My understanding is that the main traction motor freewheels, so to speak, when encountering a sudden externally caused change in velocity in order to avoid damaging the transaxle. If the force is too sudden or extreme, the Power Split Device (essentially a planetary gear set) could be damaged. The motor cuts out briefly to avoid transmitting the impact of the sudden velocity change to the PSD. It does this during traction control, as well, resulting sometimes in unintended deceleration! This protectiveness suggests a certain fragility of the PSD. I haven’t yet found a link to support this notion, however, and I can’t remember where I read it. But I agree that burning out the motor seems unlikely.

The computer is supposed to compensate for regen dropping out by increasing friction brake pressure, but it doesn’t always succeed in matching the difference fast enough to avoid detection.

A possibly related factor is that reports of **accidents** resulting from alleged unintended acceleration occur overwhelming (>80%, if I recall correctly — See The Truth About Cars web site on the whole Toyota Unintended Acceleration subject) in parking lots, ie rather low speeds. Only a few report at highway speeds. And it is at lower speeds that the braking discontinuity tends to occur. So possibly that is part of it. The rest is most likely caused by people pressing the go pedal instead of the brake. See TTAC again for reference to some studies that show poor awareness by some people of leg and foot positioning, esp when turning the head to one side.

It seems like a semi-plausible explanation, but I don’t understand how a bumpy road could cause an electric motor from being overloaded.

From what I’ve read there is a brake-override system in place, but it works as follows:
– if there is moderate pressure on the brakes
– and the accelerator is depressed more than 50%
– the brake override engages and shuts off the engine

GreasyMonkey,
Thanks for your post.

This happens with ABS brakes too, by the way. NOT Unique to the Prius!

The reason why Toyota engineers programmed the hybrid computer to momentarily cut out the regenerative brake when going over a bumpy road is to protect the car’s electric motor (being used as a generator while braking) from being overloaded and getting burned out.

You can actually bypass the regenerative braking cutout and immediately engage the hydraulic disc brakes by pressing on the brake pedal HARDER. Past 50% application of the pedal, the disc brakes will engage.

This is a predictable behavior in the Prius that you can learn to compensate for. NOT an issue if you understand what is going on and how to deal with it.

It’s still likely that:
– there is a minor problem in a small number of vehicles
– most reported cases were actually due to human error (the case in most vehicular accidents)
– the media totally blew things out of proportion and ran with bad information (as has happened time and time again)

Really, the news stations should have:
1.) dug up the accident data (e.g. accident report + black box data);
2.) found an auto engineering expert from the NHTSA, IIHS, ASRI, or VTTI;
3.) had them analyze the data to verify the claims of malfunction;
4.) THEN report on the findings, whether they indicate a vehicle problem or not.

But all of this takes work and time. Most journalists are just too lazy to do legit background research and lack journalistic integrity. They just want to be the first to break a sensational story. And then after that the media turns into an echo chamber.

I still love my Prius, and will buy another one when this one wears out (120K and going strong!). But I wish that Toyota would acknowledge that there is a problem (and admit that they don’t know why if that is the case).

At this point, everyone will want to feel better, and they will come out and say “we didn’t find anything”.

But it’s not a surprise they didn’t find anything – if some glitch was giving the computer bad data, it’s not a surprise that the black box was also recording bad data. There simply is no way to say for sure, after the fact, whether there was unintended acceleration or not.

One good thing – this whole incident has probably caused nearly everyone to add a “brake override” system to their cars, a simple safety feature cars should have had in the first place. Unintended acceleration has happened for years with purely mechanical cars (something in the carburetor gets physically stuck open – yes I know a modern car doesn’t have one, but it still has a number of mechanical parts that cause the car to accelerate) and there’s no reason why cars with electronics haven’t had this.