Regen Braking not working. No DTCs

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by koco, Aug 8, 2022.

  1. koco

    koco Member

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    Hello people. On a 2005 Prius the regen braking is not working and shows no diag trouble codes.

    The hydraulic part of the brakes does function and the B on the shift lever does activate the regen.

    The brake is grabby and only works near the bottom of its travel. The part where the regen should work is doing nothing. It has been intermittent lately; sometimes powering down and restarting the car has worked.

    The button switch on the brake pedal arm does seem to work ok. I'm looking into what part of the system triggers the regen to activate when the brake is pushed.

    I haven't hooked it up to techstream yet, torque pro was the scan tool so far. It doesn't register any value with the PIDs for regen.
     
    #1 koco, Aug 8, 2022
    Last edited: Aug 8, 2022
  2. ChapmanF

    ChapmanF Senior Member

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    Are any dash lights illuminated?

    There is a hydraulic pressure sensor in the brake actuator, detecting the pressure of the brake fluid in the stroke simulator resulting from your foot on the pedal. From that pressure, the skid ECU knows how hard you want to brake. It sends that information to the hybrid vehicle control ECU. The HV control ECU uses information from the battery ECU about the state of battery charge and how much current could be accepted. Based on that, it computes how much braking to do by regen, and how much to do hydraulically. It uses the inverter under the hood to regenerate power from the MGs, and sends a message back to the skid ECU with the computed amount of hydraulic braking needed. The skid ECU operates the valves in the brake actuator to apply the computed amount of hydraulic braking. The ECUs continually recompute and reapportion the two kinds of braking for as long as you are still applying the brake.

    If the skid ECU is aware of a problem reported by the HV control ECU, it will assume it can't get any regen contribution, and that it has to do all the braking hydraulically.
     
  3. koco

    koco Member

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    No dash warning lights are lit, but if I have the parking brake on there is no brake warning light either so that doesn't work.
     
  4. mr_guy_mann

    mr_guy_mann Senior Member

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    So it sounds like maybe (some or all?) of your dash warning lights don't work.

    You need a better scantool in order to check this out. AFAIK, Torque can only pull codes for the ECM and (maybe?) the hybrid control ecu.

    Techstream is a good option. Also check out the sticky thread at the top of the Gen2 Technical Discussion forum for reviews of several apps and OBD2 devices.

    Also, putting car shifter in "B" mode tells the hybrid system to use the engine to slow the car instead of regeneration. This would be used when braking down a long steep road. Eventually regen would "fill" the battery to max capacity & the hybrid system has to cease regen. That would leave only hydraulic friction brakes.

    Having the option to "engine brake" via B-mode lets you keep speed under control without overworking the friction brakes.

    Posted via the PriusChat mobile app.
     
    #4 mr_guy_mann, Aug 9, 2022
    Last edited: Aug 9, 2022
  5. koco

    koco Member

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    Also peculiar is that now when I open the driver door there is an immediate quick blip of the abs actuator motor running and its not the typical two second buzzing of it but just a short, less than a second running.
     
  6. koco

    koco Member

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    I have techstream but not today. I have to drive home for that 100+ mi. I will probably go to a hybrid shop for scan in the meantime.
    The rest of the warning lights are working, just not the brake one.
     
  7. ChapmanF

    ChapmanF Senior Member

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    Maybe you've been opening the door more often because of working on this?

    The pump only runs as long as it takes to replace the pressure that leaked off while it was off. If it runs for a couple seconds after sitting overnight, it won't take that long if it hasn't sat as long.
     
  8. koco

    koco Member

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    That's the odd thing; it usually runs for longer than a half second and seems to be random or when the brake is used, but it now runs every single time the driver's door is opened.

    B mode engages the smaller MG1 for regen braking so it can be employed along with MG2 for even more regen braking fun.

    At this point, I lean towards the actuator assembly being at fault, it would be good to narrow it down to something more specific, but it would take a whole big-picture analysis of many things.

    The stroke simulator has something like a potentiometer and some hydraulic lines on it, and these must be there to measure pedal travel and send fluid pressure to another part as part of the calculation of how much regen force is needed. If one of those things went bad it seems like regen would stop working.

    It sure would be good if some aftermarket company would produce some new abs accumulators since these second gen cars will be around for a while.
     
