Model 3

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Sudden Slow Down with Cruise Control

Today I made a long highway drive. The speed limit is 75mph, the car correctly noted the speed limit, and I set the adaptive cruise control at 75. I was not using autosteer.

Suddenly, for no apparent reason the car abruptly slowed. This really caught me by surprise. I glanced at the screen, it showed speed limit 35mph although there was no actual speed limit change on the road. What was much worse was that the setting on the ACC changed itself to 35mph! The deceleration was so fast that the car slowed to about 55 before I was able to act by accelerating and cranking the ACC setting back up to 75. Lucky there weren’t many cars on the road or this might have caused a bad accident.

I can’t fault the system for having an error in its speed limit files. But any cruise control should stay where I set it unless there is a slow car ahead or I make the change.

Is there some way I can stop this behavior so that the speed I set always remains until I change it? What it did today was scary and just plain dangerous. I’ll have a hard time trusting this basic system in the future.
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Comments

  • Sometimes the car will read a geofenced speed of an adjacent exit ramp, a crossing road or a construction detour. Mapping errors can also occur. It’s good to be ready to override this with the accelerator. Fortunately, it’s pretty easy to catch a decel before it’s slowed the car more than a few MPHs.
  • What software version are you at? Did you save the dash cam video? Where exactly were you? Highway and mile marker. Did you do a bug report?
  • This is one frustrated and angry passenger writing as we are driving. We are on our second S and first 3. I ordered the 3 with no auto pilot given the trauma I’ve experienced in my husband’s newer S. Never experienced it in his first gen S. The phantom breaking issue is terrifying! Going from 75mph to 50 on I 95 is dangerous and needs a fix, regardless of all the excuses! I might expect that a lawsuit enjoining Tesla may ensue should a nasty pileup occur.
    Thank you for allowing me to vent. I have not run out of four letter words as some other writers suggest, but have spared you all that. :)
  • The car shouldn’t slow down in response to speed limit changes if you’re using TACC without Autosteer, unless the car thinks you’re on an exit or onramp. I’m not sure what happened in the OP’s case.
  • glenndunk10_44596

    What software version are you at? Did you save the dash cam video? Where exactly were you? Highway and mile marker. Did you do a bug report?
  • This is by far and these days really the only time I see this. On a non highway it cannot do more than 5 over the limit and mapping or GPS issues make it think you are not on a divided highway.
  • Phantom braking doesn’t cause the car to register a new speed limit. It decelerates 2-3mph and resumes the posted speed limit. Disregard trolls and idiots. Happened twice in my 11k mile trip where I charted such things.
    RUSirius is the banned trill, FISHEV. Flag and nice on so the @Forum_Admin can block access, for the fifth time.
  • that also is not phantom braking. Phantom implies it is unknown.

    When you can see the speed limit drop on the screen, you know why the car did it, and the car did it for a specific and correct reason. It just has bad data.
  • Lets try this a 3rd Time.......

    FrankyMN,

    What software version are you at? Did you save the dash cam video? Where exactly were you? Highway and mile marker. Did you do a bug report?
  • Our local state highway has some mapping issues with regards to incorrect speed limits. One 55 mph zone is mapped at 45 and another 30 MPH area is mapped to 45 as well. Knowledge is power. Google is not always right. The car is just following bad code. Respond accordingly.
  • > @HAL2001 said:
    > Lets try this a 3rd Time.......
    >
    > FrankyMN,
    >
    > What software version are you at? Did you save the dash cam video? Where exactly were you? Highway and mile marker. Did you do a bug report?

    I think I already replied to this in an another thread. I was in the last update 2020.35.something, US 101, don't remember the mile. I didn't reported the bug, as I was not aware that this is something that happens often. At the time I had no idea what happened. I was with a passenger, and we both got quite scared. That night I joined this forum to see what people knew about, and found out that is quite common. Haven't used AP (apart areas with no traffic) since that time to be honest, I am quite scared
  • > @FrankyMN said:
    > > @HAL2001 said:
    > > Lets try this a 3rd Time.......
    > >
    > > FrankyMN,
    > >
    > > What software version are you at? Did you save the dash cam video? Where exactly were you? Highway and mile marker. Did you do a bug report?
    >
    > I think I already replied to this in an another thread. I was in the last update 2020.35.something, US 101, don't remember the mile. I didn't reported the bug, as I was not aware that this is something that happens often. At the time I had no idea what happened. I was with a passenger, and we both got quite scared. That night I joined this forum to see what people knew about, and found out that is quite common. Haven't used AP (apart areas with no traffic) since that time to be honest, I am quite scared

    Sorry 2020.48.35, the Christmas one with the boombox
  • > @Bighorn said:
    > Our local state highway has some mapping issues with regards to incorrect speed limits. One 55 mph zone is mapped at 45 and another 30 MPH area is mapped to 45 as well. Knowledge is power. Google is not always right. The car is just following bad code. Respond accordingly.

