2012 Prius v (lowercase v)...brakes failed tonight

Well that is disappointing on Toyota's part. I am in Canada so the laws are a bit different, but I am definitely speaking to a lawyer the beginning of next week (this incident just happened yesterday). I have young kids and no longer have confidence in this vehicle being able to stop and do not want anything to happen to them. I've read through quite a few threads saying this behaviour is considered "normal" for regen braking, but I'm sorry, I am a safe driver, have no accidents or tickets on my record, and when I say the brakes did not work pressing them as hard as I could, they did not work.

I've reported it to Transport Canada (similar to safecar.gov) but I have little hope they will help me as they say they receive hundreds of complaints per day.

 

did you mean to say you were going less than 1 kph when the brakes failed?

 

Pretty much... it was bumper to bumper traffic and a transport was in front of me so I was keeping my distance, about 10m, then the transport started to move so I let off the brake to coast ahead and got up to a whole 4km/h then coasted down to 2km/h and that's when I attempted to apply the brake and it did nothing.

 

what did you drive before the prius?

 

By 'hard as you could', did you use your full body weight, or at least 112 pounds of force (metric 500 newtons)?

In the US, that is a magic number. A number of power assist failure mechanisms are allowed, but the underlying manual hydraulic system must still work at that force level. Failure without some form of error warning would be prima facie evidence of failing federal passenger car brake requirements, of the sort requiring a recall. Though note that operator failure to address prior system warnings could vacate manufacturer responsibility.

There have been hints of transient problems in the past, but not enough to form a critical mass to help track it down. Please follow through on reporting this. If you can't get it adequately reported in your own country, I'd try reporting through the U.S. system.

 

Thanks for that info. I have no method of measuring how much force was on the brakes at the time, but I was pushing as hard as I could to the point my foot hurt from pressing so hard. I am expecting Toyota to pull any diagnostic data from the crash they can to let me know what exactly happened. Hopefully the can find something. Even if it turns out to be a fault of mine I just want to know what occurred so it won't happen again (maybe there was fuel on the road for example and VSC did indeed detect wheel slippage, for example).

I will report to the US system as well since they are basically the exact same vehicles.

I was driving a Honda Civic before that.

 

19 Complaints: 2012 Toyota Prius v Service Brakes Problems

More info about this.

 

this is about traction control and regen/friction brake transition on rough surfaces.

 

That may be but there is no proof that this is the same thing the OP has suffered. In situations like this the OEM will measure hydraulic pressure at all four wheels using certified pressure gauges to measure the PSI on stop tests. Sometimes they have a third party with no interest in the outcome to do this certified test. In this case it probably will not. There will be other factors as to who worked on it as well as the maintenance schedules. Lots of speculation here until more facts reveal themselves. But in times like this it does put a chill in the air as to a possible defect in the car but then how many of them are out there? Quite a few no doubt. I guess when the OP reports we will see.

 

Something I don't understand is how you had time to realize the big rig was stopped, apply the brakes, realize you weren't stopping, remove your foot from the pedal and reapply the brakes all at 6-7 MPH as you traveled only 10 feet.

 

you haven't been around long enough to ignore the 'chill'. (aka noise)

 

It's always good to keep in mind that the simplest and most likely explanation is driver error.

 

I don't know much about the Prius braking system but is there a direct hydraulic connection like normal cars?\ Or is it completely brake by wire? If the latter, I can see how a glitch would cause this. I would imagine Toyota built some sort of redundancy into a braking system though.

The only thing I can think of is the OP was going down a very long steep hill beforehand and boiled the brake fluid by overheating. Then when the brakes cooled down, they worked normally again.

