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brake valve fail open conditions/consequences

Discussion in 'Gen 2 Prius Technical Discussion' started by noa, Apr 19, 2023.

  1. noa

    noa New Member

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    Alright I've got a 2005 with about 150,000 miles on it, a copy of techstream, and I'm stranded in Manhattan New York about 150 miles from where I live. If you have a prius guy out here please let me know.

    DTA CODES
    C1252 Hydro-booster Pump Motor
    C1256 Accumulator Low Pressure

    I think C1256 is irrelevant, all the pressure is just discharged and can't be refilled. For C1252 the repair manual diagnostic flow chart is linked below.

    My prius's braking system is in "failsafe" mode where the isolation valves are failed open (valves between the stroke simulator and FL caliper, master cylinder and FR caliper), I only have front brakes, the beep is sounding, the ((!)) ABS VSC lights are on. I'm not sure if regen braking is truly off, haven't tested as I don't want to drive.

    If anybody has a complete knowledge of causes and consequences of this failsafe mode PLEASE share. I'm wondering if it engaged because of a bad motor or if something else caused it to engage and now the motor is prevented from functioning. Especially because the brake system has a recent history and recent repair:

    Full story: my rear brakes were grabbing in the rain, I opened them up to put silicone grease on the contact points, it turns out the friction material on my rear right shoe got stuck and upon taking off the rotor it all fell off. I drove the car like this for a while maximizing regen braking, letting the shoe just grind on the rotor otherwise, and it was fine. Later I have my first encounter with "failsafe" mode where the brakes were hard, its beeping, etc, and I cleared it with the 8 pump paperclip jump trick and had no problem. I figured the system was overreacting to something that WAS weird with that rear passenger drum brake. A month passes and last Wednesday this happens again but the paperclip clear doesn't work, so I resolve to carefully drive to Autozone for new shoes and a new rotor, as I drive it clears. Now the mode is back, I've changed the brake shoes, and the C1252 cannot be cleared.

    I've tested the pump relays MotorRelay1 and MotorRelay2 and they're fine. I wish I could directly stimulate the motor with techstream but I don't think I can. I wonder if bleeding the brakes or initializing the linear solenoids could possibly help. Perhaps I damaged the cylinder while working back there?

    The pump showed no signs of dying, it wasn't running for abnormally long or too often. My suspicion is that the recent history with the brakes caused confusion in the system somewhere along the line. I want to narrow things down completely before I cave and replace the brake actuator module.

    Knowing exactly the consequences of failsafe mode (does it disengage the pump motor?) and causes would be very helpful.

    Thanks

    Btw my 12V battery is 12.4 measured while off in the back, not sure if that's low. Tried to measure it in the front in the fuse box but it seemed to be oscillating, if you know whats up with that let me know.

    Links: nevermind i cant add links as a new poster!
    DTA C1256
    paperclip clear
    helpful video on brake system
    There were many helpful forum posts on this topic but none seem to address my exact issue beyond this point.
     
  2. dolj

    dolj Senior Member

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    The thing with "this trick" is you should do that after reading and recording the codes blinked out on each respective dashboard light.

    See here: Blink (a/k/a Flash) Codes – How to. for details on how to do that.

    Failsafe mode is designed to allow you to safely pull to the side of the road and get help. You will not have regenerative braking and you will only have unassisted braking on the front brakes.

    You should get all codes by the blink method above, or by connecting Techstram and listing them here.

    A couple of things I noticed in the diagnostics for those two codes were the detection condition for the C1252 is that the motor relay is ON for at least 5 min. That sure would have an impact on drawing the 12 V battery down low if the car was OFF. Secondly, the repair manual notes that DTC C1256 (accumulator low voltage malfunction) may be memorized if the power source voltage drops. Food for thought? I would think the logical step might be to work through the diagnostic for C1252 to figure out why the pump runs for so long.

    FYI, In addition to not being able to post links, your next 3 posts will be reviewed by a moderator to ensure they are from a real person and not a spambot. We will not see your posts until this moderation takes place just so you know we're not slow to answer. (y);):)
     
    #2 dolj, Apr 19, 2023
    Last edited: Apr 19, 2023
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  3. noa

    noa New Member

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    First off I meant "DTC" not "DTA".

    Anyway I realized I was only scanning the ECU for the ABS/VSC/TRAC. I did a full "health scan" and it revealed that in the Transmission Control ECU there were DTC codes C2300 (Actuator System Malfunction) and C2318 (Low Voltage Error (Power Supply Malfunction)). For what it's worth there is a B1421 in the Air Conditioner ECU.

    I cleared them, rescanned and got just my two ABS codes back. But it makes me wonder about the 12volt battery triggering something strange.

    I also noticed some weird behavior from one of my ABS motor relays. It would be on when it shouldn't've been and when I'd test it it'd continue to behave sort of haphazardly. I swapped it around with the other ABS motor relay and it continued to act that way in the new location. Possible this is related to low 12v battery? I took it out to test it and it functioned fine.

