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TACC target speed spontaneously changes

Discussion in 'Model 3' started by Cogarch, Nov 2, 2019.

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  1. Cogarch

    Cogarch Member

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    Apr 27, 2019
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    I have in the past had problems with sudden deceleration caused by my accidentally brushing the RH scroll wheel determining TACC target speed. That was disconcerting enough but at least there was an obvious reason for it. Last night however towards the end of a 300 mile drive, much of it in pouring rain, my Tesla decided to change the target speed all on its own. I was driving at 65 just coming off the M25 on to the M3 eastwards when the speed suddenly shifted down to 45 mph. I then scrolled it up to 55 because I was slowing down anyway - but then just as I was about to merge onto the M3 it suddenly jumped to 65 mph which was too fast for current road conditions.

    This was one of the trickiest moments I experienced in my long 600 mile round trip, which included several panic-braking incidents caused I think by gantries over the road (reminds me of the Young Ones sketch where the aircraft pilot suddenly screams in terror, “Oh no — a cloud!”).

    Quite apart from the dangers it creates of following cars rear-ending me, it is just so embarrassing when you have passengers..!
     
  2. swaltner

    swaltner Active Member

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    TACC will automatically adjust the speed down in a couple cases:

    - while driving on a non-divided highway, it will lower the speed to 5 above the speed limit if autosteer is on.
    - while taking highway interchanges or exit ramps, it will automatically slow down if the corner is sharp enough that “a slower speed is required”.

    in both cases, it will speed back up to the previous set speed after that situation passes IF you don’t manually adjust the set speed in the mean time.
     
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  3. ewoodrick

    ewoodrick Well-Known Member

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    Or if the speed limit drops or rises.
     
  4. brkaus

    brkaus Well-Known Member

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    I don’t believe it will speed back up after a lower speed limit. Also, I believe this is only in autosteer. TACC only won’t adjust for speed limits (USA anyway).
     
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  5. SomeJoe7777

    SomeJoe7777 Marginally-Known Member

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    I have had a lot of spontaneous speed reductions occur due to map errors.

    The Northwest Freeway in Houston has been under construction for a couple years. Some of the new lanes that have been put down are now actually the same GPS location as the service road was before the construction. Telsa has not updated the maps for the new construction.

    When driving in those lanes on the freeway now, the car will periodically think (due to the old map) that is it on the service road instead of the freeway, and will correspondingly lower the TACC speed to the speed limit of the old service road. It's very disconcerting, and almost makes TACC and Autosteer useless on that freeway.

    The notion that this car is going to do Full Self Driving any time soon is a pipe dream. Right now, the car cannot determine exactly where it is with any high degree of accuracy. If the car doesn't know where it is, it is impossible for it to execute full self driving, regardless of how good the algorithms for navigation are.
     
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  6. swaltner

    swaltner Active Member

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    Yes, it will speed back up if you don't fidget with the speed in the mean time. One section of road I drive does this all the time. It's a two lane road with oncoming traffic. The speed limit for most of the road is 55 mph. Typically, I'm driving this at 58 mph with autosteer enabled. One intersection with slightly heavier traffic than the rest is posted at 45 mph. The Model 3 slows down to 50 mph (5 above the speed limit because autosteer is enabled and the road has oncoming traffic without a divider). After 1/4 mile, the TACC set point resumes to 58 mph when the speed limit goes back to 55 mph. If I disengage TACC/autosteer in any way or even touch the right scroll wheel to try to adjust the set speed, it will not resume back to the previous set point when you exit the lower set speed. The only time it does this is when a lower speed limit forced it to reduce your set speed and you didn't mess with it during the time the speed limit was lower.
     
    • Informative x 1

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