Odd braking issue.

rich1068 · January 7, 2019

Noticed this a couple of times.

Slowing down to (for example) a junction with my foot on the brake and the car suddenly surges as if the brakes have momentarily been released and then reapplied. Not a gentle feeling but for a fraction of a second a little worrying. Something to do with the regenerative braking?

Any thoughts?

Herbie · January 7, 2019

I'd say you'll probably need Techstream to evaluate it properly but have you noticed what the state of charge of the traction Battery was when it occurred?

I'm thinking that regenerative braking came on as normal but if the traction Battery was fully charged and nowhere to put the electricity being generated, there would have been a change from regenerative to hydraulic braking so maybe you felt the changeover?

Just a guess.

Britprius · January 7, 2019

1 hour ago, rich1068 said:

Noticed this a couple of times.

Slowing down to (for example) a junction with my foot on the brake and the car suddenly surges as if the brakes have momentarily been released and then reapplied. Not a gentle feeling but for a fraction of a second a little worrying. Something to do with the regenerative braking?

Any thoughts?

With the hybrid system normal braking is done by regen. With the RX on all 4 wheels. However if the system detects an under rotation of one wheel such as may occur when braking over a wet uneven man hole or drain the system changes to full mechanical brakes cancelling all regen. This gives rise to the feeling for a split second that the brakes have been released.
The gen3 Prius had a recall on the brake software to reduce this effect because there were so many complaints about this problem. I encountered it with my gen2 Prius. On rare occasions on a particular stretch of road where I did a left turn, and braked in the wet over a man hole that could not be avoided unloading the nearside front tyre. I was able to replicate this if I tried quite easily. At first it is a little alarming, but after a couple of times I got used to it.

John.

Farqui · January 7, 2019

Hi Richard, I also experienced this transition when I first picked up my GS and adapted to it for a short while.

But after I renewed disks/pads, flushed the brake fluid via Techstream I can barely feel the transition anymore.

So how old/how many miles have your disks/pads covered and has your brake fluid been flushed in the last couple of years?

WESTIE · January 7, 2019

Hi Richard

If you do a search on this forum you will find that there has been a lot of correspondence on this, but as far as I am aware no resolution. There was even more about it on the American Lexus forum and I believe there was talk of a class action against Lexus.

Tony

Brian26 · January 7, 2019

Hi Richard, yes it is a well known problem with Lexus regenerative braking. More apparent on the 400h then the 450h if my memory serves. I can sympathise as it can be a bit off putting at times but once you get used to it it is not really an issue as if you are braking heavily then you are effectively on hydraulic braking only.

Bri

rich1068 · January 7, 2019

4 hours ago, Herbie said:

I'd say you'll probably need Techstream to evaluate it properly but have you noticed what the state of charge of the traction battery was when it occurred?

I'm thinking that regenerative braking came on as normal but if the traction battery was fully charged and nowhere to put the electricity being generated, there would have been a change from regenerative to hydraulic braking so maybe you felt the changeover?

Just a guess.

To my simple brain this makes sense. I think I'm correct in saying the Battery was at full charge (all green bars) each time it's happened.

3 hours ago, Britprius said:

With the hybrid system normal braking is done by regen. With the RX on all 4 wheels. However if the system detects an under rotation of one wheel such as may occur when braking over a wet uneven man hole or drain the system changes to full mechanical brakes cancelling all regen. This gives rise to the feeling for a split second that the brakes have been released.
The gen3 Prius had a recall on the brake software to reduce this effect because there were so many complaints about this problem. I encountered it with my gen2 Prius. On rare occasions on a particular stretch of road where I did a left turn, and braked in the wet over a man hole that could not be avoided unloading the nearside front tyre. I was able to replicate this if I tried quite easily. At first it is a little alarming, but after a couple of times I got used to it.

John.

This makes sense too but I'd be very surprised if it's the same situation because each time it's happened I've not had any traction issues. Straight line, sub-30mph, approaching a junction.

2 hours ago, Farqui said:

Hi Richard, I also experienced this transition when I first picked up my GS and adapted to it for a short while.

But after I renewed disks/pads, flushed the brake fluid via Techstream I can barely feel the transition anymore.

So how old/how many miles have your disks/pads covered and has your brake fluid been flushed in the last couple of years?

This I'll have to look into. And why do you reference Techstream for a fluid change?

1 hour ago, WESTIE said:

Hi Richard

If you do a search on this forum you will find that there has been a lot of correspondence on this, but as far as I am aware no resolution. There was even more about it on the American Lexus forum and I believe there was talk of a class action against Lexus.

Tony

I'll certainly look into that.

Our colonial cousins do like their lawsuits don't they?

Great responses as usual guys 👍

rich1068 · January 7, 2019

2 minutes ago, Brian26 said:

Hi Richard, yes it is a well known problem with Lexus regenerative braking. More apparent on the 400h then the 450h if my memory serves. I can sympathise as it can be a bit off putting at times but once you get used to it it is not really an issue as if you are braking heavily then you are effectively on hydraulic braking only.

