Life threatening brake and B mode failure

My 20115 Leaf S is 3 weeks old with less than 700 miles.

I was descending an 8% grade hill in B mode with a sharp bend at the bottom. As I went to apply the brakes, the pedal went to the floor and B mode cut out. The car was now rolling down the hill and accelerating under gravity with no brakes and without B mode slowing the car. I was expecting to hit a brick wall at the bottom of the hill when suddenly, after no more than two seconds, the brakes and B mode came back on and I safely stopped. I was a bit shaken and felt very unsafe driving the car although the problem didn't recur and I haven't been able to reproduce the problem on similar grade hills. When I told my wife, she said that the day before she was going down a hill in B mode when B mode suddenly stopped working although the brakes were OK. B mode later came back on although she couldn't remember exactly when.

On reflection after the incident, it was as if the whole computer/electronics momentarily shut down and then rebooted. I didn't notice if the dash lights went off or not.

That was yesterday and today I took the car to the dealer today. After an hour or so I was introduced to a mechanic described as the only Leaf qualified mechanic they had. He said he had driven the car and there was no problem. I asked if he had driven the car in B mode downhill. Incredulously, he asked what B mode was! I asked if he had pulled the diagnostic codes and he said there was nothing recorded. I told the service manager this mechanic was not qualified to work on the car since he didn't know about B mode and they produced another mechanic who did know about B mode. We went for a test drive together but could not reproduce the problem. As the problem is intermittent, who knows when it could recur or with what consequences. The car is now with the dealer for further diagnostics.

I'll update this thread if I get anything useful from the dealer.

I don't know how much reaction time you had, but remember you have an emergency brake and the main brakes can still be pumped mechanically to stop. You don't have to slam it and send your car into a skid, but you should be able to depress it enough to start the mechanical braking to get the car to stop (keep the foot on the main brake of course).

Now, that aside, I agree, very disturbing issue. Was the vehicle nearly fully charged when you went down this very steep hill? It's been known that when the vehicle has a full battery, you can't regen very much into the battery, even in B mode. This is to protect the battery for over-charging. Now, even if the regen kicks out, your mechanical brakes should still work as they are like the power brakes on any car. The only thing that would kill those is if the brake pump were to turn off. It does sound like car hit some type of shutdown mode if both Regen and Power Brakes failed.

How fast were you going down this steep hill? Was the Regen B mode helping you coast at a consistent speed or were you speeding up the entire way down the hill (before things shutdown). I actually have a long "straight" section of about a 1 mile road that goes down with this grade level near where I live. If you can give a good description of everything you remember, I might be able to perform a similar test but with no killer road curves at the bottom. :mrgreen:

Also, where in the country do you live?

Going over a rough patch of road can make the ABS engage and make it feel as if the brake has stopped working, although the pedal doesn't hit the floor, IIRC. I really dislike the ABS in the Prius for that reason, and you can't easily disengage it. If you hit rough road (or a patch of ice) just as your battery got topped off, you might see that situation. Ice would also defeat the B mode even with it working properly, if there were no traction.

Battery was about 70% charged when the brake and B mode failure occurred. At the time of the incident I was doing maybe 25 at the most. B mode was engaged and had been slowing the car until I hit the brakes. I use B mode a lot because I live 800 feet above sea level and B mode gets back a lot of what I lost ascending.

When B mode failed for my wife, it was close to fully charged. So are you saying that B mode is ineffective when the battery is fully charged? That doesn't seem right as the purpose as I understand is not just to recharge the battery but replicate engine braking as in a gas powered car.

I live on Maui, HI. No ice here

Ok, so no ice! Water on the road might have the same effect, to some extent. And yes, while B mode does simulate engine braking, that isn't its main purpose as it is on the Prius. Its main purpose in the Leaf is to increase regeneration. Braking is secondary, and with no ICE to spin for braking, there is no way to use it if the battery can't charge.

At 70%, you had enough regen braking, but it cut out.

And the road condition? Might ABS have come on?

Call Nissan and open a case. Possibly NTSB as well?

Hard to have any confidence in your dealer. Is there another dealer on Maui?

silverleaph said:

So are you saying that B mode is ineffective when the battery is fully charged?

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Correct. By design.

Haven't you noticed the car barely slows down in B when fully charged vs. say below 90% SoC? At high state of charge (esp. 100%), notice the almost complete lack of double circles on the regen area at the top of the dash until the battery depletes somewhat?

DNAinaGoodWay said:

Call Nissan and open a case. Possibly NTSB as well?

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NHTSA's at http://www.safercar.gov/" onclick="window.open(this.href);return false; is the right agency for those. For legitimate safety defects and not user error, file a safety complaint there as well as contacting Nissan formally, as directed in the manual. I see a section in the '14 owner's manual on page 9-15 "Technical and consumer information" about reporting safety defects that says something to this effect.

Along these lines are the brakes not physically connected to the break pedal? I fortunately haven't had this issue, but every other car I have had either had no power brakes or if it did have power brakes and the engine stalled you could pump or stand on the brake pedal and it would stop. Does this not work with the leaf? If so that seems like a MAJOR safety design flaw.