    I understand, but this sounds like pushing the problem to someone else. The car is made by Tesla, and I am expecting the problem to be fixed by Tesla, not Google. If the issue originates from Google then Tesla should find a way to fix this.
  • You were so scared you forgot where it happened? Just US101? In CA, OR, TN? Near? It actually doesnt happen often, it does happen but its rare. Most of the phantom braking posts on this forum have been proven to be hit and run trolls.
  • > @HAL2001 said:
    > You were so scared you forgot where it happened? Just US101? In CA, OR, TN? Near? It actually doesnt happen often, it does happen but its rare. Most of the phantom braking posts on this forum have been proven to be hit and run trolls.

    You can think whatever you want about my experience. But it happened, it was real, no troll. It was scary, luckily didn't crash. If you have to bully people because you are braver then them cause you don't get scared, and this things are totally acceptable to you because you are always in control, fine by me, but leave us alone to report issues and be constructive. I am not attacking Telsa, I don't regret to have bought one, but this is a real dangerous issue, and must be fixed. Period.
  • HAL2001 -
    Relax. It's a known problem, it happens to all of us, and it has several different causes. I've had it happen a bunch of times in my 3 nearly 3 years of ownership, but I couldn't tell you precisely WHERE any one of them happened.

    There's no sense in attacking people who come in and report this issue, even if it's the hundredth time we've seen a post about it, because to them it exists, it's new, and it scares the crap out of them the first time it happens. Yes, people should search for existing topics (but that's a very advanced skill on this forum), but my 30 years on the Internet have taught me that people won't; and bitching about people who don't doesn't solve anything either.

    And I'll throw my hat in with those who really hope that the next major rev of AP handles phantom braking, and phantom speed limit changes, a whole lot better than the current code does.
  • > @HAL2001 said:
    > What software version are you at? Did you save the dash cam video? Where exactly were you? Highway and mile marker. Did you do a bug report?

    Sorry about my delayed reply. We’ve been a little distracted in Texas lately.

    Here are my answers and thoughts about this:

    Using the most recent software. It just downloaded and installed yesterday. I was on Texas 71, about midway between Austin and I 10 when this occurred. I was shocked by what happened and no, I did not notice the mile marker. No dash cam saved and no bug report.

    As I stated, I can understand if the software has a mapping error and an incorrect speed limit. However, I do not expect the cruise control “speed limit” to override my setting on the cruise control. If the software wants to warn me that I might be speeding, that’s OK. The software has no right to suddenly slow my car on its own to match the speed limit. Tesla is not the police!

    IMHO, this is analogous to what the software did in the Boeing 737 Max to cause those two crashes – the “brilliant” software overrode the pilot’s judgment - with disastrous results!
  • @FrankyMN
    I was responding to the OP, not you.
  • > @FrankyMN said:
    > You can think whatever you want about my experience. But it happened, it was real, no troll. It was scary, luckily didn't crash. If you have to bully people because you are braver then them cause you don't get scared, and this things are totally acceptable to you because you are always in control, fine by me, but leave us alone to report issues and be constructive. I am not attacking Telsa, I don't regret to have bought one, but this is a real dangerous issue, and must be fixed. Period.

    Re-read my post I did not call you a Troll. And where did I bully you? Where did I say it was acceptable? Why so defensive? You stated you were scared and your lucky you didn't crash but the only info you provided is US101 which practically runs runs across the entire US. To identify the cause a location is needed. I am simply confused that you were so scared and almost crashed but you cant remember where it happened. It was just 3 weeks ago.

    When you say leave "us" alone to report issues, who is "us"? Your on a forum where users help users so who are you reporting your issues too by posting here? Are you not here for help from other users? If so you need to give some helpful information.

    Phantom braking is a target topic for the Trolls here. They constantly make up fake posts. Never did I say your problem was fake. I simply countered your statement that it "happens often" with my statement that Trolls frequently fake it. That is not calling you a Troll.

    So if you want help, Pretend this post is Tesla and your reporting the problem to them so they can fix it. Your version is 2020.35.x and you where going 65 MPH, and it reset to 35 MPH. It happened on January 23rd 2021. Where did it happen?
  • 737 Max analogy is not close to being apt. That program fought pilots to the death. Tapping the accelerator is all this takes to override. Not everyone is cut out to be an early adopter.
  • > @Bighorn said:
    > 737 Max analogy is not close to being apt. That program fought pilots to the death. Tapping the accelerator is all this takes to override. Not everyone is cut out to be an early adopter.

    You are correct. Overstatement on my part.

    Still, there should be a setting that allows me to manually set the cruise control and not be overridden by the car’s assessment of speed limit.

    I will state again: Tesla is not the police. They are not my mommy. They should not reset the cruise control to the speed limit.
  • My 2013 Model S has old school cruise control, It will not adjust the speed, but then again, it will crash into things. Pick your poison.
  • This whole point of FSD is that the car drives itself. That includes going the speed limit or whatever overage buffer you’ve programmed in. So you’re free to let it speed. This is a mapping glitch and not a programming shortcoming inconsistent with the ultimate goal of self driving.
  • > @Frank99 said:
    > There's no sense in attacking people who come in and report this issue,

    Seriously, where did I attack him? Please point out my attack and I will apologize immediately.
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