Another Brake Actuator Question

Umm, the worse that can happen is that you lose braking function completely. Under normal conditions, the brake actuator does ALL the work. You pressing on the pedal just tells the computers that you want to slow down and they decide how much regen vs how much hydraulic force to use. The actuator creates the pressure needed to apply the discs and drums. Your foot is not connected to the front or rear brakes.

If the abs actuator fails it SHOULD default to a manual mode where the master cylinder (your foot) directly applies the front brakes only. This is an emergency backup mode that has no boost.

Short form, based on your description, your braking sytem IS failing and I wouldn't drive it anywhere- at all. Besides the high possibility of having to depend on a backup system, there is the situation of what could/ would happen if braking performance was to suddenly degrade because of actuator failure.

Ask any airplane pilot- they have backup systems on the plane but hope to NEVER have to use them.

Posted via the PriusChat mobile app.

 

No, not exactly. You will lose all POWER assist and any other additional braking features (abs, traction control, etc) but you will still have manual, un-assisted braking ability. The brake pedal will feel VERY hard and VERY unresponsive but it can still provide limited braking force.

"In manual mode, there will be no boost so the driver will have a very hard/stiff brake pedal. The driver will need so much braking effort in this situation that it may prompt them to tow the vehicle for fear of having no ability to stop at all. Complaints of “no brakes” or “barely stops” is the often how customers will describe a brake system that has entered manual mode (fail-safe mode)."

Undercover: Hybrid Brakes -

Yes, this is an unsafe condition and the vehicle should not continue to be driven.

Here's some more info on the overall condition that every gen 2 owner should become familiar with and stay ahead of the issue (because in many cases one gets a lot of warning):

Brake Actuator Failure | Hometown Hybrids

 

There's more than one possible failure mode.

The normal operation is that the pressure accumulator is kept near full pressure with fluid at all times, and the actuator valves send the fluid to the brakes for you, fully under computer control.

If the pump fails, but the valves and computer still work, there is enough fluid in that pressure accumulator to give you twenty or thirty stops (typically) still with pressure assist. Before you have used up many of those twenty or thirty stops, a constant high-pitched warning buzzer will come on. In Gen 2 there is a big box of electrical capacitors in the back of the car to keep the electronics and valves powered for some time if the car has an overall 12 volt power loss. I do not know for sure if the pump can also be powered from those capacitors; my s.w.a.g. is 'no'.

If the electronic controls fail, or if you have used up your twenty or thirty strokes with the pump not working, you will have a fail-safe mode where your pedal effort directly pushes fluid to the brakes, and you will be able to brake, but the pedal will feel high and hard, and you have to really push. In a Gen 1 or Gen 2, the fail-safe mode only works the front brakes, which might give you car-control challenges depending on the road surface, how the car is loaded, and whether you are cornering.

If there are gas bubbles in the system, most of the time you may not notice, because the pressurized accumulator is able to supply enough fluid to work the brakes even when it has to squeeze the bubbles first. However (COMMA!), if the system ever goes into fail-safe mode with gas bubbles present, it will be just like any conventional brakes with gas bubbles: the pedal will feel soft, may go to the floor doing nothing but squeezing the bubbles, and the car may not stop.

Floored-pedal-no-stoppie problems are nowhere near the most common ones reported here, but they do get reported from time to time.

So it's worthwhile to find out, as specifically as possible, not just that you have a problem, but which problem you've got.

 

True enough- but I never like to assume that a failing complex part will always fail exactly like the engineers think it will fail. IF the internal valves correctly seal or open as designed when in default mode then you have some manual braking. I hope to never have to find out.

Given that the OP's car has a noisy pump (so it's worn out and failing) that's cycling on every 10 seconds or so (so there's a massive internal leak as well), he may find out firsthand.

 

At the risk of repeating #4, it's also known that there's more than one failure mode, and "you still have pedal, it's just hard" is only one of them.

 

It is not uncommon for PriusChat readers to throw together an old beater laptop and an inexpensive J2534 dongle and have a way to read their own trouble codes for themselves whenever necessary. It never stops paying for itself.

From your description of the symptoms, it does seem unlikely that you'll find a solution other than actuator replacement.

 

Mr Chapman, A FURTHER QUESTION ONCE BRAKES REPLACED - continued buzing from actuator?

