Unintended Braking?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Dave T, Apr 30, 2016.

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  1. Dave T

    Dave T New Member

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    I've heard about unintended acceleration, but we're having a problem that I haven't seen discussed and I'm looking for help.
    We have a 2011 Prius Five. It has the Advanced Technology Package, which includes features such as Dynamic Radar Cruise Control, Pre-Collision System, and Lane Keep Assist among other stuff.
    My wife drives the car most of the time, and she loves it. Except for one major issue.
    Every once in a while - it can be months between events - the car momentarily slams on the brakes when driving at highway speeds while nothing else is going on. We know it's not operator error. It's happened to my wife several times and me once. When it happened to me I was driving on a dry, lightly traveled divided highway. Everything on the dashboard lights up, alarms go off, the car decelerates like I suddenly stomped on the brake pedal though my foot never moved off of the gas, it lasts for maybe a second and then the car is back to normal. It's the same each time it's happened. Luckily each time this has happened we haven't had anyone tailgating us.
    We've taken it to the dealer multiple times. The first couple of times they looked at us like we were loony. They checked it out and we got it back with a message that said the technician checked the computer and test drove it for 20 miles and the car was operating as designed. Except it wasn't and it happened again. We finally convinced them that this was maybe a legitimate issue and they said they brought in a Toyota Field Specialist who checked the car out and they said that they found the issue and it was the PCS wave radar sensor that was slightly out of alignment. That didn't fill me with confidence - if a car can behave that erratically when a sensor is slightly out of alignment it sounds like a poorly designed car. But the car behaved itself for about five months. Until yesterday...
    Anyone out there have any ideas or suggestions? Thanks.
     
    #1 Dave T, Apr 30, 2016
    Last edited: Apr 30, 2016
  2. Bill the Engineer

    Bill the Engineer Senior Member

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    Does the car still have the original 12 volt battery?

    Bill
     
    #2 Bill the Engineer, Apr 30, 2016
  3. Dave T

    Dave T New Member

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    I don't think. I haven't replaced it and would have to go back through the service records to make sure the dealer didn't replace it during one of the times it was in for service. Why do you ask? Is there a reason that a 5 year old battery could cause this?
     
    #3 Dave T, Apr 30, 2016
  4. Dave T

    Dave T New Member

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    Actually a 4 year old battery because the first time it happened was early last year.
     
    #4 Dave T, Apr 30, 2016
  5. bisco

    bisco cookie crumbler

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    low 12v in a prius can cause a lot of computer headaches. i think i have heard other people report this as well, not sure there's a fix though.
     
    #5 bisco, Apr 30, 2016
  6. Former Member 68813

    Former Member 68813 Senior Member

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    if that happens when you drive a congested road and a big truck is behind you, that can get you killed (and some one else too).
    time to report to NHTSA: File a Vehicle Safety Complaint | Safercar.gov | NHTSA
     
    #6 Former Member 68813, Apr 30, 2016
  7. Maarten28

    Maarten28 Active Member

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    It happens to me too once in a while. Only when I'm driving with the adaptive cruise control and it does give a warning then.
    I assume it is some kind of situation that the radar thinks something is right in front of the car. Maybe the way the sun is shining or a leaf blows in front of the radar, IDK.
     
    #7 Maarten28, May 2, 2016
  8. macman408

    macman408 Electron Guidance Counselor

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    Most people encounter it when there's a metal plate of some sort in the road, such as at the start of a bridge, or as part of an expansion joint somewhere. The only time I think I've ever encountered it (and I don't think it actually got to the point of braking for me) was when I was traveling down a steep hill towards some railroad tracks. In that case, I can't really blame the car, because in the direction I was heading, I *would* have hit the tracks, if not for the fact that the hill leveled off just before them, and then continued going down just after them.

    You could ask them to do the sensor alignment again. Maybe they didn't tighten it enough, or maybe whatever it's attached to is slowly drooping? Maybe you could even request that they set it a little on the high side of the normal range?
     
    #8 macman408, May 2, 2016
  9. dhancock

    dhancock 2 Prius Family

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    We've had this happen several times with my wife's 2011 back in 2012. In our case, it was hot outside and the car was always passing a big interstate truck. We theorized that the satellite communication system on the truck was interfering with the DRCC system. We think that it was related because in all cases (about 7 in total) the car was ALWAYS in the same position with respect to the truck cab (front of the car even with the cab door). When this happened, the radar "locked up" and showed a car immediately in front (even though there was no car there). Turning off the cruse would stop the braking, but we had to pull over and shut the car down in order to "clear" the phantom car in front.

    Of course, we could not get the problem to re-occur for a dealer and Toyota would do nothing unless we could make the problem re-appear. I did file a report with the National Highway Safety Authority (and reported it here), but nothing ever came of it.

    The next year (2013, I think) we had no problems with it, and haven't had any since. I don't think that we had any sort of alignment problem as I have noticed that the radar beam seems pretty narrow (as observed by noting the vehicle in front icon disappearing as I pull out t pass. Car now has 96K miles on it.
     
    #9 dhancock, May 4, 2016
    Last edited: May 4, 2016
  10. bisco

    bisco cookie crumbler

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    i can't tell you the number of times this car has saved my life. possibly the safest car i have ever owned.
     
    #10 bisco, May 4, 2016
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