Braking problem

It's a very annoying problem, you just have to mash the pedal and hang on. I would be curious to see if the 2nd gen did this

 

This behavior is one more reason to make your stopping events
as long and even as you can -- in other words, plan early. You
get the best benefit of regen and least energy loss in the
electrical system that way, too -- less pack heating, less
chance of exceeding the current limit. The behavior over bumps
is simply the system protecting itself against excess battery
currents, and its recourse is to fall to hydraulics.
.
Most people within Toyota will likely have little idea how the
braking system works, let alone subtleties like this. Time has
proven that it's up to the owner community to maintain the info
and reassurance on this, and try to make sure new drivers know
what's going on and how to moderate their driving habits in
beneficial ways to compensate for it.
.
_H*

 

The Gen II does this. I have a 2006.

Tom

 

I am a UK Prius driver having owned my gen3 Tspirit for just five days. Yesterday I experienced this problem when I was heading down a slight hill and applying light brake pressure. The car suddenly felt like it was accelerating and I instinctively hit my brakes hard to regain control. Someone on the Toyota owners forum pointed me to this thread and having read all the posts I'm glad to know it was not just my imagination that this happened. Co-incidently the air conditioning died at the same time. The fan runs, but is not cooling the air. I have it booked in with service next week and will also mention the braking problem.

I agree with most of you who have experienced it, that it is pretty scary when it happens.

 

This doesn't sound like anything associated with the main problem discussed in this thread. Perhaps you have had both issues, but I don't see that they are related in any way..

 

Brakes are a bit quirky, and seem to be a compromise to get the regeneration advantage. The above discussed, "pause of terror", as well as trying to gently brake while going in reverse down a sloped driveway (jerky shuddering).

A new one I haven't seen discussed happened to me. Traffic on the freeway suddenly stopped and I was distracted a bit and then jammed on the brakes. ALL FOUR WEELS LOCKED UP AND SCREECHED! I'm glad to have stopped in time, but where were the anti-lock brakes? a similar experience in my '97 Acura yielded the anti-lock shudder, but no lock-up of the tires. has anyone else experienced the lock-up of brakes on the Gen III?

Lee

 

You still had anti-lock even with the screech. I don't believe your wheels locked. The Prius has a smother ABS that doesn't shudder. Tires will still screech while rolling if braking is done at a high enough level. The ABS system tries to allow for the maximum braking force while still assuring the wheels turn.

Tom

 

My gen II has done this periodically throughout its life. At 88K, it's been thru plenty! It's just something you get used to. You float for a sec or two, seem to lose traction (I call it the scramble) and now that I know what to look for in road patterns, I know when to expect it most of the time.

 

+1

When the car is new and you are unfamiliar with all the subtleties of
the car you don't realize just how often this happens. As you learn the
car and relax, you'll feel various degrees of Dianne's "scrambles" in all
sorts of non-critical braking scenarios.

When I first became aware of this behavior, I was shaken even though
I hadn't and haven't experienced it in a critical driving situation. So,
much to the displeasure of my commuting partner, the next day I
looked for road conditions when there was no other traffic near by
where I could induce the 'scrambles." Yup, it sure feels weird, and it
sure feels scary. Yup, you shouldn't be following too closely as you
need a split second to get on the break pedal to get solidly into pure
friction braking. (You should do this exercise too.)

The best response is realize that the Prius needs a generous
following distance, real time awareness of road and traffic conditions,
and an active anticipation of what's ahead. This also is a part of safely
and responsibly driving any vehicle.

 

Whew! I've been there too. Was slowing down from about 30 to 20 mph going over some SLIGHTLY bumpy railroad track and FELT like it accelerated while crossing. Scared the bejeebies out of me.

 

Just hit the brakes harder should do the trick (hopefully anyway)

 

I tried yesterday, it happlened when braked at a slight slope.

after experiencing this several times, I am not surpised as the first time.

also remember to brake in advance.