Thank you for excellent detailed responses. You are a breath of fresh air in a covid world. I have read about 30 chats about Brakes - so geting cleaer about the pressure and the many bells and whistles that the techstream handles. Duly noted your advise to acquire dongle / techstream, for next time... Car scheduled for Mechanic later today, to glean codes and maybe flush fluid. But in the meantime... What are the problems caused by not following the unhooking procedure?

BACKGROUND
DIY Back brakes (shoes, drums, cylinders) on my 2004 prius 7 (183k mi) without software nor with out undoing 12v. Opened door to pump brakes with out car on at first, then with igon..

Anywho - bleeding went no where and he thus, did a gravity bleed. As you can imagine, the Exclamation, ABS, VSC and red brake lights came on. Now there is a faint, continuous buzing sound when the car is in ready mode. .

Test ride about 1 mile - braking forwards and backwards, below 25 mpg. . Brakes feel a little bit better than before.
We cleared the codes via the 4 /13 jump, but the buzing sound still there and sound coming from actuator area.

Did we bust the actuator?

What are next stepps ? sound we try to undo 12 v and maybe hv plug for 10 or 20 minutes?
WIll mechanic who has software be enough? Any other adivce appreciated... Do you accept tips?
Bless you!

(my brother bless his heart, spent 4+ hrs doing this, but when given procedural research, If its not in video form, he ignores it - dyslexia. I cant complain = he already saved my butt by changing 1 HV module) He may have thought this old brake technology could not come with anything more special to do than what hes been doing all his life.... and toyota mechanic walked him through it without mentioning the 12v etc.... i guessed why$$ ) .

 

H᠎i,

It's not easy for me to say anything about the buzzing sound or the sound coming from the actuator area, unless you can describe them in much more detail or (better) post a recording.

Wouldn't be any need to remove the HV battery plug in any case; nothing in the brake system runs on 200 volts, and the output of the HV battery is completely cut off whenever the car's off anyway.

If it were my car, I would just occupy myself with other projects until a tool for reading the trouble codes was at hand, and then proceed to identify the issue.

By the way, if you say you "cleared the codes via the 4 /13 jump", you were this close to knowing what the codes were. The purpose of the "4/13 jump" is primarily to get you the codes (by making light blinks you can count). Secondarily it gives you a way to clear them, but why that keeps being what everybody thinks it's there for is beyond me.

 

ok thanks, yes i messed up on codes... ill upload video if i can get small enough file to upload.

As i said mechanic said ibuzz was normal but i never heard a constant buzz before, and its coming from actuator area.

resourvoir filled to max after brakejob yesterday but after driving it 12 miles, its down midway b t min max.
Front brakes changed about 2015 and last months inspection said 5mm.


Aside, we changed coolant pump day before brake job. Triangle thrown when i was on highway. i drove about 12 miles after that.

should i fill resourvoir to max or is better to leave as is to compensate for fronts?

uploaded small file here engine running normally cold start... buzz not evident ig on when not in ready mode. oops its not taking mp4... t b a

 

The only time the level should be at max is when (a) the pads are at brand-new thickness and (b) the pressure accumulator is zeroed down. Under all other conditions the level is lower. Techstream makes it easier to zero down the accumulator for checking the level. (Otherwise, it'll often be pretty close if you can sneak up on it after a long overnight park without opening the driver's door or doing anything that wakes it up.)

 

i copied video if buzz to u tube.
here is the link. hope it works.

 

The sound I'm hearing prominently in your video is almost certainly this. That's exactly what the throttle body sounds like in my 2010. You might not believe me until you get your hands on a mechanics' stethoscope and touch the throttle body with it, and then you probably will.

Unrelated to the brakes, and harmless.

The engine coolant goes through the throttle body, and water is a great conductor of sound and there are other pipes and hoses going everywhere in the engine compartment, which is why the sound kind of seems to be coming from everywhere. But even in your video I can tell it is louder when your phone is closer to the throttle body, and not nearly as loud anywhere near the brake components.

 

ok, my bro has such a scope and we will check it. relieved thus far. We are Doing Shocks / Sway next. last year she started feelin like a jack hammer... oh joy.

btw im in north east nj , but lived, shortly, in se indy. oldenburg... farm. it was heaven, but every other month had to go to Cincinnati city to get recharged... good memories in my youth! Have great respect for yall and your state...
thanks again...