Brakes "mutate" to become grabby in stop & go traffic?

[Ingineer]

This seems to be not grabby brakes, but rather the motor winding protection algorithm. To simulate a normal automatic transmission car, the Leaf puts a creep function in the motor drive system. When an electric motor is taking current in, but not producing kinetic output, it is no longer a motor, but rather a heater. Most modern inverters perform winding thermal modeling in addition to measuring the actual temperature with sensors in order to protect the stator windings. It may indeed be a bug, but it seems like after a while of extended stalling the drive goes into a mode with reduced creep bias, and this, to the driver, seems like grabby brakes when in reality, it is a power cut.

-Phil

 

[Coffee_Slurry]

Interesting theory -- that the car has to give up on "torque converter" motor creep, to avoid overheating. I'm not convinced, though.

If you find a sufficient downhill grade, you can really feel when the powered creep kicks in. From a stop, the brakes release the wheels and the car rolls a bit *before* the creep engages. It feels like a bit of surprise thrust. Amusing, but not very dramatic.

This thing with the brake mutation is more pronounced. It's like the brake booster goes into overdrive, or some kind of panic mode. (Very small amounts of added brake pressure cause non-linear amounts of stopping power.)

Unchecked, I think I could get the car to go from coasting to locking up the brakes with just a steady press. More experiments needed.

Very slow stop & go traffic on a downhill grade -- I believe any driver would notice something amiss.

 

[garygid]

As long as your foot is on the brake, shouldn't that stop the creepy-power to the motor?

But, they are talking about unexpected too-strong braking when the brake pedal is pushed gently, NOT an unexpected acceleration when the brake pedal pressure is gradually being reduced, right?

 

[dcarter]

Right- unexpected sudden halt.
I always marveled at the modulation between regenerative breaking and actual disc braking, and have continually wondered where the regen stopped and where the discs began.
Now that I know that the Regen in the LEAF is quite conservative, I must assume that the computer expects the Regen to be helping out at these circumstances where the brakes grab, but at low speeds, the Regen is phased out- incorrectly so.

So yeah, I think it is a software issue.

dcarter

 

[garygid]

Control Software (firmware) issue, yes most likely.
However, at low speeds there should not be any (significant) Regen, because the motor is just turning too slowly.

 

[Ingineer]

As long as your foot is on the brake, shouldn't that stop the creepy-power to the motor?

But, they are talking about unexpected too-strong braking when the brake pedal is pushed gently, NOT an unexpected acceleration when the brake pedal pressure is gradually being reduced, right?

At some brake pressure, I'm sure it does, but not at light pressure. Particularly when in stop-and-go, you might be using some creep torque (in other words, no accel pedal pressure), but still are "riding" the brakes. The Prius has a similar system, and you can see easily see when creep is disengaged by watching the flow arrows on the energy screen. If you don't press the brake hard enough, the car is still wasting power trying to creep, and will do so for a bit before realizing you aren't moving and then shutting down creep. I'll have to do some more experiments in the Leaf to see how this is implemented.

Keep in mind if you have some amount of brake pressure, and creep is suddenly removed, the car will stop abruptly just like brake pressure was suddenly increased. It may be difficult to tell without some real instrumentation.

-Phil

 

[TomT]

Having experienced this for an extended time, and now knowing how to reproduce it, I would have to say that it definitely is not a motor protection issue. I am familiar with what you are referring to and it would, I believe, have both a very different feel and also not be invoked in this situation...

This seems to be not grabby brakes, but rather the motor winding protection algorithm. To simulate a normal automatic transmission car, the Leaf puts a creep function in the motor drive system. When an electric motor is taking current in, but not producing kinetic output, it is no longer a motor, but rather a heater. Most modern inverters perform winding thermal modeling in addition to measuring the actual temperature with sensors in order to protect the stator windings. It may indeed be a bug, but it seems like after a while of extended stalling the drive goes into a mode with reduced creep bias, and this, to the driver, seems like grabby brakes when in reality, it is a power cut.
-Phil

 

[Herm]

Put a temperature probe on the motor, it may be detectable when this happens.. I would hope liquid cooling would keep this under control.

 

[FairwoodRed]

Has anyone actually talked to the mechanics at the dealer about this? It's frustrating to see people pointing out other threads on the same issue as if it's just some newbie issue. Those threads don't have any resolution and are a waste of time to read. This is a real problem and needs to be addressed by Nissan.

