[bwilson4web]

Hi folks,

Many have already been following the buzz about the Prius brake problem so I wanted to post a summary. The good news are reports that Toyota has a fix and been putting it in new cars since January.

The symptom is momentary loss of braking force when lightly braking and running over a road surface imperfection. However, it doesn't always happen and about 50% of the PriusChat owners reported they've not experienced it. Only 20% call it a severe problem. In truth, I hadn't experienced the problem until Friday but even then, our Huntsville pavement becomes so super slippery when it rains that I would not have otherwise even noticed it.

Trying to understand what is going on, I collected photos of locations that triggered the braking problem:



. . .


The common elements were:

• slight, downhill grade
• light braking
• some discontinuity of the road surface
• intermittent - slamming the brakes worked RIGHT THEN!!!
• many NHW20 users report similar experiences but not as severe
• initial startle is soon replaced by irritation

So I bought a pair of accelerometers so I could loan one to folks having more luck replicating the problem. But the accelerometers arrived the same day we learned Toyota will be sending out a fix for the ZVW30. No word on the NHW20. Regardless, this is what the Gulf Coast Data Concepts accelerometer looks like in the car:


So this is my first capture of a speed bump:

The red line shows vertical acceleration of the speed bump. The blue line shows braking action with the brakes working right after passing the speed bump. This is normal braking behavior.

Then Friday it was drizzling and the first speed bump at the Redstone, Martin Road gate gave a "slip-n-slide" feeling. Now Huntsville streets are notorious for being slippery in the rain but I was recording the data:

You can see the two speed bump and the momentary pause when the guard checked my badge. It was the first bump that showed the problem including an 800 millisecond, delay in return of braking:

You can clearly see the braking action holds off for nearly 800 milliseconds. Integrating the acceleration gives velocity:

So here you can see how the velocity remains for 800 milliseconds at a speed of a little over 7.6 m/sec. This would add a little over 6 meters to the stopping distance. If a driver wasn't expecting this it could be bad news. But like I said, this is sort of how we expect wet Huntsville streets to behave.

We're expecting the ZVW30 braking fix to be a software download. No word on an NHW20 fix, yet.

Bob Wilson

[geodosch]

I experienced this issue in my 2007, under the conditions described above. In addition to what's listed, I also noticed the traction control indicator on the dashboard flash. However, I did not experience the 'irritation' phase. I was braking on an uneven surface, so I chalked it up to the TCS/ABS system taking a moment to adjust to what was going on. The first time it occurred it surprised me for a moment, but within a fraction of a second the brakes slowed the car. I had it happen several other times, always under the same conditions, and after the first time it no longer surprised me.

To be honest, it may or may not have happened in my 2010... it's such an unremarkable event that I don't even remember. When I first heard there were brake issues in the Prius I quickly checked it out, and as soon as I saw what they were referring to I thought "They've got to be kidding!" I've had several friends and coworkers ask if I knew about the problem. When I described it to one of them, she said "Oh, that? I've had it happen in my Four Runner."

If they have a recall / service event / whatever they end up calling it, I may not even have the work done, depending upon the details of what exactly the fix is going to do.

[jdenenberg]

Guy's,

There was an indication from Toyota that the improved braking response time "Fix" will also help in other circumstances. Perhaps this means that the related acceleration hesitation will also be improved. That's the one I am concerned about in my 2004 as it requires me to be very careful when merging into traffic (The brake hesitation never bothered me.). Does the 2010 exhibit the same acceleration hesitation on a wheel slip?

JeffD

[bwilson4web]

Hi,

I haven't experienced any acceleration delay but that could be driving style and quality of roads. Is there a specific surface ... say gravel covered roads?

Bob Wilson

[jdenenberg]

bwilson4web said:

Hi,

I haven't experienced any acceleration delay but that could be driving style and quality of roads. Is there a specific surface ... say gravel covered roads?

Bob Wilson

Bob,

The acceleration delay issue is a variant of the TC issue "I can't get up my driveway". TC cuts all motive power when there is a wheel slip. This symptom was reduced in the 2007 models where the motive power was reduced, but not completely cut off.

Yes, if I try to accelerate while turning (turning seems to aggravate the issue) on gravel, sand, or snow my 2004 will just stop. This makes me fair game for a broadside. I've adapted by not to taking advantage of our speed off the line when merging.

Jeff

[bwilson4web]

Hi,

We have snow in the forecast. My last snow and ice tests failed to show any problems. Regardless, I now have the A0B patch and will do some regression testing.

I can't claim the A0B patch fixes the 'brake pause' because my test conditions require a drizzle to slick the streets. However, I was about to gather some preliminary data from a street by the dog park:

Pothole specifications:

• near side used for ~20 mph, slight braking testing
• length ~7 ft.
• depth > 4-6 inches

Prior to getting the A0B patch to the skid control ECU, I ran multiple, braking tests through the pothole. An accelerometer running at 160 samples per second recorded the data. During the first braking tests, I noticed a distinct drop in the HSI regeneration bar. Typically from ~75% of the bar to ~10-15% of the bar. Regardless, here is typical deceleration graph from the raw data:

I have multiple runs and each shows a decrease in braking force but not the full-up pause seen 2010/02/05:

We know this is both rare and intermittent. Many have reported not being able to repeat the same brake pause over the same bump. During my testing, I tried a run at 25-30 mph and saw no evidence of the regenerative current drop. The Prius documentation mentions 19 mph as a threshold speed in the control laws.

After the A0B patch, there was a change in the deceleration behavior:

I have multiple recordings show no reduction in the braking force. I noticed the regeneration bar did not change before and after the pothole. However, this is not the same brake pause seen in a drizzle rain. Otherwise the brakes felt pretty much as before.

If anyone is interested in the data, send me a PM and I'll forward the raw data.

Bob Wilson