No regenerative braking?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by sergbot, Sep 4, 2021.

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  1. sergbot

    sergbot Junior Member

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    Hey, I got the parking brake light staying on and not going out. In addition, the car won't regenerate energy when pressing the brake pedal. However the car will regenerate energy slightly whenever the gas isn't pressed. But again when I press the brake pedal, there is no additional regeneration, like there used to be 3 days ago. I got the coolant switch error code 1121, but I'm sure it's unrelated. I used to see the Regen braking occasionally intermittently quit, but then next press it would work again.

    My best guess is that the abs unit is on its way out the door. Cos it's supposed to send the 4 volt signal to say how much to Regen.

    I noticed with a specific car scanner app that it had two readings, mg braking torque and mg requested braking torque, and these numbers would just stay 0 when pressing the brake pedal.

    Is there any fuses or relays or anything I can check related to the parking brake light/regeneration control module

    I know the parking switch isn't actually broke otherwise it would beep at me like the way it does when you try to drive with the parking brake engaged
     
    #1 sergbot, Sep 4, 2021
  2. SFO

    SFO Senior Member

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    Which device/app are you using to retrieve OBD2 codes (DTCs) ?
     
    #2 SFO, Sep 4, 2021
  3. dolj

    dolj Senior Member

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    There seem to be a lot of assumptions in your analysis. It would be a good idea to scan for DTCs as suggested by SFO in post #2.

    The advice that could be offered would be then more definitive rather than best guesses.
     
    #3 dolj, Sep 5, 2021
  4. sergbot

    sergbot Junior Member

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    So I checked out the brake pedal switch and it's a dead end. Shorting th connector manually didn't make the aux brake light go away.

    I mentioned in the post the error code I have is dtc p1121. So basically no codes and just aux brake light on the dash won't turn off
    Oh I'm using the torque pro app, and the elm whatever app
     
    #4 sergbot, Sep 5, 2021
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  5. TheLastMojojomo

    TheLastMojojomo Active Member

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    By parking/emergency brake light you mean the red "BRAKE" light correct?

    The red Brake light will come on if the brake fluid is low.

    This will likely disable regenerative braking.

    Usually you'll notice the brakes getting spongy occasionally at this point as well too.

    No codes will be thrown either.

    Pop the hood and see where the fluid level is at.

    Just something to check.



    Posted via the PriusChat mobile app.
     
    #5 TheLastMojojomo, Sep 5, 2021
    Last edited: Sep 5, 2021
  6. ChapmanF

    ChapmanF Senior Member

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    P1121 is not a code from the brake ECU; you are right in thinking it is unrelated.

    If the scan tool you are using is not also showing you the codes from the brake/skid ECU to explain why that warning light is on, there are still a couple of options available to you for finding out why:

    • use another scan tool, such as a beater Windows laptop with Techstream, that can read all of the relevant codes, or
    • bypass the use of a scan tool entirely, by reading the brake system blink codes.

    I often recommend the jumper-wire-and-blink-codes method in a situation like this, because it is always available, and sidesteps any question about what a particular scan tool is or isn't able to read.
     
    #6 ChapmanF, Sep 5, 2021
  7. stlprius

    stlprius Junior Member

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    I have a 2010 that intermittently is flashing the brake light. Would this fluid level be low enough to cause it?
     

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    #7 stlprius, Sep 5, 2021
  8. TMR-JWAP

    TMR-JWAP Senior Member

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    I believe the line molded into the plastic directly above the word "MAX" is the max level and the line molded directly below the same word is the minimum recommended level.

    So, ...yes.
     
    #8 TMR-JWAP, Sep 5, 2021
  9. ChapmanF

    ChapmanF Senior Member

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    I shall file a dissenting opinion. That upper line is indeed the MAX line. The MIN line is rather hard to spot, being below the 'seam' of the reservoir.

    maxmin.png

    The unlabeled shorter line in the middle there is known in the repair manual as the "fluid level support line". It is the only line to which you should "top off" if you ever feel the urge to do that, and you don't have Techstream or a way to "zero down" the system.

    Brake fluid should only ever touch the MAX line if the system pressure has just been zeroed down and all of the brake pads are new. The fluid level will immediately drop from that level whenever the pump runs to pressurize the system, and will slowly drop further over the years as the pads wear. In the absence of leakage, there won't ever be any real reason to "top off", and one future day when the system is zeroed down and the pads are replaced, the level will be right back at MAX.

    So, the photo in #7 shows an absolutely normal fluid level for a system that isn't currently zeroed down and doesn't have brand new pads. It is well above the MIN line, and it is not a reason for the brake light to flash.

    Reading any brake system DTCs would be a good next step toward finding out why the light may be flashing.
     
    #9 ChapmanF, Sep 5, 2021
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  10. sergbot

    sergbot Junior Member

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    Hi again, my brake fluid level is full.

    I wanted to see maybe it's the float switch that is defective, so I unplugged the switch. No result. And I shorted the two pins in the switch.
    No result,

    Maybe it takes a restart to make the light go away. I'll try that next. I'll also use a continuity meter to see if the switch is supposed o be always on when full or always off when full.


    Was just about to clear codes in an attempt to reset the computer so it takes the new brake reservoir level into account maybe clearing the problem, and a p0606. Just popped up.

    Not sure if that's an exciting problem or if it's because I stuck a paper clip in the brake float switch connector

    Edit2. Cleared the 0606 and it went away without problems. Weird.

    anyway there is a reading called brake master cylinder torque, and friction pad torque and regenerative brake torque.

    And usually those readings show numbers when you press the brake pedal. But they don't now. So it makes me tht the pressure transducer In the brake unit went bad, idk if it would cause my symptoms though

    I'm also realizing that I had brake assist with no regenerative braking. Now that I messed with the brake fluid level sensor, I lost the brake assist. So I'm guessing if your brake fluid level hits low, you loose brake assist. So it was probably smart to ask me if I still had brake assist before suggesting it was the brake fluid level sensor. O well, hindsight is 2020 I suppose
    Edit 5

    So I plugged the float sensor back in to try to get brake assist back, and I got brake assist back instantly, and regen brakes are back and the BRAKE light is out.

    Sooo I'm shocked and don't know what to think. I guess unplugging and plugging the float sensor made the brake actuator reset?
     
    #10 sergbot, Sep 5, 2021
    Last edited: Sep 5, 2021
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  11. TMR-JWAP

    TMR-JWAP Senior Member

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    I learn something new every day.......
     
    #11 TMR-JWAP, Sep 5, 2021
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