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ABS or speed sensor question

Discussion in 'Gen 2 Prius Technical Discussion' started by kens97uber171, Aug 5, 2019.

  1. kens97uber171

    kens97uber171 Active Member

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    Have noticed my cruise control is a bit erratic.
    When I look at OBD speed output in Torque..
    It seems to vary 3mph or so.
    Is there a single speed sensor in the transmission or does it use wheel speed sensors..
    I've got access to Techstream if needed.
    Torque did not seem to have individual wheels speed data.

    Thanks.

    Posted via the PriusChat mobile app.
     
    #1 kens97uber171, Aug 5, 2019
  2. Skibob

    Skibob Senior Member

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    This was recently talked about here. Basically the speedometer lies. It reads about 2-3 MPH higher than the actual speed with the original tire size. The computer knows the actual speed however so it can correctly record the mileage.
     
    #2 Skibob, Aug 5, 2019
  3. sam spade 2

    sam spade 2 Senior Member

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    All of that has NOTHING to do with the cruise control not holding a rock solid speed. :whistle:

    A cruise speed variance of 3 MPH is NOT considered a problem........with any vehicle.

    One designed for high fuel efficiency in particular tends to be slower to correct a speed variance.

    Why are you worried about it ??
    If your model has an ECO mode button......turn it OFF and the cruise might hold tighter, if that is REALLY a big concern.
     
    #3 sam spade 2, Aug 5, 2019
  4. kens97uber171

    kens97uber171 Active Member

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    Well.. seems I did not explain myself well.
    The speedometer on the dash stays Rock solid.. and I'm talking straight flat road at 65mph.
    When I look at the OBD speed PID in Torque
    It varies 63-64-65. And back and forth.. while the speedo on the dash stays at 65.

    It tends to jerk and be erratic when going from off throttle down a hill to on throttle up a hill.. it does not do this if you use you foot .

    Also.. I've never had ANY car vary 3mph in actuall speed on cruise control going straight and flat.. and I've owned over 40 cars in my life . And my Prius does not show any speed difference either.. but one of the speed sensors IS showing erratic speed output. And I'm thinking that's what's making the cruise control behave badly.

    My basic question is about the speed sensors.. does it just use the 4 wheel sensors.. to know it's speed, or is there another sensor in the transmission.
    Many cars have both, and use the data from them for different things or to compare them.

    Thanks.. I hope that clears up my question.

    Posted via the PriusChat mobile app.
     
    #4 kens97uber171, Aug 7, 2019
  5. ChapmanF

    ChapmanF Senior Member

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    I'm pretty sure it starts with the four wheel sensors. The New Car Features manual describes how the speed gets to the combination meter, which has a bit of the Rube Goldberg to it. The skid ECU gets the readings from all four wheel sensors. It then generates a pulse signal where the speed of the pulses represents the vehicle speed, and the combo meter reads that (as does the nav ECU, etc.)

    It seems that pulse signal has a longer history as a way of carrying speed information, and might have come straight from a transmission sensor in some older model, but now it just comes from the skid ECU synthesizing a compatible signal.

    You can put the skid ECU into a wheel-sensor test mode, which is more sensitive than normal driving. You enter the test mode and do some specified maneuvers in the manual, ending in driving in a straight line for a certain distance and speed (so you have to pick a starting place where you can do that), and then it tells you if any of the wheel sensors are iffy.

    That's supposed to be done any time a wheel bearing or sensor is replaced, to confirm the repair, though not everyone does.
     
    #5 ChapmanF, Aug 7, 2019
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