Directly from the factory shop/repair manual for the 2008 Prius. 2. OPERATION DESCRIPTION (a) Electronically Controlled Brake: The skid control ECU recieves signals from the pedal stroke sensor, master cylinder sensor and wheel cylinder pressure sensor. Based on these signals, the skid control ECU calculates the necessary braking force for each wheel. The necessary hydraulic pressure braking force is sent to the hybrid control ECU via CAN communication. The skid control ECU receives a braking force (regenerative braking force) signal from the hybrid control motor via CAN communication. The ECU calculates the necessary hydraulic pressure braking force based on the necessary braking force and regenerative braking force. Rephased/structured for clarity: 2. OPERATION DESCRIPTION (a) Electronically Controlled Brake: The skid control ECU recieves signals from the pedal stroke sensor, master cylinder sensor and wheel cylinder pressure sensor. Based on these signals, the skid control ECU calculates the necessary (TOTAL) braking force for each wheel. The necessary (TOTAL) hydraulic pressure braking force is sent to the hybrid control ECU via CAN communication. (In return) The skid control ECU receives a braking force (regenerative braking force) signal from the hybrid control (ECU) motor via CAN communication. The (skid control) ECU calculates the necessary hydraulic pressure braking force based on the necessary braking force and (Less the) regenerative braking force. So what happens if the hybrid control ECU is "out to lunch" and never sends the results of its calculation back to the skid control ECU...?? No braking at all...?? Your can press down on the brake pedal with all your weight but unless the skid control ECU opens the ABS/TC manifold ports to allow master cylinder pressure to reach each individual wheel...
I assume there's a timeout mechanism which comes into play in that scenario. Now what happens when the SCM goes "out to lunch" , what is the nature of the backup mechanism which opens the "ABC/TC manifold ports" ? Online sources refer to the presence of this backup.
Agreed. My experience in conventional cars when turned off the brake pedal could be pumped maybe 1 1/2 strokes before getting stuck. In the Prius the pedal can be pumped as many times as you want. After the same 1 1/2 strokes or so you'll hear a pump and have the same braking feel.
If the skid control ECU goes out, the system fails over to manual hydraulic friction brakes. Nice first post, btw. Tom
The documentation clearly implies that the skid control ECU will NOT open the ABS/TC manifold ports to allow hydraulic braking to the wheels if the HSD control ECU "says" it will provide 100% of the braking. So it might not matter how hard your depress the brake pedal the resulting hydraulic pressure may not be ported to the wheels or maybe only a portion provided what the HSD control ECU "reports". Yes, clearly there should be, SHOULD BE, a "time out" period if the HSD control ECU doesn't report back within a very short time limit, say <10 milliseconds. But if you look at the 2010 Prius TSB regarding the fix for delayed hydraulic braking in the event ABS disables regenerative braking then it becomes quite clear that NipponDenso has overlooked, until now, the need for this type of time out. It appears, from reading the TSB, that the early HSD control ECU was sometimes LATE (to busy, too many other more important tasks..??) in responding to the skid control ECU's query and the skid control ECU simply stood by and waited. Meanwhile......NO braking.
Okay, you're pushing on the brake pedal and that means the master cylinder MUST be generating brake fluid pressure. Yet we know that in this circumstance 100% of the braking force might be provided by the HSD control ECU using regenerative braking. Have you a better conclusion or explanation...?
A better conclusion or explanation for what? I still don't know what you are driving at. As for Sikes, we know none of this applies. He had plenty of braking force, which is why he burned the brakes. Tom
By manual, I would think the electrical system is completely bypassed and there is a special outlet in the master cylinder which is activated when the brake pedal passes a certain threshold and enables a direct pedal to brake hydraulic mechanism. Is that correct ?
Yeah, it's called "heartbeat". If the braking ECU loses communication [in the positive, negative, or indifferent] with the hybrid ECU, it immediately falls back to standalone operation, which is normal hydraulic braking with full boost still enabled. It doesn't sit on its thumbs waiting for an answer that never comes. All the ECUs in a prius have very short timeouts for losing network communication; try shorting pins 6 and 14 of your DLC and see how fast you get a "christmas tree" of error lights and a blinking shift-state display that means "no network". . _H*
In that regard my guess would be that since the skid control ECU monitors the brake fluid pressure sensor it might open the ABS/TC manifold hydraulic ports if certain predetermined pressure level is readed. On the other hand you may be correct. The ABS/Tc manifolds may have a pressure relief into the calipers if the brake pressure exceeds a certain level. This latter method would help to explain why Sikes was not able to apply full braking force.
I don't see how the Prius could meet federal light vehicle brake requirements if the base hydraulic system did not function when the overlying electric power assist system loses power.
You may be on to something. This would explain the 2010 issue as well as the problem that some of the 2nd Gen drivers experience when ABS begins its cycle and some drivers have reported a 'lurch' or 'surge' of forward momentum - that I believe is a total losss of braking control (however short lived)! But I offer: Toyota claims if you were to kill the power to the vehicle (hold START buttom for 3 secs or more) that you would still be able to stop the car. It would require more foot pressure on the pedal than your're accustomed to using, but you should be able to stop the car (like loosing power assisted boost in the traditional power brake system in other cars). This means to me that the in the no-power mode, the valves are open allowing the user the ability to stop the car.
There is a big bank of capacitors in the boot next to the 12 volt battery to supply power to the brakes if the system is shut down. When that runs out the solenoids in the brake controller will return to resting locations allowing brake pressure from the master cylinder to activate the brakes manually, unassisted.
Those of us research types love the experiments, here is some fun... After making the car Ready, unhook the 12 volt battery. Now take the car up to 40+ mph and then shut it off (hold the Power button 3+ seconds. With the 12 volt battery unhooked and the inverter disabled, there is no way the ECU is getting or sending any data. Press on brake peddle and release it several times to dissipate any capacitor saved charge. Does car stop or keep coasting out of control? If the brakes still work it must be that there is a physical linkage that engages at some point in the peddle path... See, I'm thinking that the car will still stop.
"..immediately falls back..." Apparently not, according to the recall, in the case of the early versions of the 2010 Prius' skid control firmware. The way the recall reads some revision was made to the HSD control ECU firmware that resulted in the need to respond to the skid control ECU query with a low priority. And there no indication/evidence that in the Sikes incident that the two ECU's weren't still communicating but passing incorrect information. The HSD control ECU, due to an instruction execution "race" condition/fault, may have been "locked" in the cruise control "set/accel" mode. What happens, for instance, if I inadvertently apply the brakes while holding the control in the "set/accel" position, or move the control into "set/accel" while the brakes are applied..?
