Sudden drop in speed while in cruise control

Sudden drop in speed while in cruise control

Our month-old Model 3 has a habit of randomly dropping the speed while in cruise control, going from 65/70 mph down to 50ish mph. There isn't any pattern to this behavior, aside from the car being in cruise control (the basic cruise control as well as autopilot). Sometimes there wasn't a car in sight, and yet this happened. Sometimes we were in heavy traffic, so we felt rather unsafe.

Both my husband and I, two very different drivers, have noticed this. We took this car on a 1800+ mile round trip last weekend and observed this behavior together. So I booked an appointment using my phone. Today, someone from Tesla contacted us asking for additional information. After my extended description of the problem, he responded with this message:

"I looked at the data from your vehicle and there are no hardware faults that would be indicative of an issue requiring service. The autopilot and other driver's assistance features require the operator to always be alert and ready to take over control for these exact scenarios. Traffic aware cruise control may misinterpret vehicles or other environmental factors as encroaching on your vehicle even if they don't appear to be. This can cause the vehicle to slow down when you feel it should not. These features are constantly being improved over time with updates and will continue to improve."

I did my best to explain to this (young) person what my husband and I independently experienced, but he stopped responding.

So my questions are:

1. Have you experienced such behavior with your Model 3 (or other Tesla models)? Was it fixed? What was the problem?
2. Any tips on getting through to Tesla that even though the diagnostics were negative, the car was indeed behaving in a way that might be dangerous?

howard | 18. Juli 2019

1. Yes numerous times. No it can not be “fixed” in the traditional sense. Tesla is correct the hardware is functioning properly. It is a programming issue that we all hope will be fixed in future updates.

2. Tesla is aware but has not been able to correct it.

I don’t use TACC “adaptive cruise”, AP or NoAP in part for this reason. I try it every new update hoping but no joy.

yumei.leventhal | 18. Juli 2019

Thank you, @Howard! Frustrating! The cruise control on the Subarus works perfectly -- so the technology is there.

The other thing is: A week after we got the car, we still couldn't get autopilot (specifically, lane change) to work. Took it to a service center and they discovered a faulty/loose cable holding one of the sensors. So we are sorta wondering if we have remnant issues with sensors.

vmulla | 18. Juli 2019

Oh boy!!

Long time AP user/fan/critic here.

Yes it happens, but there's a lot more to it than what you get out of reading messages on the forum. Bottom line that few incidents aside the system is fantastic. Be careful, understand the nuances of the system and enjoy.

vmulla | 18. Juli 2019

FYI... The cruise control on the subaru is respectable, but it's nothing compared to the capabilities of Autopilot (even with it's current rough edges)

howard | 18. Juli 2019

yumei.leventhal | July 18, 2019
Thank you, @Howard! Frustrating! The cruise control on the Subarus works perfectly -- so the technology is there.

It works incredibly well on my 2019 Volt as well. That should get this thread deleted quickly.

Mike UpNorth_ | 18. Juli 2019


howard | 18. Juli 2019

vmulla, AP is not in question. Basic adaptive cruise is.

howard | 18. Juli 2019

Mike UpNorth | July 18, 2019

Sorry, Mike forgot it helps you feel better. 2019 Jolt has better adaptive cruise than my 2018 twice as much Tesla. There now I feel better. Win win

Mike UpNorth_ | 18. Juli 2019

Sad face.

howard | 18. Juli 2019

Me too!!

leo33 | 18. Juli 2019

"These features are constantly being improved over time with updates and will continue to improve."

I've had my car a bit more than half a year, and I can personally attest to this. Yes, pay attention, and be ready to intervene, but it's been getting better and better.

Devilstower | 19. Juli 2019

Every time I’ve experienced a slowdown, it’s been near an intersection.. and when I’ve allowed the car to do what it wants (in a no traffic scenario) it has resumed previous speed a few seconds later. Which has made me think the issue, for me at least, isn’t lunging at shadows, so much as picking up speed information from other roads.

And yes, my wife’s Subaru CrossTrek sails through the same intersections with no problems, but then, it also doesn’t know what the speed limit is. And if you try to treat it’s LKAS system like auto steer, you end up slowly ping ponging back and forth down the road. And that LKAS won’t even turn on in areas the Tesla steers through without an issue. The Subaru is clearly a much, much simpler system doing a lot less “thinking” about how it handles speed. It’s just that right now, there seem to be LOTS more situations where it actually turns out better, because Tesla’s system generates ‘false positives’ for slowdown.

dsvick | 19. Juli 2019

There are two types of unexpected slowdowns I've encountered. One is what people call phantom braking, this is where the car, for seemingly no reason, brakes hard from whatever speed you were cruising. The other one I've experienced is where the set cruising speed is actually lowered from whatever you had it set at. Every time I get either of them a file a bug report to Tesla, they'll pull the logs and see what is going and either ignore it if there was a valid reason or they'll add it to the NN as another learning example.

