It is a definite repeatable phenomena for me: Use accessory mode (by pressing on Power button without foot on brake) in the parking lot (like when listening to the radio, waiting for someone) => when the waiting is over, press on the brake and press the Power button again => put vehicle in reverse => grabby brakes (explain to wife that you are not doing this to spill her drink on purpose) I don't want to repeat the earlier discussion, but was wondering .. what was the resolution to this?
Why use accessory mode? If you are stopped in a lot listening to the radio, your engine will be off anyway. I have sat in the car for 30 minutes waiting, with engine ready and the ice did not kick in and the HV battery did not discharge even one bar. IF I had to wait longer than 30 minutes for someone, they better be bringing me dinner not just coffee.
That is a different discussion: Why did Toyota bother with the Accessory mode? The point is: whay are the brakes grabby? Is it worth the worry?
What could be causing it? Bad hardware design? Bug in brake-pedal-interpretation software? If so, maybe it will help if NHTSA+NASA+NAS knew about it.. it might help them diagnose/identify issues with the system software robustness.
i think some of it has to do with the cooling and the wear direction and mode of the brake material. i'm starting to think it is inherent to some rotor/caliper designs as i have experienced this in a number of vehicles.
My theory is that it's related to the power brakes; the same noise you hear when you open the driver's door. I don't know exactly how the mechanisms work, so I won't speculate on the details of what's happening, but presumably it does that either as an intentional part of the design (for example, it reverts to using a mechanical braking mechanism to avoid draining the battery too much for the power brakes) or as a side effect of the design (for example, it keeps pressurizing the hydraulic system until it builds up too much pressure). It sounds to me like if you just turn your car on and go, you'll never experience the issue.
Yes, starting the car normally and reversing is always normal. Your logic of engaging power brakes vs mechanical brakes etc would be implemented in the control logic, which is something that the agencies are looking at.
Here is a good place to start, it has helped me immensely. http://priuschat.com/forums/newbie-forum/48705-what-every-newb-should-know.html
While it may be repeatable with the Accessory -> Ready sequence, that is not the only sequence that it occurs under. I have it happen regularly without first using Accessory mode. It's annoying and I hope it's fixed someday but I don't see it as a safety issue in any way.
Prius does not use "brake by wire". They are hydraulic brakes, with electrically generated hydraulic pressure to give them more power. That electrically generated pressure is stored in a bottle, and the pump doesn't run for more than a few seconds. While going forward the system is more complex, due to regeneration, from a stop in reverse it's a simple hydraulic brake system. I suspect the grabby brake issue is not with the system design, but rather the pad material or perhaps warped rotors. I'd look at the rotors first. Warped rotors are very common. Note that in reverse you are using electric propulsion only, so you have a lot less power to overcome brake friction, so a warped rotor will be very noticeable. Further note that the GII didn't have this issue, but it has drum brakes in the rear. That's the only difference in the two systems, as far as I can tell. So that furthers my suspicion of warped or contaminated by rust rotors, particularly in the rear.
For me, it never happens (that I remember) in normal use, only when I have used the Accessory mode before starting up and reversing. That won't get explained by purely mechanical issues like pads or warped rotors.
To wrap this up, it is better to NOT use ACC mode as that will drain the small 12V battery. The ON mode will use the larger traction battery and ICE as needed. So, I should be using ON mode for radio etc when parked. However, this is OT for this thread.
