Brake pedal hard & warning lights on at start up/No Codes

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by JeffOxroad, Apr 16, 2022.

  1. JeffOxroad

    JeffOxroad Junior Member

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    2009 Gen 2 Prius, 100,000 miles, has intermittent brake issue at start up.
    I depress brake, push start button, "ready" light comes on, put the car in reverse or drive, ease off the brake to allow the car to move, but try to continue to use foot brake pressure to keep the car at a speed below the idle speed in driveway.
    The brake seems to release more than usual for the small amount of pressure I've taken off the pedal.
    When I push back the brake to slow the vehicle to speed below idle speed, the pedal is hard and it does not seem to respond and does not stop the vehicle.
    If it were an ordinary gasoline engine vehicle with power brakes, I would describe the feeling as if the brake booster had lost all vacuum pressure and the brake pedal takes much more pressure to slow the vehicle than normal.
    Sometimes (but not always) in this condition three Amber warning lights stay illuminated in dash: amber brake light with circle and exclamation point, ABS, and VSC.
    If I have the space and clearance and let the condition continue while pushing the hard brake pedal with little to no response, the car will generally continue for about 12 feet then come to an abrupt stop.
    If I don't have space, I press the parking brake to stop and hold the vehicle until the brake pedal returns to normal usually after a pump for the pedal.
    In the episodes where the lights have stayed illuminated and I allow the car to go as far as it wants while I'm pushing hard on the non-responsive brake pedal, the amber lights seem to go off at the same moment the car abruptly stops.
    The stop under this condition feels like the car has hit an object, all be it at a slow speed, and stops suddenly with a bang of sorts implying the brakes have locked up. (to be clear there has been NO impact)
    No Codes are shown on Scan Gauge.
    Dealer says the issue can't be diagnosed and therefore can't be fixed until warning lights stay illuminated and a code is logged in the system.
    If anyone has experience and/or solution with this issue please advise. Thanks
     
  2. Peter Stricklin

    Peter Stricklin New Member

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    Abs accumulator is leaking down, at some point it will become enough out of spec to set a code. Eventually you will just have no power brakes. It's a fairly involved repair as you have to pull the inverter to do it. It took me a lazy day, if you aren't comfortable doing it, find a tech in your area who is knowledgeable about the prius. If you mechanic doesn't know what's wrong with it now, he is not your guy.
     
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  3. mr_guy_mann

    mr_guy_mann Senior Member

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    It seems that ScanGauge is only capable of reading codes and data from the ECM (engine control module). There are a whole bunch more ecu's on a Prius. Now if you use the search feature on this forum you can learn about "flash codes" - where you can use a small jumper wire to (carefully) connect two terminals of the OBD2 data port, turn the car on, and some of those ecu's will flash the warning lights on the dash if there are fault codes present.

    Usually, the ecu's will keep those codes in memory as "history" codes for quite some time (or until a scantool or loss of 12V battery power erases them).

    Count which lights flash how many times ("long" flashes are for the tens digits and "short" flashes are the ones). Then you can post what codes you find, and can look them up in the service manual (search for how to get that as well).

    That said, your symptom sounds like a serious failure in the braking system- loss of high pressure that's needed to well, operate the hydraulic brakes and perform ABS and traction control functions. If that is what happens you would have a fail safe system that "connects" the brake pedal to the front brakes - no boost, no ABS, no rear brakes. Braking takes much more force on the pedal and stopping distances increase alot.

    Please don't drive this car until you have it fixed.


    Posted via the PriusChat mobile app.
     
    #3 mr_guy_mann, Apr 17, 2022
    Last edited: Apr 17, 2022
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  4. JeffOxroad

    JeffOxroad Junior Member

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    Thanks for the replies and guidance. The Toyota dealer has had my Prius for 2.5 days and I provided them with a video similar to this one showing illuminated warning light and backing the car with pressure on the brake pedal that does not stop the vehicle.
    Dealer says there are NO codes and they can't recreate the issue I described.
    Dealer says I should come pick up the Prius, pay the diagnostic fee, and drive the car, "it's safe to drive."
    But....you don't diagnose...the issue... :(
    The dealers position is what I explain is not happening because the OBD would be measuring a code and time warning lights are present. Hmmmm. Interesting tack.
    I'm contacting Toyota Corporate to find out if they recommend another dealer or if I should take the vehicle outside the Toyota network.
    Obviously there is an issue.

    4 out of 5 lights - YouTube
     
  5. JeffOxroad

    JeffOxroad Junior Member

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    I should add for the entire video I am pushing the brake as hard as possible with my foot. I do NOT increase that pressure although after about 12 feet the Prius stops with what feels like a full engagement of the brake and a clunk. After this initial failure at start up the brakes seem to work as designed. Thanks.
     
  6. ChapmanF

    ChapmanF Senior Member

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    I think there are only long and short pauses between the flashes ... the flashes themselves all have the same length. Short pauses go between the digits of a code, longer pauses between codes.

    So flash flash (pause) flash (longer pause) flash flash flash (pause) flash flash would be 21 and 32.

    I think it can do better than that; I believe it uses the simple strategy of broadcasting a "hey, anybody got codes?" on the car's CAN, then showing you whatever comes back.

    It was semi-crippled on the Gen 1 Prius, which was such a crazy pre-CAN mishmash of different comm networks that some of the ECUs had to be reached by resetting the connection with different baud rates, and some by passing messages to them through other ECUs, and pretty much nothing but Techstream had a realistic chance of doing all that.

    The later generations adopted CAN and are more standard, which should be an improvement. On the other hand, there are still (at least in Gen 3, I don't remember for Gen 2) multiple different CANs in the car, with some ECUs connected to more than one and acting as bridges. So some weirdness is still present.

    ScanGauge also has this "XGAUGE" feature where you can teach it to send additional queries and commands if you know how to encode them. I'm pretty sure PriusChat members figured out how to use it to get HV ECU codes (and even INF codes) for Gen 1, for example.
     
  7. Peter Stricklin

    Peter Stricklin New Member

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    I'll just say this, as many of these cars had this problem, if they can't diagnose it, you don't want them touching your car. There is even warranty extension documentation about it. I would talk to the service manager about the diagnostic fee, pay enough to cover the tech's time though they obviously gave it to the wrong guy, and find an independent in your area that can handle it.
     
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  8. ChapmanF

    ChapmanF Senior Member

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    There are some known problems with the brakes, but this doesn't sound quite exactly like the ones I'm familiar with.

    It really ought to be possible to get trouble codes though.

    With access to Techstream, I would consider setting up datalogging/graphing of the brake fluid pressure sensors, and getting a capture of those during this initial startup weirdness.
     
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  9. mr_guy_mann

    mr_guy_mann Senior Member

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    Just going by the ad copy from the scan gauge website - I didn't see anything about scanning other systems beyond engine controls. Lots of "features" that all seem to be based mostly on generic OBD2 engine data. Could be wrong (I often am).

    Said it before, if a scan doesn't return a list of what specific ecu's that it was able to communicate with, then "no codes found" is pretty much useless in my book.

    Posted via the PriusChat mobile app.
     
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