such a coincidence that while bad news is spread about Toyota, good news is spread simultaneously about FORD! Sounds to me like someone at the NHTSA is getting some extra pocket money from Mr Ford.
        • 5 Years Ago
        You are pathetic, TOYLETA fanboy

        Toyota is building junk lately, get over it
      • 5 Years Ago
      The issue of reduced braking ability when hitting a bump was certainly a problem on Toyota's MR2 Spyder. Then again, do ABS systems that account for this kind of situation even exist? PS Toyota never acknowledged the fact that the close-coupled catalytic converters on 00-02 MR2 Spyders tended to fail catastrophically usually leading to engine failure. Owners made it a habit of ridding themselves of the potential problem in a very non-environmentally friendly way...
      • 5 Years Ago
      damn...another hit for Toyota.

      and to everyone who is saying autoblog is mudslapping toyota..c'mon..every time something comes out about chrysler/gm/ford they post it.. it's been a long time since Toyota recalls & investigations have been really publicized because everyone thinks Toyota is bulletproof. Get over it whiners. You don't hear me complaining when something is posted about any auto maker. Idiots!
      • 5 Years Ago
      Evidently, nobody in Japan ever heard of Murphy. Multiply his effect by the complexity of the mechanism and wonder how this (and worse) hasn't happened before now.
      • 5 Years Ago
      We had two Ford Escape Hybrids at my place of work. One vehicle had two brake failures, on the other, it happened once. I was driving during the first failure. I was able to stop with about 10% remaining brake power. After the car was turned off and restarted, the brakes worked fine. After the third incident the vehicles were returned. Google: ford escape hybrid brake failure....you will see that these failures have been occurring for a few years. No action from NHTSA so far. I guess that someone has to die first.
        • 5 Years Ago
        nope. you may want to read or inform yourself. No action generally means no complaints. Did you file a complaint for that vehicle? did your place of business on all of them?


        Great, then there will be no action. Man accountability is inconvenient huh?
      • 5 Years Ago
      Toyota has come under the cross-hairs of many lately, but I don't believe for a moment that Autoblog is out to get them. What is happening is some Toyota owners are feeling the sting, on a personal level, from the growing criticism regarding transparency and quality. It's coming from a variety of places, and maybe it's about time some new light was shed on Toyota's unassailable reputation for greatness. Hyundai was the butt of jokes for years, ask Jay Leno and Rosie O'Donnell, but they persevered and won some respect. Toyota had been so long associated with "quality" that it was almost heresy to suggest that there were problems. And there have been problems, most of which have been mentioned, and argued over, on this blog. Toyota's problems have nothing to do with Autoblog. They are fair game for criticism, like every other car maker, because none are entitled to a pass or a coronation.

      • 5 Years Ago
      Nothing like a Toyota article to bring out the idiot fanboys on both sides. It is entertaining though.
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