The brake pedal going to the floor sounds suspiciously like another thread here. In that thread, the 12V battery was failing. This seems to have some correlation in this case, especially if the car's computer blinked out for a moment.

1. Check and re-check the battery connections.
2. Check to make sure the ground from the battery goes to the car's chassis and is secure. Even consider taking it off, cleaning it, polishing the metal with sandpaper, and re-attaching it.
3. Have the dealership thoroughly test the 12V battery.
4. Consider running LeafSpy and watching the 12V battery level.

LeftieBiker said:

Going over a rough patch of road can make the ABS engage and make it feel as if the brake has stopped working, although the pedal doesn't hit the floor, IIRC. ...

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This has happened to me three times in one year. I'm lucky there wasn't a steep incline in any of the cases. It feels like a software bug.

LeftieBiker said:

... And yes, while B mode does simulate engine braking, that isn't its main purpose as it is on the Prius. Its main purpose in the Leaf is to increase regeneration. Braking is secondary, and with no ICE to spin for braking, there is no way to use it if the battery can't charge.

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Well, no it's purpose is to simulate engine braking (hence calling it "B" for braking mode) and control speed on downhills without having to use the brakes. Secondarily, people here use it just because they like more regen. Unfortunately, if the battery is full, no regen is available to use. Nissan could have switched in some sort of resistive load (which would have produced lots of heat that would have to be dissipated) to take the place of the battery in this situation and maintain the braking effect, but didn't.

On the Prius, "B" mode actually does use the gas engine to brake, and so you don't lose the function because the battery is full.

Check for loose 12V battery terminals.

Of all the problems caused by the 12V in these cars, if loss of braking is one of them, it's got to be the worst. Not be able to start is bad enough. Not being able to stop . . .

davewill said:

Nissan could have switched in some sort of resistive load (which would have produced lots of heat that would have to be dissipated) to take the place of the battery in this situation and maintain the braking effect, but didn't.

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Yeah, it's called the Brakes because that is exactly what they do, convert mechanical energy into heat that slows down the car. :mrgreen:

If they did do a resistive shunt for the extra power, you would end up putting more wear on the motor than necessary because whether driving forwards or regen, those forces are still acting directly on it. Regen is one thing, but turning your motor into the main brake is not a good idea. I do see one benefit of a resistive load, perhaps the heat would be sent back into the cabin for cold weather driving. But it would only be of benefit if you start at the top of a large mountain with nearly full capacity charge and the rest of the time it is just extra weight and complications added to the vehicle.

knightmb said:

davewill said:

Nissan could have switched in some sort of resistive load (which would have produced lots of heat that would have to be dissipated) to take the place of the battery in this situation and maintain the braking effect, but didn't.

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Yeah, it's called the Brakes because that is exactly what they do, convert mechanical energy into heat that slows down the car. :mrgreen:

If they did do a resistive shunt for the extra power, you would end up putting more wear on the motor than necessary because whether driving forwards or regen, those forces are still acting directly on it. Regen is one thing, but turning your motor into the main brake is not a good idea. I do see one benefit of a resistive load, perhaps the heat would be sent back into the cabin for cold weather driving. But it would only be of benefit if you start at the top of a large mountain with nearly full capacity charge and the rest of the time it is just extra weight and complications added to the vehicle.

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A resistor bank capable of dissipating 40kW is not a trivial device. It would be quite large and require a large amount of airflow and the ducting that would entail.

I'm not comfortable with brake by wire. I certainly hope my brake pedal is directly connected to a hydraulic cylinder that has the ability to directly apply the brakes if I push on the pedal hard enough. If your pedal really did go all the way to the floor, they shouldn't be looking at your electronics, they should be looking at the brakes.

The brakes and the B mode came back together so it was just a momentary glitch. Pretty sure it was electrical or software related.

Thanks for all the helpful responses. I understand B mode better now and why my wife's experience may have been because the vehicle was fully charged.

I'll file with NTSB and Nissan as suggested.

No response from the dealer yet (only one within a hundred miles across the ocean and no ferry).

johnrhansen said:

I'm not comfortable with brake by wire. I certainly hope my brake pedal is directly connected to a hydraulic cylinder that has the ability to directly apply the brakes if I push on the pedal hard enough.

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It's not brake-by-wire, but rather it is electrical assist. The pedal does have the direct ability to push hydraulic fluid to the brake cylinders in the wheels.

johnrhansen said:

If your pedal really did go all the way to the floor, they shouldn't be looking at your electronics, they should be looking at the brakes.

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That is the logical conclusion, but it is not the case with the LEAF. Somehow, without sufficient 12V voltage, hydraulic pressure is lost and the pedal goes to the floor. We have discussed several possible paths for the hydraulic fluid to take, but no one is sure of the exact mechanism that relieves the pressure.

This thread is one of several discussions where the LEAF's brake pedal has gone to the floor. The need for 12V power is why the LEAF includes an ultracapacitor to hold up the 12V voltage long enough to stop the car in the case of an electrical failure. Unfortunately, if the battery voltage drops, so does the voltage of the ultracapacitor.

