Odd "rev" sound when declining down hill

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by eseerc, Apr 23, 2013.

  1. eseerc

    eseerc Junior Member

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    2010 Prius
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    Today while driving, I was declining down a rather steep grade. I had my cruise control set at 65mph. As I was driving, I heard an odd noise, almost like my Prius was "revving" in a neutral gear. The battery was at a full or near full charge when this happened.

    While searching for a information on this noise online, I found a forum post which read

    "What you're hearing is the engine braking - just what you'd expect when using B. The engine spins (pumps air) to help slow you down. At some point, if you have heavily charged your battery on the descent, the engine will be spun by the electric motor just to bleed off that excess electricity."


    Only problem being, I was not in the "B" gear, I was in "D". Is the noise I heard still explained by the above explanation? Furthermore, if so, is this a "bad" thing? Should I no longer use cruise control while declining down a steep grade?

    I will also note that this is the first time I have ever heard the noise and I have declined down similar grades before with no "noise"

    Thanks for the help!
     
    #1 eseerc, Apr 23, 2013
  2. 2k1Toaster

    2k1Toaster Brand New Prius Batteries

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    If your battery is full, the only thing it can do is burn energy spinning the engine as an airpump. So it is doing exactly what it is supposed to.

    When you are in B mode, it does this preemptively. That way if you know you are going down a hill, you can burn a bit along the way instead of waiting until it is full and burning it off. Because once you are full your regen brakes stop working and you are on friction braking.
     
    #2 2k1Toaster, Apr 23, 2013
  3. cwerdna

    cwerdna Senior Member

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    #3 cwerdna, Apr 23, 2013
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