Prius 4th GEN Brake diagram

Hi ,

Those exact diagrams are found in a manual called the New Car Features manual (its name in English anyway); here is some information on how to obtain it, in different regions of the world.

 

sure, why not?

 

What are you wanting to do with it? Yes, seems very complicated, but we ask more and more of our brakes these days.

 

Here’s the same diagram (from New Car Features, under Brake: Brake Control/Dynamic Control Systems: Brake Control System: Brake Control), in which I’ve replaced the reference numbers *1–*32 with text labels for easier reading:

The same section of New Car Features has text, tables, and twelve other versions of the diagram, which explain how the brake system and its valves operate under various conditions. I highly recommend it.

 

i only ask one thing of my brakes, the rest is icing...

 

It's actually got 3 "normal" functions - plus about 7 others which are happening from time to time, possibly without you knowing, as well as 2 emergency functions (if one brake circuit malfunctions, plus if there's a Power Supply malfunction.

 

Goodness gracious, what's with the new small pumps and reservoirs inside the actuator, and what's a gap hold valve, and why does the stroke simulator cut valve look like it's normally-open and in the wrong place to cut the stroke simulator?

 

must be why there's a recall

 

I believe they are return pumps. New Car Features doesn’t explain, but see, for example, U.S. patent 6,079,799 to Sawada et al., reference numbers 9, 10, 39, and 40, and Bosch’s book Brakes, Brake Control and Driver Assistance Systems: Function, Regulation and Components (2014), page 75:

If the degree of slip of any of the wheels increases further despite [switching the solenoid valves to the ‘maintain pressure’ setting], the pressure in the brake(s) concerned must be reduced. To achieve this, the solenoid valves are switched to the “pressure release” setting. The inlet [S__H] valve is still closed, and in addition, the outlet [S__R] valve opens to allow the return pump integrated in the hydraulic modulator [brake actuator assembly] to draw brake fluid from the brake(s) concerned in a controlled manner. The pressure in the relevant brake(s) is thus reduced so that wheel lock-up does not occur.​

The gap hold valve SGH (to its designers, 離間室カットバルブ, separation chamber cut valve) is open during normal operation, so pressure from the gap hold chamber behind the input piston in the master cylinder can reach the stroke simulator. If the brake actuator or power supply fails, SGH is de-energized and closes.

Similarly, the stroke simulator cut valve SSA is closed during normal operation, but if the brake actuator or power supply fails, it is de-energized and opens.

 

To be clear, the (two) pumps shown inside the actuator here are new with this Prius generation. All previous Prius generations have had ABS functions, but have relied only on the pump and accumulator shown at the top of the diagram here, which all Prius generations have had and which are still present in this one.

So something new is going on here. It probably does have something to do with the patent Elektroingenieur linked to, which begins by describing some deficiencies in prior systems when trying to provide not only ABS but also "sideslip prevention control".

 

I'm speculating that they are trying to make a system that has finer modulation under ABS braking, as an improvement over older systems that just switch valves on and off. For example on a surface with uniform traction, a skilled driver can probably stop a little faster than ABS by threshold braking. If they can make the ABS as good as that, it would improve stopping distance and the ability to control the car.

The Prius sometimes has a different ABS sound than older cars I've driven. Older cars make a fast clunking sound, or almost a grinding sound as the valves switch. On ice the Prius makes a buzzing sound that increases in frequency as the car comes to a stop, and continues for a moment after stopping or releasing the brakes. I'm not saying that's the sound of the two new pumps. It might be, or maybe they just switch the valves really fast, but either way it's different. I haven't heard that in other cars, but I've mostly driven cars sold in the late 2000's before this one.

 

All the Prius generations have had linear solenoids in the brake actuator, so they can modulate ABS pressure; they are not Grandpa's ABS that just switched on and off.

The extra pumpy bits in Gen 4 are new. I would guess that when the patent says it's about combining ABS with"sideslip prevention control", they didn't just slip those extra words in to make a minimum page count.

 

It is now dawning on me, another thing they changed here with Gen 4. When I first saw that diagram I noticed the two new strange pumps, but until now I didn't notice the change to the way the four brakes are grouped. You can see in a Gen 3 diagram the two fronts are grouped together and the two rears (the same as the previous generations), but Gen 4 has gone with two diagonal pairs.

Huh ... and in this design, of the two hydraulic lines shown linking the "Brake Actuator" dash-dot-dot box and the "Brake Actuator Assembly" even-dashes box, neither one is a straight return line to the reservoir. They are both supplying pressure, one to each diagonal wheel pair. When fluid needs to be returned, it looks like it is up to those two return pumps to push back.

 

This design seems like they could be contemplating making that even-dashes "Brake Actuator Assembly" box into a part that can do ABS duty in many different vehicles, with the hybrid/share-braking-with-regeneration magic all contained in the dash-dot-dot "brake actuator" box.

But still, when I look up that even-dashes box, its "what this fits" listing only shows Prius and Prius Prime, so it doesn't seem to be quite universal yet.

 

From what I see on that diagram, I would agree 100% with your assessment. The "brake actuator assembly" looks to be laid out as a normal everyday (non-integral) ABS unit.

Apply pressure goes straight through the assembly to the wheels until the ecu decides it needs to perform ABS modulation. Inlet valves shut to isolate each wheel as needed, then outlet valves open to dump pressure into (low pressure) storage accumulators. At the same time, a single motor drives both pumps to push any fluid from the accumulators back upstream of the inlet valves.

How refreshingly conventional. And reliable (I occasionally see a leaky outlet valve on one of these but not very often). All the gee-whiz stuff is in the M/C-pump (actuator) assembly.

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