    I didn't write these numbers down but I checked my 12v voltages using the headlight flip diagnostic menu and I believe it was 12.4 in ACC, 12.0 in IG-ON, and 14.4 in IG-READY
     
  4. noa

    noa New Member

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    I did this, it's pretty explicit in the OP. Blink codes are redundant given the DTCs but for anyone referencing who doesn't have DTC scanner access, they were ((!)) 57, ABS 42, VSC 45.
     
  5. dolj

    dolj Senior Member

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    Yes, I picked up on the fact you have Techstream after I originally posted. Sorry about that.
    Yes, there definitely seems to be a theme there. I alluded to that in my comment about the C1256 being set. The thing to remember is if you are getting these codes when the car is READY, there would have to be a significant draw to bring the 12 V line voltage down to the trigger points for each code - enough that the DC/DC converter was not able to keep up with demand. You should have FFD (freeze frame data) for each (or some of the codes) to see what the various 12 V reading were when the codes triggered. Figuring out why will be key (he said stating the obvious :)) to solving this problem.
     
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  6. noa

    noa New Member

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    Thank you this is a great point. I did screenshot the freeze frame data and it is telling. We're talking ~6.4v in the C2300, ~8.3v in the C2318. I looked at the current data after and it was a normal ~14.2v. Pics attached for these datapoints.

    I don't understand the 12v electrical system well at all (eg DC/DC converter and all that) and I'm not sure why it would've dipped like that.
     

    Attached Files:

  7. noa

    noa New Member

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    What worries me is that it's brought up often enough as a possibility, but I don't recall seeing many success stories for this brake system issue as a result of replacing the 12v battery. Perhaps it just wasn't so strongly on my radar when I was browsing days ago though.

    I think my order of operations after I get the car towed home will be replacing the 12volt battery then replacing that bizarre ABS motor relay (I wonder if I damaged it while testing it, holding the oversized jumpers and the relay all myself was a balancing act that led to the relay turning on and off extremely rapidly; the circuit was closed but still floating in the air -> a brief very rapid series of oscillations that maybe it wasn't made for). I can travel down the C1252 diagnoses (I should be able to link these tomorrow once I'm less of a new member. It's a lot of circuitry connectivity stuff).

    Anyway, from there maybe initialize the linear solenoids? Maybe bleed the brakes / make sure I didn't damage the rear brake cylinder? Work my way up the line until I may need to replace the whole actuator assembly. Suggestions welcome.
     
  8. dolj

    dolj Senior Member

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    Yes, it is brought up so often, usually by people that do not have a clue, that it has achieved folklore status. Invariably it does not end up solving anything and the person with the problem is out several hundred dollars for a battery they didn't need.
     
  9. ChapmanF

    ChapmanF Senior Member

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    I'm not sure I'd go so far as saying it never solves anything; stopped clocks are right twice a day, and especially if the trouble codes you have include codes about supply voltage, it can be a reasonable thing to check. And if your former battery was pretty old, at least you get a newer battery out of it.

    But even the supply-voltage codes can be caused by things other than the battery, such as wiring or connection issues on the wiring path that supplies the voltage to the brakes.

    Typically, the troubleshooting steps shown in the manual were already put there in an order that Toyota thinks will get you to the solution with few wasted steps.
     
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  10. noa

    noa New Member

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    I just found that failsafe brake mode does not inherently mean that regenerative braking won't function. I'd seen the opposite claimed.

    I wonder if the mode directly prevents the motor that charges the accumulator from running.

    And thanks ChapmanF. Havent tried anything yet this week.
     
  11. adam Boudili

    adam Boudili New Member

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    I had same exact problem. Same lights same brake feeling everything.
    1252 1256 where the same codes I had. Checked for sensors/ leaks / oil/ brake fluid… everything else that could cause abs problems that’s minor. Ultimately I found a honest mechanic and he changed The ABS CONTROL MODULE AND ABS ACCUMULATOR ! my Prius v 2012 is now perfectly running. No lights, no hard brakes, nothing. You are wasting your time if you don’t get the accumulator and control module because trust me I looked for every single possible way to avoid it. Got both to be installed flr $1800 altogether. I was blessed with such a honest mechanic. Good luck.
     
  12. ChapmanF

    ChapmanF Senior Member

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    Is that with the 2012 Prius v mentioned in your profile? I see that you've made this identical post on several threads, some Gen 2 and some Gen 3.

    There are some differences between the generations: if you meant a Gen 3, it sounds like your mechanic replaced two assemblies under the hood. In Gen 2, the accumulator is integrated with the actuator. In Gen 3, they're separate assemblies and you don't always need to replace both; sometimes you can diagnose which one is at fault.

    Also, there are other possible causes of the codes. If the pump can't run for an electrical reason, for example, there will be too-long-pump-run and low-pressure codes.

    So a person wouldn't be wasting time, to go through the recommended diagnostic procedure before going straight to the big-bucks replacement. If the replacement is what's needed, the diagnostic procedure will tell you that. If you want to save time and just bet that way, and you win the bet, that rocks. But it'll suck for the guy who jumps right to the big-bucks fix and it still doesn't work, and then finds out the 5¢ wiring fix takes care of it.

    NB when I say "go through the diagnostic procedure", I just mean the steps in the manual for pinning down what's causing the problem. I don't mean the kind of further wishful thinking people sometimes go through before finally getting it fixed.