• Feb 3rd 2010 at 10:29AM
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2010 Toyota Prius – Click above for high-res image gallery

The pain just keeps on coming for Toyota. First came complaints of cars running away without the driver pressing the accelerator. Now the Japanese government has ordered Toyota to investigate the braking system on the 2010 Prius. The U.S. government is also looking into the matter after receiving over 100 complaints about the brakes on the third-generation hybrid. Like most other hybrids, the Prius uses an electro-hydraulic brake system that blends the regenerative and friction braking functions. There have apparently been dozens of complaints in Japan and North America of a momentary loss of braking power at low speeds on slick or bumpy road surfaces.

We can't say for sure what is or is not happening on the Prius, but we can offer some possible insight. On hybrids vehicles, the brake system monitors the state of charge of the battery, the available amount of regenerative braking and the driver's brake demand (as measured by the brake pedal apply). As the vehicle comes to a stop, the amount of regenerative braking is ramped down to zero since you can only capture kinetic energy when the wheels are rolling. Doing this smoothly requires an accurate software model of the friction braking behavior. If the friction brakes are generating less torque than the model says it should be, the brake system will apply less pressure to the brakes. The result would be a momentary loss of deceleration during the hand-off from regen to friction braking.

Because the behavior of friction brakes varies over time depending on temperature, moisture, wear and other factors, it is very difficult to calibrate these models. The other part of this equation is the fact that this is happening at low speeds. The wheel speed sensors become less accurate at low speeds as the time period between sensor pulses is extended. Rough or slippery surfaces exacerbate the problem, which could make the brake system think there is slip that might not be there, resulting in lower brake pressure. The result is a perceived loss of deceleration.

If these are the problems that the Prius is experiencing, they could be corrected with an update to the control software. However, that will take time for Toyota to calibrate and validate.

[Source: New York Times]

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    • 1 Second Ago
      • 5 Years Ago
      "We can't say for sure what is or is not happening on the Prius, but we can offer some possible insight."

      hahahhah was "offer some possible Insight" intended?
        • 5 Years Ago
        You beat me to it. I was just going to say that I'm sure Honda is glad to hear that.
      • 5 Years Ago
      Could this get any worse???... Looks like it's getting there...
      • 5 Years Ago
      I only see one douchebag right now, and that is you rf.
      • 5 Years Ago
      • 5 Years Ago
      What I don't understand is that if this is only an issue with the 3rd Gen Prius, why don't they look at what they changed from the 2nd Gen? Is the braking system that much different that the control software needed to be overhauled completely?
      • 5 Years Ago
      I wouldn't call them horrible, either. We have some friends that own two Gen2 Priuses (bought them used), and they're about as far across the political spectrum as you can get from the "typical Pruis owner", whatever that is. They bought the cars to get out from under the gas bills for two high-mileage Suburbans (they've been handed down to their high schoolers, who drive very few miles), and the husband also has a 50-mile plus (each way) daily commute (he now gets over 50mpg, versus about 20 for the Suburban).

      For them it was more about saving money, not making some kind of political statement.
      • 5 Years Ago
      They should check previous generation Avensis too...
      A friend of mine had one about 2 years ago. When it was driven at very slow speed on ice or snow brakes didn't always work. Sometimes when braking abs started doing something but car wouldn't brake at all. It was quite scary when it did that first time for me. Braking again helped every time and luckily nobody got hurt. Toyota service didn't find anything wrong..
      My friend is now driving happily with his Mazda 6.
      • 5 Years Ago
      Now the Pious??? Say it ain't so!!!!

      Toyota management has a new slogan for their workers:

      "The beatings will continue until morale improves..."
      • 5 Years Ago
      I have a Toyota that has an ABS problem of kicking in while braking
      over bumps in the road.

      As others have stated, this ABS modulated braking sends LESS braking
      pressure to each brake cylinder so you don't lock the brakes up.

      However, as I have experienced, this INCREASES the stopping distance,
      --and that is very dangerous when there are cars stopped a short distance
      ahead of you. Again, I have to stress that ABS does not stop the car
      faster, which everyone has that false notion. How can it? - When it is pumping
      less hydraulic pressure to each tire. So it's supposed to allow you to steer
      around the cars that have jammed their brakes on and are about 100 feet
      ahead of you, and your car in ABS mode will take its' good ole time in
      stopping your car. Suppose there is no where to steer around any obstacle
      in front of you?

      ABS has a very limited function on the road, but in everyday driving on dry
      roads it should not be kicking in while braking over bumps.
      • 5 Years Ago
      I've owned both the 2nd and 3rd gen Prius and can say that this is a common problem, and agree it might be an issue with both the regen braking and ABS; the only time I wasn't able to stop properly was when the road was wet, it was on a downward slope and my tires weren't in the best of conditions
      • 5 Years Ago
      Sucks to be Toyota right now.
      • 5 Years Ago
      Not so mad GM for taking so long to launch the Volt. Note to GM: Take your time and get it right.
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