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I have a two and half months old Toyota RAV4 hybrid. I have noticed that occasionally when pressing the brake pedal when being stopped, I can hear sound that seems to come from the ABS. It is definitely similar to the sound you can hear when braking hard on a slippery surface. The sound lasts for about half a second, and if you continue to press the brake pedal, the sound does not occur again.

I haven't found any logic to the sound. It certainly isn't caused when you press the brake hard: I have many times pressed the brake hard when being stopped and the sound does not occur, and many times pressed the brake very lightly when being stopped, and I can hear the sound. Most often, the sound occurs after starting the car, but occasionally I have heard the sound while being stopped at a stoplight.

My own theory is that this is some kind of self-test for the ABS that the computer occasionally performs to see everything is working.

Apparently, RAV4 hybrid is not the only vehicle with the sound. 2008 Ford Escape hybrid has also the sound according to a comment to my answer in 2008 Ford Escape hybrid brake and computer problem.

However, another comment to my answer has a different theory. According to it, "it's the actuators in the bypass modulator valve activating to allow pressure through. It should only happen if you press hard on the pedal when stopped". This certainly isn't in line with my experience: I have heard the sound many times when pressing lightly on the brake pedal, yet not heard the sound many times when pressing hard on the brake pedal.

So, my question: what is the sound? Is it a self-test as I suspected, or is it something else?

  • I would bet, since this is a hybrid and hybrid braking relies on using the electric motor for regenerative braking to charge the batteries, there is a switchover to physical braking at some point. If your RAV4 is set to use regenerative braking when you press the brake pedal and the system switches, you are going to hear/feel some sort of mechanism switching it over. IIRC, Ford uses Toyota's hybrid system so would be the same there. I don't know if this is actual, because I don't have any direct experience with hybrid systems, so leaving this as a comment. – Pᴀᴜʟsᴛᴇʀ2 Feb 11 '17 at 14:05
  • @Paulster2 Yes, there has to be a switchover mechanism. But when at standstill, it should use only the traditional brake pads, as regenerative braking is not effective when at standstill. Perhaps it's a self-test for the switchover mechanism instead of self-test for ABS? – juhist Feb 11 '17 at 14:11
  • If the mechanism is still set to hybrid when you push on the brake pedal, there would be a switchover. This could very well be the case if the system was still using it as you came to a stop. If the system didn't need to use the brakes between when it was set as regenerative and when you press the pedal, it wouldn't have changed. I don't think it's a self test, because that happens once during startup. The antilock braking system has to pass a POST or the dash light comes on. I don't think there are any other tests besides this. – Pᴀᴜʟsᴛᴇʀ2 Feb 11 '17 at 14:15
  • More support for self test hypothesis: opensiuc.lib.siu.edu/cgi/… refers to "pad apply self test" and brakeandfrontend.com/tech-feature-ford-hybrid-braking refers to a self test in an older Ford Escape hybrid as well. However, these are for Ford, not for Toyota. – juhist Feb 11 '17 at 14:18

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