 

The Prius brakes work by applying hydraulic pressure from the master cyl to the ABS unit. Now if the ABS is working properly the hydraulic pressure is modulated to the wheels accordingly by the ABS control thru the computer controlled valves. You can check your ABS functions by going 40 MPH and stomping on the brakes as hard as you can and as long as the pavement is even, not bumpy and surface not wet you should stop but not skid your tires. On a car where the ABS lite comes on you have no ABS functions and to put that to a test do the same and you should be able to lockup your wheels and skid. Basically when not working your valves become non-functional and are clear to let hydraulic pressure thru the ABS uncontrolled other than pressure thru the Master cylinder.
As far as boiling brake fluid if that had happen the heat would have to come from any surface which causes the friction. Your front brakes do the majority of the stopping so it would be the rotors. Those rotors and brake pads would have to smoking to boil brake fluid. Thats why if you are going down hill it is best to step hard and slow down suddenly and resume than to step gradually and not release the pedal. Constant pressure on the pedal creates heat while momentary stepping causes speed slow down but upon release cools the rotors down.
Another factor is the amount of brake pad thickness you have. The thinner the pads the quicker they heat up. Thicker pads absorb heat.
As far as a malfunction of the brakes - is it human or machine?. I reserve every incident on its own merits. Unless there is a KNOWN design defect each must be tested in a procedural fashion to pin the blame. Thats if it can be duplicated each and everytime. I have been in the tranportation industry a long time and we have drivers who claim brake failure. Those vehicles undergo a test and sometimes results do not pin the blame on the driver. Thats why dash cams and accident event monitors come into play. But in this case it is a known event but is elusive at times to pinpoint and figure out. I guess thats why we have lawyers...
merged.

Yeah...sometimes its hard to seperate the noise from the meat lol! Thanks for the heads up!

 

It's good to be careful generalizing about how the Prius brakes work, because there were significant changes between the generations:

All generations have an electric-pump-driven pressure accumulator.

In Generation 1, the accumulator pressure is used partly for master-cylinder boost: that is, it gets modulated in behind the master-cylinder piston, adding to your foot pressure. That's a lot like the booster in a conventional car, just using hydraulic pressure instead of engine vacuum. But what happens from the master cylinder output is a bit different.

In fail-safe mode (actuator valve SS closed, SMC1 and SMC2 open), it acts directly on the brakes, but only the front ones and with a bit of strange feel because the stroke simulator is still part of the fluid path. There is no rear braking in this mode. Closed for SS and open for SMC1/SMC2 are the normal positions for these valves with no voltage applied, so any loss of power naturally goes to this mode.

In normal operation (voltage is present, the computer has gone through self test and declared itself sane, it signals SS to open and SMC1/SMC2 to close), the direct master cylinder output is only routed through the "front brake" side of a (completely conventional) P&B valve, then to a spring-loaded "stroke simulator" to give you the feel of a normal brake. That fluid circuit doesn't act on any brakes at all. But the same modulated accumulator-pressure circuit providing boost to the master cylinder piston also supplies the ABS control valves, of which there are only three pairs, because Gen 1 ABS controls the two front brakes independently, but the back brakes only as a pair. In other words, the Gen 1 system is a little funky, and it's shown on page 106 of the 2001 New Car Features Manual. One thing cool about it is, even if the ABS control goes wonky, you still have boosted non-ABS operation of the (front) brakes, for as long as accumulator pressure holds out.

In Generation 2, the mechanical P&B valve is gone, the "boosted" master cylinder is gone, there's another pair of ABS valves so now all four brakes have independent control, and in normal operation you are not doing anything at all with your foot except squishing the stroke simulator. The computer measures your effort (via the master cylinder pressure sensors), calculates appropriate pressures for all four brakes, and uses the four ABS valve pairs to directly modulate brake accumulator pressure to the four wheels.

There is still a fail-safe mode, the power-cut positions for the valves are: open for SMC1/SMC2, closed for SLAFR, SLAFL, SLARR, and SLARL, closed for SLRFR and SLRFL, open for SLRRR and SLRRL, and closed for SCSS. (SCSS is a new valve they added to seal off the stroke simulator in fail-safe mode, so all your pedal effort can go to the brakes.) In fail-safe you're still only braking the front wheels, and in Gen 2 you don't really have boost. See 2004 New Car Features Manual pages CH-43 to CH-61.

