Brake Issue

welcome! that's a tough call over the internet. do you think you can duplicate it for a service tech on a test drive? a lot of things can affect prius braking at low speeds, especially moisture while sitting, the rotors rust up,so it's hard to know if yours is normal or not.

 

When this transition happens, is it consistently at 7 mph? And only with light to moderate braking, not hard braking?

If so, it may be the low speed transition from regenerative braking to purely friction braking. Regeneration doesn't work at lower speeds, so is shut off, and the transition to pure friction-only is not necessarily seemless in my Liftback. It is merely an annoyance, not a safety issue. With harder braking using both regeneration and friction, I don't feel the change.

If this thing is happening at other speeds, then something else is happening. Even if it is this transition, if the change is excessive, then a brake inspection may be warranted.

 

Check out post #4 in this thread. Brakes Grab When Wet | PriusChat

 

I am the OP on the referenced thread. My C still does that.

SM-G900V ?

 

I realize this is an old thread, but I've been searching for information on braking mode transition and how it almost always feels unpredictable to me at low speeds. It's actually pretty unnerving for me, as I've had way too many close calls, almost lightly rear-ending cars at stop signs, for example, where I might be timing my slow-down speed to coincide with the car in front's departure from the stop sign.

There's (1) engine braking, whose response changes with the CVT and stops altogether when the engine cuts out, (2) regenerative braking, which is speed sensitive and managed for battery state of charge and probably other factors, and (3) friction braking, which is electrically-hydraulically operated, possibly computer-controlled. Managing the transitions between these various and changing modes has to be a real engineering challenge. Unfortunately, it's not as smooth and predicable as I'd like, and I feel like it's only a matter of time before I bump someone.

Do other people have issues with precise control of slow-speed braking? The best way I can explain it is to say when I'm rolling up on a stop sign and apply the brakes, at some point during the slowdown, I've applied the brakes and, let's say, I've calculated that I should come to a stop in about 100 ft. But as I reach about 30 ft from my desired stop point, the braking is insufficient and might take another 45 ft to stop. Mild alarm sets in, and I gently press the brake pedal harder with little effect. Now it's too late to stop smoothly, and I press even harder and come to a much more abrupt stop than I wanted to. This happens EVERY TIME, and I have yet to get used to it.

Another problem I have is inching forward while parking. From a stop, I let up on the brake pedal to inch forward and lightly press down again expecting to stop in 12 inches or so. But either the electric motor is powering through the braking or the brakes just aren't responding as expected, so the car basically keeps going even though I've applied more brake pedal force. So I press a little harder, and it's again a very abrupt stop. It's like pressing the brake a little harder gives nothing, nothing, nothing...then too much. It's annoying and begging for a small accident.

I asked the dealer about this, and the veteran service writer had no idea what I was talking about. He was happy to do a several hundred dollar workup for me, and if it turned out there was a known problem, submit it for an extended warranty claim. I declined, given his lack of familiarity with the problem.

Is there a software update or something for this?

 

First issue only: is this happening with light braking only, on rough or very slick surfaces only? If so, it could be the regen-to-friction transition with a sometimes disconcerting momentary pause, when traction is momentarily lost (either in reality, or from some apparent wheel speed mismatches detected by ABS) on the bumps. The car needs to switch from 2-wheel regen-only to 4-wheel friction brakes, and the switch is not seamless. And means that in susceptible conditions, timing to coincide with the bumper of the car ahead, with little or no safety margin, is a rotten idea. (This habit is still a rotten idea on non-hybrids, but is much less likely to bite you there.)

This particular issue will never happen on good smooth dry surfaces, nor with hard braking, nor produce a pause lasting even a whole second. If your case is violating any of these conditions, then you are experiencing something different.

I have no useful comment for your second problem, other than it adds suspicion that your first problem may not be just the usual regen-to-friction transition.

 

This happens regardless of surface, mostly because I'm never getting close to traction limits. It's definitely not a traction issue.
I suspect as you said that it's the transition from regen to friction braking, possibly where the friction braking isn't coming on hard enough as the regen is losing its efficacy due to speed. I'm talking like 15 mph or less. Hard / emergency braking is outstanding on this car, and I have no problems with braking predictability at speeds above 20 mph.

The second issue might be related to how much torque the electric motor is producing at inching along speeds, and when the computer decides to cut power to the motor because it "thinks" you're legitimately stopped. It's all very mysterious behavior at very low speeds, and I suspect it's software controlled, so I may never get around these problems.

 

I agree that the behavior around the transition region is kind of fiddly and involves a learning process on the human's part to stop as nicely as possible, but my experience was that the learning process didn't take me very long before I was making decent, nice, consistent stops.

A known complication is that a layer of flash rust from a damp environment does make the transition even fiddlier and grabbier, only until the brakes are applied a couple times in neutral to clean the rust off. That's been known and posted about forever, and was likely the complete answer to the first post in this thread.

Another, smaller complication is that in any driving gear, there is a little bit of creep force applied by the transaxle at low speeds and while stopped, which prevents you from fully feathering out your pedal pressure as the car slows to make a perfect chauffeur stop. When I feel like showing off, I'll just lean the shifter gently into N somewhere below about 6 mph, make the perfect chauffeur stop, then lean it back into D. The liftback shifter, of course, is completely silent (if you don't bang it to its stops), so your passenger never notices, unless they're watching carefully.

Maybe the c shifter is more mechanical and will make a giveaway kerchunk noise when you shift to neutral? Gen 1 did that. Annoying. I considered concealing an electric pushbutton somewhere that would just force neutral when pressed, but never ran out of less-nerdy projects.

So I guess what I'm saying is that if your braking transition is really so fiddly that you actually aren't learning it with reasonable practice, it might be worth taking your wheels off and just doing a good once-over of your friction brake conditions. There could be something that could use a little TLC.