I'm happy to be the one who actually goes to the service department with this, I'm just wondering if anyone has tried that yet.

I was in my local service department this weekend to have them look at my grabby brakes. The tech took a test drive (6miles) and said that he could reproduce the grabby brakes, but that the system was operating as designed. I wasn't happy with that answer and I'll probably end up calling Nissan corporate to further my complaint. I'm also planning to call the service department manager to give him a chance to either back up his tech, or address my concerns, before I decide if the dealership is good/bad.

 

[JustinC]

I was in my local service department this weekend to have them look at my grabby brakes. The tech took a test drive (6miles) and said that he could reproduce the grabby brakes, but that the system was operating as designed. I wasn't happy with that answer and I'll probably end up calling Nissan corporate to further my complaint. I'm also planning to call the service department manager to give him a chance to either back up his tech, or address my concerns, before I decide if the dealership is good/bad.

Thanks. Sounds like it may take a handful of us doing the same thing before Nissan takes this seriously. I'm sure they didn't design the brakes to be grabby. It might be something they decided to live with though.

 

[johnr]

I've noticed this effect a few times when pulling into a parking space. The stop-and-go effect that results must be amusing to bystanders.

 

[Wattson]

Greetings, I've had the same issue since the beginning and have lived with it for just under 6000 miles. I finally decided to look on this forum to see if anyone else had the same issue (being unable to smoothly come to a stop when in stop and go traffic; grabbiness). The funny thing is that the problem has now gone away completely.
I read someone mentioning they felt it had to do with the stability control, so I experimented driving with it turned off. The brake grab was only happening slightly, and I wanted to be sure I could repeat for a service rep, so I tried a few things that normally made it worse. I pushed the brake pedal down firmly (when stopped with stability control off), and at first it felt a little spongy. Holding the pedal firmly a little longer it became pretty solid, so I let off. Now the car has behaved completely normally, with or without stability control on, for about 200 miles.

 

[TomT]

Actually, I don't believe you can turn off stability control on the Leaf, only traction control... I can't see how turning that off would make any difference unless there is an obscure software bug...

I read someone mentioning they felt it had to do with the stability control, so I experimented driving with it turned off.

 

[Wattson]

Sorry my mistake, it's the traction control button on the left. It may not be related, but that's how I fixed the problem for me. If it is part of the problem perhaps it has something to do with anti lock brake control not being set right for almost zero speed. Also, since doing this I don't feel the odd sponginess in my brake pedal which I'd experience in stop and go traffic.

 

[TomT]

I'm certainly willing to give it a try and see what happens... I suppose it could be something as obscure as that recalibrating the brake pedal position sense algorithm or the like...

Sorry my mistake, it's the traction control button on the left. It may not be related, but that's how I fixed the problem for me. If it is part of the problem perhaps it has something to do with anti lock brake control not being set right for almost zero speed. Also, since doing this I don't feel the odd sponginess in my brake pedal which I'd experience in stop and go traffic.

 

[garygid]

Maybe the "baby" just needed to be "burped". :lol:

I will try this "fix" also:

1. LEAF not moving, probably in "D",
2. Turn Traction Control Off
3. Press brake firmly
4. Hold
5. Vary pressure a bit to see if it is spongy or firm
6. ... notice any results!

 

[DarkStar]

Even after following the proposed "fix" I can immediately replicate the issue as I previously described.

 

[evnow]

I see this happening on long descents with nearly 100% charged and thus no regen.

 

[GPowers]

The breaks can surprise you some times. They do not always respond the same way.

So can the acceleration or lack of acceleration. Once I was pulling out of a parking lot in to traffic. I was making a right hand turn to merge into traffic and the Leaf failed to respond to a heavy pedal. I just flopped over the steering wheel and when the Leaf decided, it slowly started to accelerate. Not the normal burst of torque I was expecting.

 

[Wattson]

It might not matter, but I did my "fix" while in ECO mode.
Also, I pushed that brake pedal with pretty good force (imagine a hard emergency stop) applied slowly for about 30-40 seconds without letting up. Then I drove around the block a couple times with stability control still off. Then I switched it back on before turning off the car.

It could be electronic, or maybe the brakes just need to be bled. The brakes were still nice and smooth today when gliding forward at stoplights (which I couldn't do before with this car).