In Generation 3 (which the original poster here has), well ... I know there were additional changes, but somebody who has the 2010 New Car Features Manual would have to fill them in, because I don't have it.

Naturally, the fail-safe mode is always based on the positions the control valves will take when power is cut. So you dependably get fail-safe behavior if (a) the power is actually cut, or (b) the computer has declared itself insane and actively disabled its outputs, to bring on fail-safe mode.

The one situation that needs to be handled more carefully is: the computer firmware has gotten hung up somewhere, and hasn't detected that fact and disabled the outputs. That case can be trouble.

A common approach to that possibility is to build a "watchdog timer" into the computer: just a simple, hardware timer that is always counting down. When the firmware is functioning properly, it continually resets the watchdog, often enough that it never counts down to zero. If it ever hits zero, the firmware must be hung up, and the timer forces a hard reset.

In Bookout v. Toyota (which was a 2005 Camry acceleration case), the software expert who got to look at Toyota's code found that there was a watchdog timer, but he was seriously critical of the way the firmware handled resetting it (page 41).

However, on the same page, he said the watchdog design in the 2005 Prius was better than what he had seen in the Camry ... so we can hope that by the time of the 2012 Prius it's at least that good or better ....

-Chap

 

Appreciate the verbose write up. Much in revision and improvement has taken this issue steps closer to a fail safe system. It is very complex maybe to the point it may be a danger to itself in order to stay in control yet some firmware or hardware fails will trip it up and make it unable to recover. When things like that happen it is always litigation and their fines which will change the design of any said component to the satisfaction of the shareholders and the legal powers who who extract the monies. Hopefully no lives will be lost in the meanwhile. But int he meanwhile...it seems this is an ongoing situation which rears its ugly head every now and then. For every event TOY obviously does not publish their findings upon any examination of the complaint car in question. Lawyers will not want to reveal their hand for obvious reasons. I suppose their are no class action suits or recalls to this matter at this point in time. Thank you again

 

Others have already given longer answers, I'll give a shorter version from a different viewpoint.

When Gen3 was released, a full brake-by-wire system was not yet legal for passenger cars. That will eventually change as self driving cars take over, but I'm not yet aware of that being ready yet.

In Gen3, a drive-by-wire brake boost system is layered on top of a more traditional hydraulic system. The normal operation is thus a drive-by-wire system controlling the hydraulics. Should that fail, the driver can stomp through the wire layer to push the underlying hydraulic system manually. That is why I have asked posters if they applied at least 112 pounds (metric: 500 newtons) of pedal force. That is a magic number in US vehicle regulations, for all types of power assist brakes.

The concern is whether or not an electronic control failure reliablity goes into fail-safe mode, so as to not obstruct manual operation of the hydraulic system. It seems that there could potentially be a transient glitch that doesn't fail safely, but happening so rarely that nobody has been able to separate it from the more common operator errors and mechanical/electrical failures that do trigger warnings demanding repairs.

The mere existance of such a glitch, if demonstrated, would force a recall under NHTSA passenger car equipment regulations. That is why all suspected incidents should be reported to both the manufacter, and to safety agencies. Any such glitch will not get fixed until enough cases get reported to get it actually identified.

 

keep in mind that all auto mfg.'s go through this. if there is a real issue, you will see a regular member(s) report a problem. watch carefully, it's always someone new.

 

Because I was paying 100% attention to driving and not distracted. I saw the truck was stopping/stopped so I applied the brakes and expected to slow down, but instead did not.

I appreciate all the verbose explanations and will be getting a copy of the data recorder data once I get my car checked out by Toyota. I will post updates once that happens and will not say 100% that it's the cars fault until I can see it in the data.