noise when braking, even when engine is off

We've noticed this too. My searches on the forum indicated it was something to do with the brake system as well.

Mine seems to be more prevalent when I'm carrying more passengers or cargo, but it definitely happens from time to time on both braking and acceleration when it's just me in the car as well. I'll be interested to see what the more experienced say. It definitely makes me nervous.

Great description of the noise, by the way! I always think it sounds like a Vibraslap (most like the LP 209 soundclip, but not as metallic, on this site: Latin Percussion: LP Vibra-SlaP II ). Ours lasts about 1.5 seconds each time.

 

The noise likely is from the brake pressure accumulator pump. If you repeatedly depress the brake pedal while the car is READY, you should hear the pump running to rebuild pressure in the system. If that is the same sound that you expressed a concern about, then no worries.

 

This is the brake accumulator pump. The power brakes on the Prius are driven by an electric pump, since the engine does not run continuously as it does on a normal car. The accumulator pump kicks on about every third time you use the brakes. It builds pressure in a little tank, which is then used to power the hydraulic system. The accumulator pump makes a buzzing or ratcheting sound.

The other noise you hear after powering off is the coolant transfer pump. The transfer pump saves hot engine coolant in a thermos bottle, which is then used later to preheat the engine. The coolant transfer pump makes a whirring sound.

Tom

 

It's the brake accumulator pump. I have this comment on a hot key, so I can press it once to answer this question. It's one of the most common.

Tom

 

Thanks, Tom! for the clear reply to the question.
I have found this in various forums, so I will ignore this noise, unless it gets worse or I feel it affects performance. It's just unusual to have this noise when you hit the breaks (as you say, approx every third time or so and sometimes when accelerating out of a stop).
But I guess it comes with driving around with a lot of technology under the hood and the seats and the trunk and everywhere.

 

The noise from my brake accumulator pump now occurs every 8 seconds when driving and about 20 seconds when stopped. I just had the clock spring replaced and the also my sway bars. (and my instrument cluster) The pump has always made that noise but now it happens what seems like too often. 1) could it have been something they did 'wrong' at the shop?; 2) should I be worried? I am about to drive for 5 hours away on a two week trip to a mountain town in North Carolina. Closest big city 1 hour away

 

If the brake pump runs periodically even when you don't push on the pedal, then something inside the brake actuator assembly is bleeding off (leaking) pressurized fluid.

Normally it should stay pressurized for a long time - my 06 will barely drop any pressure after 30 minutes until I use the brakes.

If yours is getting worse then it's time to research replacing the brake actuator assembly. No one can tell how long a failing actuator will last before the pump dies- but it should start throwing ABS codes when it gets worse.

Loss of the pump will result in the "last stage" fail safe function- the brake pedal will directly operate just the front brakes- no boost (hard pedal), no ABS, no regen, long stopping distances.

Posted via the PriusChat mobile app.

 

Thank you! So it is safe to drive until the ABS last stage event occurs? and even then, but then it will brake like an old truck?