Brake Quirks Under 5 mph?

[DrDeke]

I am having occasional brake quirks on my 2012 FFH and I'm not sure whether this is normal or whether there might be something wrong with the car.

 

Most of the time, at least 95% of the time, when I use the brakes to bring the car to a stop, the braking process "feels" completely smooth and normal. I understand that, at a certain point, the braking system shifts from regenerative braking to hydraulic/friction braking, and I can see the regen indicator on the dashboard turn off, but the "feel" of how quickly the car decelerates relative to how hard I am stepping on the brake pedal feels basically the same through the entire process.

 

Once in a while, though, once I've got the car going quite slowly (maybe around 5 mph) and am maintaining steady pressure on the brake, the car will quit slowing down as fast as it was before. When this happens, I go "oh, ^&@#!" and I have to quickly push the brake pedal harder in order to avoid hitting the car in front of me or sliding into the intersection.

 

Now, when this happens, and I *do* step on the brake pedal harder, the car does stop just fine at that point. But it's kind of unnerving to have to bring the car to a semi-panic stop like this at random intervals, and it sometimes makes passengers ask "what, didn't you see that guy or something?" Unlike in the well-known 2010 FFH braking problem which a TSB was published about, I do not get any warning indicators on the dashboard when this happens.

 

I did quite a bit of driving this weekend, and it seemed like this happened more often than normal. I also noticed something new: on three occasions when this "problem" happened, the ICE (which had previously stopped when I took my foot off the accelerator) started up for a few seconds right around the time the braking strangeness happened. I don't know why that would happen; the HVB was at a "normal" (between 40-60%) charge as indicated on the dash, the outside temperature was about 40 F, but the engine was fully warmed up at the times this happened.

 

I am a bit hesitant to ask my dealership about this, because they probably already suspect I'm a kook, not having (yet) been able to reproduce my HVB drain problem, so I'll be curious to see what you guys have to say.

[Griswald]

It's been well argued one this forum. Seems to be a transition software issue.

[Waldo]

It's been well argued one this forum. Seems to be a transition software issue.

 

That discussion only concerns the 2013+ though, I doubt this is related.

 

To the OP - have you noticed any relation to the environmental conditions, like rain, hot, cold early in a drive or only after you've done a few miles?

[lolder]

This is a new behavior to the 2010-12 models. Below 5 mph, only the mechanical brakes slow the car. Take the car where you can "slam" on the brakes hard a few times from slow to moderate speeds. They might be sticking or something. The mechanical brakes are first applied by an electrically powered piston in the brake master cylinder. If that doesn't work the brake pedal arm contacts the piston mechanically an inch or more further down and applies the brakes. I would think that would throw a DTC. Until you get a DTC or it happens most of the time the dealer will have difficulty fixing it.

[hybridbear]

Wet brake pads/rotors can impact brake feel.

[lolder]

Yes, I know about the wet brake phenomenon and have experienced it many times. It is more of a "grabbing" effect in the 2010-12s instead of a "loss" of braking. Many hybrid cars have had braking anomalies including Prii which was similar , I think, to what Hybridbear has complained about. The situation that the OP described has not re-appeared since the "Customer Satisfaction 10 B 13" was done to early cars in 2010. A lot of things have to interact properly in hybrid braking; the mechanical and regen mixing, the ABS and the traction control. I think the OP's car has a fault somewhere.

Interestingly, the Tesla S does not mix regen and friction braking. The max regen is available through the accelerator pedal in the default setting. I had recommended that the accelerator pedal be called the "Go" pedal but now it is also a "Slow" pedal in the Tesla. The Tesla brake pedal only operates the mechanical brakes. There is a large amount of regen braking available. New owners who were leery of the new behavior have said they got used to it in 1/2 an hour and now only use the mechanical brakes to completely stop the car. The new "autopilot" system apparently has full control of both braking systems.

[DrDeke]

 

To the OP - have you noticed any relation to the environmental conditions, like rain, hot, cold early in a drive or only after you've done a few miles?

 

It's hard to say... It might happen a bit more often when the engine/battery/etc haven't warmed up yet, but it also definitely happens even after everything is fully warm (car has been driven continuously for more than an hour). Other than that, I have not noticed any other relation to conditions; it happens in both warm and cold weather, and dry and wet weather.

 

Take the car where you can "slam" on the brakes hard a few times from slow to moderate speeds. They might be sticking or something.

 

That's not a bad idea, I'll find a safe place and give that a try this weekend.

 

 

I appreciate everyone's input.

Edited February 12, 2016 by DrDeke

[DrDeke]

The "give it a few hard stops" thing didn't help :(.

 

I recorded the following video on the way to work this morning, which "shows" the brake problem happening 5 times:

 

 

Really though, there's not much to show on the actual brake problem itself. However, the recorded video of the dashboard does show the engine starting during braking which happened on 3 of the 5 unusual braking events recorded in the video.

 

If you watch, you'll probably want to check the description for the times of the braking events so you can skip to them. Otherwise, the video will be very boring.

 

Now that I've caught the ICE-start-during-weirdly-needed-hard-braking event on video, I will let the dealer know about the problem and see what they say. They want me to bring it in again soon to take another look at the HVB drain problem anyway...