  9. ChapmanF

    ChapmanF Senior Member

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    B mode can't really be described as simply as that. The HV control ECU is controlling MG1 and MG2 in combination all the time. B mode changes a couple of biases in the HV control ECU firmware, so that (a) a larger braking effect is applied when the go pedal is released (to feel like a conventional car downshifted) and (b) engine braking is used earlier to hold down the regen power delivered to the battery, so that the battery is less aggressively charged in a long descent.

    No, the stroke simulator has a cutoff solenoid valve on it, which is only used in fail-safe operation, to take the simulator out of the picture. (When your foot is making the only brake pressure there is, you want it going to the real brakes, not to the springs in the simulator.)

    There is no potentiometer there, or anything in the stroke simulator that measures travel. The master cylinder pressure sensors detect the pressure in the fluid that results from your foot pressure, and that is the input to the ECU's computations. Post #2 above explained it. Those sensors are in the actuator.

    There's documentation available on how the brake system works. Not everybody feels like looking such stuff up and some will choose not to, but when choosing not to, it's good to avoid just making up other ways it "could" work. Problem-solving can get sidetracked that way.

    [​IMG]
     
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  10. koco

    koco Member

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    That's a cool brake system diagram. I don't see the part that has the pin switch for the driver's door wired to the pump motor.
     
  11. meeder

    meeder Active Member

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    Because it isn't directly wired. The ECU will detect that the door is opened and it will trigger the brake system to start building pressure.
     
  12. ChapmanF

    ChapmanF Senior Member

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    It's a little more fun than that. The door pin switches are wired to the Body ECU. So the Body ECU sends a message to the Skid ECU, "hey, driver's door just opened, get ready to drive."

    Then the Skid ECU looks at the accumulator pressure sensor reading (PACC in the diagram above) and, if it is below the threshold, the ECU runs the pump until the pressure is adequate.

    That goes to show a couple of things:

    First, the documentation of the car stretches to more than one page and more than one diagram. If there's something you want to understand about it and it's not on one diagram you're looking at, that's generally only telling you that you'll find it somewhere else.

    Toyota Service Information and Where To Find It | PriusChat

    Second, if the thing you're not seeing is something you only think should be there because you've guessed at how the system works, and you're not seeing it because the system doesn't work like that, that's the way learning happens. :)
     
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  13. koco

    koco Member

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    If you're not curious and speculative about an approach to a diagnosis, you won't get far with it as long as you eventually narrow it down to something that isn't a speculation. You have somehow concluded that I am merely guessing and wishing that the thing works only the way I Imagine it to work, and that's an awful lot to glean from what I have written. It seems like an attempt to portray another user as an inferior for thinking their own way. People have primarily three different learning styles, that's why they will not all understand things the same way.

    I don't really have time for a complete reverse engineering of the brake system even though the information here is nowhere near close to that. One of those animated videos of the entire system would be novel to look at for a few minutes. There is a transducer or similar device to measure the level of brake application or else it wouldn't work.

    I am mostly posting here to see if someone has maybe had the same malfunction of the brake system. It would be good to get a reply based along those lines since the other related threads don't have it. So if you know a guy who knows guy that had the same problem, please share.

    I will probably just change the pesky abs accumulator again and move on.
     
  14. ChapmanF

    ChapmanF Senior Member

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    It's awfully well-documented in the repair manual and New Car Features manual, so there wouldn't be any need for reverse engineering except to learn some kind of obscure unpublished internal detail; what's published is plenty to answer the questions in this thread.

    Indeed there is; post #2 covered that.
     
    #14 ChapmanF, Aug 11, 2022
    Last edited: Aug 11, 2022
  15. mr_guy_mann

    mr_guy_mann Senior Member

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    You haven't really given enough information to let others understand what is happening. Using the word "regen" here and there is getting in the way. Replacing parts based on symptoms matching up is guessing. Sometimes that's good enough. Other times, not so much. Since some faults can be expensive to repair, "we" try to push testing, which requires understanding how things work.

    If you have an accumulator problem, does the ABS pump run frequently when the car just sits there READY? Does the pump come on with every brake pedal apply? When the pump turns on, how long does it run before turning off?

    Actually, your post #5 does jive with having a failed accumulator (the actual part that stores high pressure fluid). Having some data (above questions) to back that up would be nice.
     
    #15 mr_guy_mann, Aug 11, 2022
    Last edited: Aug 11, 2022
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