Sudden Lunge while stopping

I drive a 2008, so not the same generation model, but with my vehicle, attempting to brake over a rough broken up road surface will normally result in the “tyre skid” light flashing, and the vehicle seeming to lounge forward instead of progressively decelerating as it normally should.

Other causes of this phenomena include braking on black ice, a gravelly surface, or more interestingly, having 2 different types of tyres on the front axle.
Having one tyre badly deflated can also trigger this condition.

There’s also probably the slim chance that the sensors detecting any slippage during braking manoeuvres are faulty - but I’d wager that’s practically unheard of.

Just my tuppence worth - I could be wrong on all counts, but I thought the aforementioned scenarios might offer some form of assistance.


iPhone ? Pro

 

I’d venture that you have a speed sensor issue. Probably on one of the front wheels. It’s signalling the computer that one of the front wheels is still rolling, as the other stops. The ABS computer releases the brakes, causing you to roll a couple of feet. If access allows, unbolt the ABS sensors from the hub assemblies and inspect them. The tip of the sensor should be free of major grease and debris. Wipe the tip of the sensor off and reinstall it.

 

You could always read the diagnostic codes and see what they say.

 

Unless the hub assembly(ies) have a lot of wear, I’d bet the car is not setting any codes. The trigger wheel for the speed sensor is sandwiched between both bearings in the assembly. But it’s very possible the grease in the assembly has been redistributed across the tip of the sensor, effectively changing its signal. The grease is electrically conductive, so some signal still passes thru it, just not as much as it should.

I can’t speak for Toyota ABS control logic, but most domestic car makers control logic only sets trouble code for loss of ABS sensor signal while the brake pedal is applied.

During a power on self test, the sensor would still show proper resistance, since it is still connected.

 

You can get that symptom from a healthy car if you are braking while driving over certain pavement defects. The bump or momentary loss of traction confuses the antilock part of the regenerative braking system, so it gives up and hands off to the standard hydraulic system. That's the lunge you feel, the bit where one braking system peaces out and the other one has to jump in cold.

Pretty much anything with regen brakes can do this.

 

leave more room between you and the car in front

 

Another possibility is that you have a worse than usual issue that is common in the early third gen Prius. Prius braking is a bit complicated because of the transition from regenerative to conventional braking and antilocking braking. At slow speeds it would feel like the brakes would momentarily "let go", then grab again (but not actually lunge 5 feet). Toyota issued a software update that partially fixed this. I've has the update but it still happens somewhat but not nearly as severe as you do.

A 2012 would have had this "fix" but some drivers still complain about it. I suppose that a failing speed sensor could worsen this.

 

The "sudden lunge" during mild braking when a wheel hits a bump or rut is a phenomenon people have been noticing in every Prius generation since the earliest generation, and being startled or spooked by if it wasn't explained to them when they first got the car. There's an explanation of it in this post.

While the behavior has always been present in every generation of Prius, there was something about the early 2010s that made people complain about it more, and there was a recall (A0B) that tweaked the early 2010 software to tone it down a bit (it still happens, but more like the other Prius generations).

Way back when I bought my Gen 1, the folks who told me about it (right here on PriusChat) gave me the advice to just go find a road with some lumpy spots and make it happen a few times until I got used to it and it didn't startle me, and that's still the best advice going.

Now, when that transition happens, it usually doesn't involve the traction light blinking. So that's something a little different in the OP here.

It turns out the skid control ECU keeps a record of the last, I think, four times traction control has kicked in, and everything that was going on at the time (speed, braking, turning, g force, etc.), which can be reviewed in Techstream. That might help explain what was going on there.

 

My light used to light up all the time on my Gen 2 especially with the original tires that came on the car. When I replaced the original tires with Michelin tires it decreased in frequency but still did it. My 2016 has only had it happen a few times since I have owned it. Usually braking on a manhole cover or any debris on the road especially at intersections during and after winter from salting of our roads, or really rough road surfaces when braking.

 

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I guarantee if you try and stop here you will feel the regen transition to hydraulic.

I cross this set of tracks every day and it still unnerves me feeling the transition.

 

It hardly takes much. There's a spot on my street leaving my neighborhood, coming up to stop, that'll do it every time. The pavement defect responsible is barely visible if you're not looking for it.

 

I've noticed it as a passenger in hybrid Fords, Toyotas & Hondas. I've directly felt it driving two Toyotas including our own '18 c. Once, just last week, it strobed the ABS swervy icon lamp just at the transition point. I was doing moderate braking across a rail grade crossing. That's the only time I've ever seen it.

 

Haha, yeah, that’ll do it!


iPhone ? Pro

 

New Prius owners should be aware when driving on rough surfaces at high speeds, leave lots of space between yourself and the vehicle ahead of you as if you try emergency braking, you might literally get the shock of your life!

Once you’re used to this quirk, you shouldn’t have a problem. Despite my normally enthusiastic driving, I’ve never run into the back of anyone during an emergency braking procedure.



iPhone 6s +

 

Worth remembering, too, that in clear 'emergency braking' situations (which the pedal stroke sensor detects by how fast you went on the pedal), the car goes straight to hydraulic braking anyway, so this transition doesn't occur.

The sudden-lunge-y transition is something people notice during light/moderate regen braking on an irregular surface, and then think about extrapolating to "what if that had happened during emergency braking?!".

 

As Chapman points out, this regen transition issue applies only in light-to-moderate braking. This 'shock' you mention should happen only to drivers using just light-moderate braking at low to medium speeds with intentions of stopping within a very few feet of the vehicle ahead, then freeze up or even release the pedal when the brake produces an unexpected feel. Proper drivers should have plenty of reaction time and space to escalate to hard or emergency braking, where this issue doesn't occur.

At high speeds, regen braking is capped to such a light level that you would use it only when you have more than a thousand feet of braking distance available, plenty of space to switch to hard braking if this 'pause' occurs.

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But note that this regeneration transition issue is different than what OP reports, having his skid control warning light flash on. If that isn't a tire or road surface traction problem, then there is something wrong with the car that should be addressed.