Prius Squirrely Brakes

I have a 2011 Toyota Prius with an oddity in the braking system. When I brake while going over a rough surface, like a manhole cover or rumble strips, it feels as though the brakes let go. It’s really disconcerting, but this has never been picked up during an inspection. I will bring it to the Toyota Service department, but am leery of being overcharged for something. Anybody have any experience with this issue?

Yep, the ABS “thinks” the tires are starting to lock because they are hopping from bump top to top causing an erratic signal. There have been ABS systems in the past that were much more prone to this than others. Since this is a '11 car, I have to assume this didn’t happen before?

Did you replace the tires and this started? The Prius uses somewhat special tires. If you didn’t replace them with the same type, that could throw off the ABS. You may have a failing wheelspeed sensor or worn suspension part that may be causing this. Some of these can be a bit pricey. It is impossible to comment further until the mechanic inspects the car and draws a conclusion. The dealer will be pretty pricey in any event. This is a complicated car that my be best diagnosed by the dealer. You might consider a good independent shop for a second opinion if you don’t like what the dealer says or wants to charge.

If you bring it to the dealer’s service department you are GUARANTEED to be overcharged for something.

What you describe is common for ABS systems. And, Prius has regenerative brakes, which means that when you’re braking you add to your brakes the resistance of a generator. When going over rough terrain and the wheels are sporadically free-spinning, the resistance of the regenerative braking system will make the ABS problem even worse by constantly resisting the spinning while the ABS is trying to free it up. The two systems will work against one another.

If you want to be sure, I’m recommend a reliable independently owned and operated shop.

Be aware that when you apply the brake pedal the brake master cylinder is activating a brake position sensor. The brake pedal feel is actually a simulated resistance engineered by the brake controller. The brake/ABS/regeneration controller is in actual control of the brake functions. Firstly, the retarding generators are brought into play resisting the wheel rotation. As more braking is demanded the calipers are applied in proprotion to the perceived traction of the wheels. Finally, the ABS system kicks in to keep the wheels turning i.e. no skidding. All of this is under the control of the brake and transmission controller.

If you are experiencing problems consult the service department software technician as to how the system should operate. If they say that they all do that, drive another in the same enviornment and see if the same thing happens. Otherwise, suggest that a software upgrade be downloaded.

BTW, you may be wondering what happens if the brake controller goes haywire. The failsafe for the controller is to connect the brake master cylinder to the front calipers only. There will be no regenerative braking, no ABS, and no rear brakes. One system only.

Hope this helps you understand your Prius.

I don’t see any bulletins or recalls for this problem on the 2011 Prius but the 2010 Prius had complaints of delay of brake response when the ABS engaged on a bumpy surface. Are you sure your Prius is a 2011?

Safty Recall A0B involves asoftware update to the ABS controler.

Toyota has received complaints of inconsistent brake feel during slow and steady application of brakes on rough
or slick road surfaces when the anti-lock brake system (ABS) is activated in an effort to maintain tire traction.
The system, in normal operation, engages and disengages rapidly (many times per second) as the control
system senses and reacts to tire slippage. If the same brake pedal force is applied under these conditions, in
the worst case, this may lead to an increase of vehicle stopping distance and thus raise the possibility of a crash.
Toyota dealers are requested to install a newly designed software update (reflash) to the ABS Actuator
Electronic Control Unit (ECU) at no charge to the vehicle owner.