Bottom line, I agree with those who say to carefully check out the 12V power system FIRST! I wouldn't drive the car again without first doing that.

that is a design flaw and is deserving of a safety recall.

When you pressed the brakes and nothing happened, are you sure the pedal actually travelled to the floor, or, is it possible that the pedal simply felt super stiff (almost like pressing against metal) and provided very little braking effect?

I also drive a 2015 LEAF (S trim w/QC). I never use B mode, but while actively braking (ex: exiting a freeway and slowing to stop at the red light at the end of the ramp), I "regularly" (perhaps three times a week on average) experience the regenerative braking effect suddenly cutting out (ex: instantly drops from 4 green bubbles to 1 or sometimes 2 bubbles lit). Whenever this happens, the brake pedal simultaneously gets extremely stiff, and the total braking force active at that moment drastically drops.

However, the times when this occurs isn't totally random, and I have a working hypothesis as to what is happening. When the problem occurs, it normally is observed immediately after driving over a small bump or ridge in the road (ex: such as the small ridge that exists where an asphalt roads switch to concrete or vice versa, like on my local freeway offramp).

I'm pretty sure the root cause is that the Anti-lock Braking System is getting triggered by driving over the little tiny bump in the road. ABS detects wheel lock up by monitoring the RPMs of each wheel and comparing them to each other. If one or more wheels is observed to be rotating too slow relative to the other ones, it reduces brake force to the slower moving wheel(s).

Due to regenerative braking, which only applies to the two front wheels in the LEAF, the brake force distribution between front/back wheels during normal braking is probably extremely unbalanced (I would guess something like like 98%/2%, or 100%/0% during B mode deceleration with no braking). This unbalance is more severe than in gas vehicles without regenerative braking.

One side effect of unbalanced brake force distribution, is that it makes it much easier to lock up the wheels in the front during braking. Ordinarily, I don't brake hard enough to anywhere near approach wheel lock up. However, when I drive over a small ridge in the road surface, this momentarily compresses the suspension springs, pushes my car up towards the sky, and momentarily increases the tire normal force (making wheel lock up virtually impossible). Unfortunately, moments later, the wheel will be riding too high above the pavement, and the spring needs to decompress to travel back to fully meet the road surface. During this split second, the normal force observed by the tire is very low, making partial wheel lock up really easy. The total event duration is too short to fully lock up the wheel, but it is presumably long enough for a fast acting/high resolution RPM sensor to momentarily detect the decrease.

Both the front and back wheels will experience this effect when driving over a small ridge in the road surface, but the rear wheels don't actually lock up, since they aren't actually providing much braking force in the LEAF, and therefore don't need much normal force to begin. Therefore, only the front wheels lock up (ever so momentarily), while the back ones keep spinning at full RPM.

The fast acting ABS system sometimes detects this momentary RPM error (front tires rotating slower than rear) and thinks it needs to immediately decrease brake force to the front wheels, to keep them from hard locking up. Little does the ABS system know, that the normal force on the front wheels is about to return to normal again a split second later, and no actual intervention was required.

When the ABS activates, and it detects the RPM slow down is in the front wheels, it immediately scales back on the regen braking effect, causing a drastic loss of braking force. Simultaneous with this, it presumably closes some metal valves in the hydraulic lines leading to the front two wheels, preventing any increased pressure being experienced inside the cylinders inside the brake assembly for each front wheel. This prevents further fluid flow out of the master cylinder into the brake lines feeding the front wheels, causing the brake pedal to suddenly develop an extremely stiff feel. In a panic situation, I could potentially see someone mistaking this stiff feel for the feel that might be expected when the brake pedal hits the floor and is pressing directly against metal.

That said, when the problem occurs for me, braking force isn't 100% lost, it is simply greatly reduced by the abrupt reduction in regen braking. If I immediately press much harder on the brake pedal, I can still stop, presumably since the rear brakes are still working, as the rear brake lines aren't being blocked by the ABS. The rear wheel breaks are terrible however, and can never generate more than roughly 1/3 (or perhaps less before locking up) of the total braking force that would normally be available to the vehicle when all four wheels are actively participating in braking.

In the first few weeks of owning the LEAF, I almost rear ended people at red stop light several times due to this braking issue. Since then, I have learned which intersections that I regularly drive on have bumps/ridges which tend to trigger the problem. Now, I anticipate the issue upon approach to these intersections, and immediately apply much harder brake pressure as soon as I detect the pedal getting unusually stiff. I have also trained myself to brake a little earlier and a little gentler than I would have in my previous gasser (2006 Honda Civic).

Now that I think about it, one thing that might be worth trying, and may actually make the vehicle ultimately safer, is to deliberately defeat the whole ABS system. ABS systems have self diagnostics and if they detect a problem will normally go into a fail safe mode that disables the whole system. Therefore, finding one of the wheel speed sensors and unplugging it (and taping over the connector to minimize corrosion/dirt ingress) may be enough to "fix" the issue.

I love the LEAF, and never plan to drive a gas car again. That said, the current braking behavior really is unacceptable and needs to be recalled/fixed properly.