 

I have had a 2010 Prius for a little over a month now and about 8 different times this has happened, where there is a sudden loss of brake pressure. I describe the feeling as sliding on ice. It lasts anywhere from half a second to a full second, but it is quite startling. I've never owned a hybrid before, but I couldn't imagine this to be normal. After reading the above thread, I'm wondering if what I experienced is indeed normal. Or am I experiencing something different? It's happened at low speeds (about 15 to 20 MPH) and higher speeds (55 to 60 MPH). The later is the most recent experience where I was going to exit off the highway and I'm braking at a normal but adequate and steady pressure to exit the highway. Suddenly there is a loss of brake pressure and I nearly went off the exit ramp. It was so dramatic that it's hard to say that I'm really exaggerating with that statement. I took the car into the Toyota dealership and they said all the diagnostics show there are no problems with the car. Perhaps on a related incident, my wife was stopped at a red light. Suddenly the car lurched forward. I was not in the car at the time, but she says that she was just holding the brake peddle waiting for a green light and it jumped forward 1 to 2 feet. She said if there was a car directly in front of her, she would have hit it. Could this be related to the loss in brake pressure? Was it likely an isolated incident? It sure does make me nervous anytime I have to apply the brakes.

 

If you nearly shot off the ramp, that's not normal. Usually the transition feels much worse than it is, so if it felt scary but everything came out okay, it's probably a loss of traction and subsequent transition to friction braking.

As for your wife, what happens in that situation is that the ICE starts and gives a little kick of power. If you aren't holding down the brake adequately, the car will try to move forward.

Tom

 

So, we should wait until this causes collisions or running over a few pedestrians and then get concerned? I would guess that there isn't a "I-felt-like-I-lost-my-brakes-because-the-car-went-over-some-bumps" thread on most chat forums of other vehicles. Does anyone have any knowledge that Toyota has been alerted to this phenomena? If this is normal behavior for the Prius, then put it in the brochure or on the website. The three times it's happened to me were definitely the most exciting moments so far with the car. I guess that's a feature.

Seriously, this worries me most at intersections. In Toronto, we have many intersections with bus-caused ripples or construction on-going. Also, we have pedestrians and bicyclists that do suprising things at intersections. Losing brake control for a second at an intersection will eventually cause an accident. Even a half-second is too long.

If this is just a feeling that the driver feels, and the braking doesn't actually diminish, fine. But how do we know that? Has anybody tested this? And shouldn't this be explained in the owners manual? (or is it? I confess I haven't checked.)

 

My new 2009 don't do this. I would hate to think I would buy a new car and it's something I have to get use to.

 

It will. So far you haven't experienced the correct set of circumstances, but it will happen.

Tom

 

Wouldn't your response be much the same about ABS cars in general, not just those with regenerative brakes? If one wheel or both front wheels encounter enough loss of traction, the ratcheting effect of the brakes applying and releasing gives much the same sensation. And the majority of all braking is at the front. You're still stopping better and with more control than a locked wheel, but there is a different sensation as each wheel tries to apply friction. With regenerative brakes, the form may be different but isn't the outcome basically the same?

I replaced a Lexus 400h in June after 3 years, never encountered what I considered a unique regenerative braking issue, nor on the new Prius since then. My experience with ice patches on roads in ABS cars has produced that feeling of diminished braking, but less so than a slide, and permitted me to maintain steering control. Someone has posted here that the Prius has very little pulsation feedback when ABS is activated, which is signalling to the driver to maintain or increase pedal pressure--is it also possible ABS is activating but it is not obvious to the driver? I'm just not buying into any need to alter braking habits because of driving a Prius (or regenerative brakes) as some suggest.

 

This happened to me twice last night (once in East St. Louis, not the first place I would choose to have car issues). I had already read this thread, so at least I kind of knew what was happening, but I have to say it definitely felt like acceleration, not just a sudden lack of deceleration. Both times everything was back to normal with the car within a second, followed by me returning to normal some time period later.

 

Yes, this is true. It's a bit more pronounced in the Prius, since the Prius control system is very conservative about protecting the HSD. Because of that the Prius is a little more likely to give you that accelerating feeling when it loses traction during braking, but it can happen with any car. Even old cars without ABS feel like they are accelerating when you hit a patch of ice while braking. Your body becomes accustomed to the braking force, and when you lose it you feel like you are accelerating.

A very good point.

Tom