Can ABS cause slight drag on one wheel with no hint that it is doing so?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by pasadena_commut, Sep 3, 2025 at 11:49 PM.

  1. pasadena_commut

    pasadena_commut Senior Member

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    Hypothetically...

    If an ABS sensor on one wheel was damaged just a tiny bit, for instance when a wheel bearing was replaced, is it possible that it might send a signal that was almost but not quite perfect to the ABS unit, and if so, might the ABS unit respond in such a way that the brake on that wheel was only very slightly applied? All of this going on silently, with no warning light or code?

    I'm imagining this as the sensor sending a series of evenly spaced pulses and the damage causes just one (or a small number of adjacent ones) to be noisy and so instead of 1 or 2 rising edges per unit time there are many more, let's say 5. The ABS unit seeing 5 on that one wheel and 1 or 2 on all the others starts to do "wheel is slipping" and begins to squeeze the caliper there, but before it can really get going it is back to regular pulses for the rest of that rotation. Unclear why it wouldn't light up the indicator, unless perhaps it is doing so, but at a duty cycle of 1 in 100, making it too dim or brief to see. I'm thinking the ABS unit should throw some sort of code for this scenario though.
     
    #1 pasadena_commut, Sep 3, 2025 at 11:49 PM
  2. ChapmanF

    ChapmanF Senior Member

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    For the specific scenario you're (over)thinking about, I'd say the answer is 'no'. Anything the system does in response to an unexpected wheel sensor input is going to be considered an activation of ABS or VSC (and appear in the ABS History screen) or a fault (and produce a trouble code and warning lights).

    But if you are chasing a slight drag at one wheel, why would you (over)think about it being somehow a response to a wheel speed sensor anyway? The brake actuator contains a bunch of small electrically-controlled valves that regulate brake fluid pressure out to the lines. A valve slightly impaired with corrosion or a contaminant could cause your symptom. So could a rubber brake hose that has started delaminating (hasn't happened to me, but I've read they can form a rubber flap inside that acts as a bit of a one-way valve). Also a caliper itself can be sticking, or (different from sticking) have a piston that moves freely but doesn't retract when pressure is released (a function of the square-cut rubber piston seal).

    You should be able to find a lot of threads here with ideas for troubleshooting a dragging brake.
     
    #2 ChapmanF, Sep 4, 2025 at 7:44 AM
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