Battery fan speed tied to vehicle speed

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by BryGuy, Sep 14, 2025.

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  1. BryGuy

    BryGuy New Member

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    Hey all,

    I've had no trouble codes, but realized my battery temp was high when using Dr. Prius. Realizing I've never heard the fan run, I checked and found the corroded plug in the trunk, cleaned it and the fan ran - but now its running at a speed seemingly directly tied to the inertia of the car! Not exactly wheel speed, but wheel speed +/- acceleration.

    I'm assuming the resister is bad, but I'm hoping someone can help me diagnose the circuit. I have a multimeter but don't know how to read the wiring diagrams, and I believe there is a fuse, a relay, and a controller on the same circuit. But I don't know to determine if the control signal from the ECU is bad as well.

    Any help is appreciated! Gonna see if I can capture a video later.
     
    #1 BryGuy, Sep 14, 2025
  2. bisco

    bisco cookie crumbler

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    try monitoring the battery temp and see if it increases and decreases at a similar rate
     
    #2 bisco, Sep 15, 2025
  3. ChapmanF

    ChapmanF Senior Member

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    If memory serves, there have been other posts also reporting that the ECU likes to take road speed into account ramping fan speed up and down, presumably to keep it more quiet when there's less road noise expected.

    Moments of strong acceleration or regeneration are moments where high battery currents contribute more heat, too.

    It may be operating as designed.
     
    #3 ChapmanF, Sep 15, 2025
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  4. PriusCamper

    PriusCamper Senior Member

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    That's the first explanation that makes sense as to why its so hard to hear! Up here in the PNW some battery rebuilders tell people that the fan is only for hotter climates and the fan is never needed in our region. I'm certain that's not true, especially due to climate change and all my gardens getting wrecked by increasingly hot and brutal Summers.

    And all that effort that went into making a multispeed fan never made sense to me but your explanation resolves that.
     
    #4 PriusCamper, Sep 15, 2025
  5. BryGuy

    BryGuy New Member

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    Thanks guys! I'll need to confirm it, but it seemed like the fan was running according to vehicle acceleration regardless of battery temp. The times I worked on it, the battery was between 120F-130F in the center (even with cabin temps cooled to ~72), and I believe the fan should have been running on high at those times unless the speed throttle has priority.. Does anyone know what the speed levels' corresponding temperatures are, or if there is a max temp override?

    Another thing that concerns me is that I don't really hear any definite steps in the fan speed. In the video linked , temperature was 130f at the core +/- .2 degrees the whole drive.
    (I'm aware of the bushing/shaft wear and am going to try to oil it today)
     
    #5 BryGuy, Sep 15, 2025
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