Bri

What I'm describing is much more pronounced than the usual "Oh!!" moment you get pulling up behind someone in your new 400h. Yes you do get used to that but this feels much more like something has let go before taking control again.

Farqui · January 7, 2019

Techstream ensures your brake fluid is flushed correctly and working at 100%.

When was your brake fluid last changed?

Herbie · January 7, 2019

3 minutes ago, rich1068 said:

And why do you reference Techstream for a fluid change?

Something to do with the regenerative side of things. I don't know exactly what as I've never had to do it yet, but I do recall reading on here that you do need Techstream.

rich1068 · January 7, 2019

I'd have to check but I don't think it's due or Lexus would have banged on about it at the last service.

And I'm being thick but how does diagnostic software help change brake fluid?

:edit:

Ah, got you.

Herbie · January 7, 2019

Here's a video I just found on Youtube - there's plenty more of them as well if this one isn't particularly useful (I've not watched it yet 😊 )

pedrofromredro · January 7, 2019

6 hours ago, rich1068 said:

Noticed this a couple of times.

Slowing down to (for example) a junction with my foot on the brake and the car suddenly surges as if the brakes have momentarily been released and then reapplied. Not a gentle feeling but for a fraction of a second a little worrying. Something to do with the regenerative braking?

Any thoughts?

Now this is a very interesting topic.we have had are 400h since November and the wife asked me why it does this with the brakes as she noticed it (she drives it majority of the time).i have experienced it a few times myself and it is a very odd feeling.i have passed it of as just a RX issue as are CT never done this

rich1068 · January 10, 2019

Further to this, still haven't had chance to watch the video or look into the Techstream requirement but I have looked through the service paperwork and the brake fluid was last changed in February 2016 so probably due assuming it's every two years as most manufacturers recommend.

i-s · January 10, 2019

I could be talking complete loblocks, but this might be worth a try. It might make no difference at all.

Have you tried pumping the brake pedal with the ignition off?

This forces the calipers to self-adjust to the current state of pad wear without the servo interfering.

What I suspect might be happening is that as the pads have worn but the servo has masked any self-adjustment there is then a "gap" which becomes apparent in the hand-off between regen and friction braking. I have had similar moments in my Leaf.

Free to try and nothing lost if it doesn't help.

rich1068 · January 10, 2019

I have no idea how cars work so I'm up for that

Britprius · January 10, 2019

5 hours ago, i-s said:

I could be talking complete loblocks, but this might be worth a try. It might make no difference at all.

Have you tried pumping the brake pedal with the ignition off?

This forces the calipers to self-adjust to the current state of pad wear without the servo interfering.

What I suspect might be happening is that as the pads have worn but the servo has masked any self-adjustment there is then a "gap" which becomes apparent in the hand-off between regen and friction braking. I have had similar moments in my Leaf.

Free to try and nothing lost if it doesn't help.

The hybrids do not have a servo. The only mechanism to compensate for pad ware is that there is no return pressure from the pads being pushed against the discs "unless the caliper slide pins are seized". Hydraulic fluid takes up any pad ware behind the caliper pistons.
The braking system pressurises by an electrically driven pump when you open the drivers door, and does not need the ignition "or ready mode" to be operate.

John.

Lozsta · January 10, 2019

Ah I already love this forum, I also have this issue. I will try pumping the brake pedal later.

i-s · January 10, 2019

Fair enough - the point is to force the pistons to move relative to the caliper seals (which are what pull them back when brake pressure is released) without interference from any other system.

Like I said, it might do nothing at all in this system.

Rutlandlex · January 11, 2019

I've noticed mine doing this lately too. It certainly doesn't do it all of the time but it started recently. It certainly seems like the regen brake letting go, confirmed by the needle on the power meter jumping out of the blue zone up to zero. I find it happens under gentle braking going down hill but I've yet to experience it under heavy braking.

I also sometimes get the opposite whereby it feels as if the engine is still trying to push the car forward under braking then it will relinquish and you end up with a little more braking force when it does. It's a bit like driving a manual and slowing down in gear until the rpm's fall just below the idle rpm level. When you drop the clutch pedal you suddenly feel a bit more braking force.

It's not something that I can't live with and it certainly isn't dangerous it's just a bit annoying. As for a class action lawsuit - get a life!

welland · January 17, 2019

On 1/7/2019 at 2:01 PM, rich1068 said:

Noticed this a couple of times.

Slowing down to (for example) a junction with my foot on the brake and the car suddenly surges as if the brakes have momentarily been released and then reapplied. Not a gentle feeling but for a fraction of a second a little worrying. Something to do with the regenerative braking?

Any thoughts?

Have a look at my thread with similar experiences:

Rutlandlex · January 23, 2019

I'm pretty sure that these are 2 separate issues. I'm now convinced that the loss of braking power is caused by a combination of bumps and cold, damp conditions. I have been taking more notice recently and I have been braking using the regen brake then hit a bump then suddenly no regen. I'm guessing it is the wheel speed sensors that detect a tiny momentary wheel lock so the system stops the regen brake to allow the mechanical brakes to take over in case the ABS is needed but it isn't because the wheel lock-up is over with so quickly.

I never noticed any issues in the summer with this.