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Speedo values ascend on deceleration?!!

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by landolfi, May 16, 2008.

  1. landolfi

    landolfi Junior Member

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    Hi all,

    I have an 05 Prius with about 43K miles. Tonight while driving the speedometer showed me going 45 in a 30 zone, which I thought was way wrong so I decelerated, and then I noticed that as I decelerated, the numbers were actually increasing, even at braking (43 when rolling, up to 53 as it stopped increasing in 1-mph increments)!!! Has anyone else ever experienced this, or does anyone have any ideas? This is somewhat urgent, I am far from home and although I have a portable nav system with me, it's very disconcerting to see invalid values on the dash display, especially since I have at least a 6 hour drive back home.

    I did try cycling power, to no avail. When I returned to the hotel, I put it in reverse and it seemed to show correct speed on backing up and the same when going forward, though I didn't try it for long. I also wonder what else could be flaky if the speedo is.

    I will also check with dealers in this area (Columbia, MO), but could use any advice/opinions in the meantime.

    Thanks for any suggestions,

    Bob
     
    #1 landolfi, May 16, 2008
  2. Patrick Wong

    Patrick Wong DIY Enthusiast

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    This is a really weird problem. The Prius has an ECU in the combination meter to determine vehicle speed. Apparently that is not working correctly. Bad vehicle speed info may confuse the hybrid vehicle ECU which is responsible for dividing up the powertrain workload between the engine and MG2.

    Since you have a portable GPS, I suggest that you set that to show your speed and compare against the speedometer. Note the discrepancies so that you can report to your dealer.

    If you do not notice driveability problems then you should be safe to return home; however I strongly suggest that you do not use cruise control until this problem is fixed.

    Good luck.
     
    #2 Patrick Wong, May 17, 2008
  3. landolfi

    landolfi Junior Member

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    Thanks, Patrick. Driveability does not seem to be an issue so far, but even then I'll stay away from the cruise and monitor the gas mileage on the way back. I would imagine since I'm 3+ years out and 43K, this won't be covered under warranty, so I'll report back on the repair cost.
     
    #3 landolfi, May 17, 2008
  4. Patrick Wong

    Patrick Wong DIY Enthusiast

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    Hi Bob,

    I also suggest that you set up your GPS to track your trip miles, and compare against the trip odometers in the Prius. If the displayed speed is incorrect, the odometer readings might also be off.

    If the problem is the combination meter ECU, that would be covered under the 3 year / 36K mile warranty, so I agree that you would probably have to pay for that replacement. The dealer would have to special-order a new ECU because your current odometer reading has to be programmed into the new part.
     
    #4 Patrick Wong, May 17, 2008
  5. landolfi

    landolfi Junior Member

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    DOH!! I am sooooo embarassed. . .

    On the way to the dealer, who was very accommodating in agreeing to take a look even though the Prius techs don't work weekends (Joe Machen in Columbia, MO), my wife monitored speed on the GPS. As we accelerated, the discrepancies grew quite large, showing 85 when the GPS showed 53, and then I noticed it was showing KPH, not MPH. A simple reversal of the button I bumped last night in the dark when setting the trip odometer returned all to normal. The speedo increments that occurred with deceleration were probably because I was braking on a hill.

    We went ahead to the dealer anyway, and the service manager and the mechanic got a nice chuckle.

    Thanks again,

    (Sheepishly),

    Bob
     
    #5 landolfi, May 17, 2008
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