EVRider | 19. Juli 2019

When using Autosteer, the car will slow to the speed limit plus 5mph (depending on speed offset) if GPS/map data indicate you’re on a secondary road (the data is sometimes wrong). When using TACC alone, the car shouldn’t slow down except if you start to take an exit.

howard | 19. Juli 2019

In TACC it will brake/sudden slow down when:

A car moving faster than you cuts in front of you. Does not do this with my other car.

When you move out to the adjacent lane just behind a car that is traveling faster than you. Does not do this with my other car.

When coming to a curve with a car in the center lane turning. Does not do this with my other car.

When coming to a curve with another car starting to pull out in the other lane. Does not do this with my other car.

When in traffic with a car just to your left entering a hard curve. Does not do this with my other car.

When you turn your indicator on to move around a slower car in front of you it will brake unless you are fully clear of the car. With my other car if the indicator is on I can close on the car ahead as I move around it.

If coming into a curve and the car apparently "thinks" you should be going slower when it should not. Does not do this with my other car.

Then there is the 4 lane straight away where it just brakes for no apparent reason. My other car has never done this.

In general, I find the pacing to be more abrupt/less smooth. There is less distance allowance when pacing. I cannot move in and out of traffic as freely as I can with my other car.

casun | 19. Juli 2019

@howard i have not experienced the sudden braking in most of the situations you list above and only very randomly in two of those situations. your post reads like it always brakes in those situations.

howard | 19. Juli 2019

casun, It is fairly consistent for me, but I just try it for a bit with each new software update and then don't use anymore. Most of these issues occurred when I last drove Denver to Steamboat Springs a couple of weeks ago. I was trying to use it as much as possible but my wife said enough is enough. She joked at one point that she knew it would brake. I think that was the car in the center lane turning left on a curve. I am a bit more sensitive to it since I drive both cars back to back every few weeks. That is where the irritation really comes out.

hokiegir1 | 19. Juli 2019

I noticed on my most recent update (2019.20.4.4) that when I first installed, there wasn't really any change...but now that I've had it for about a week, I'm noticing some additional improvement, particularly in the "dancing cars" area-- so I think there may be some re-calibration time that happens in the background after updates. We also noticed that going around a curve, it's getting better at not staying square in the middle of the lane -- which often puts you uncomfortably close to the car in the next lane who tends to "hug" the line. I think they are doing small incremental steps in this area to see where the optimal placement is. As far as slowing down for a curve, are we talking a gentle sweep or a more aggressive turn on the highway? Because there are sometimes "recommended" speeds and we've seen that it is starting to account for those to some degree and slowing a bit before entering them. As a driver, you might make this change more gradually and subconsciously (easing off the accelerator), but the car is still learning this and hasn't gotten to where it can anticipate it like we can looking at a map or far out in the distance to see it coming.

WardT | 19. Juli 2019

We have experienced this awkward slowing/breaking issue as well. It is very frustrating! My two cents is Tesla is reacting to their cars crashing into parked cars by programming in more safety features. I suspect they don’t have enough computing power to consider the scenarios in a timely way. I don’t think this issue will be resolved until they upgrade the CPUs. When the car randomly brakes, it thinks it sees danger. If it had more computing capacity it might be able to figure out what is going on faster and not apply the brakes.

howard | 19. Juli 2019

hokiegir1, I never have mine in AP. We are just talking straight up adaptive cruise. Adaptive cruise has not improved at all in the last year. Perhaps AP is getting better but I would not know as I don't use it yet.

WardT, adaptive cruise is the simplest of the advanced functions. There is more than enough computing power. I am not an expert but I think the adaptive cruise on my other car is more of a mature canned application if you will. In otherwords, it is a dedicated closed system. Tesla is still concentrating more on AP apparently hoping it fixes the TACC issues as well. It is a wrong approach to me but it is what it is for now. Disappointing.

yumei.leventhal | 24. Juli 2019

Update: Apparently the sudden drop in speed is known as "phantom breaking"--a Tesla technician called while on another job with the same issue(s) and said the customer's firmware was one version behind. When I checked our car, its software was "way behind", he said, and he initiated a software update which is going one as I type.

So if you have experienced similar issues, this may be one of the things that need to be checked off first.

The software version was never once discussed when I was going back and forth with the Tesla contact person. Nice as he was, he never once mentioned it, but kept on insisting future updates would fix the problem--meanwhile, I am wondering whether we'd live to see the next update, given what we had experienced.

vmulla | 24. Juli 2019

hokiegir1 | July 19, 2019
I noticed on my most recent update (2019.20.4.4) that when I first installed, there wasn't really any change...but now that I've had it for about a week, I'm noticing some additional improvement, particularly in the "dancing cars" area-- so I think there may be some re-calibration time that happens in the background after updates


I've suspected 'silent' updates to the software for a long time. I do not think it's different software that's being pushed, but rather a different configuration file that adjusts the software.
I've noticed this most recently with the screen brightness issues - there was no software update but the issue corrected itself overnight. I've also seen the same thing you did, changes in dancing cars and speed limit adjustments.
The approach would make perfect sense because not all tweaks need a new release.

charles.a.braun | 24. Juli 2019

@yumei.leventhal - dont expect the software update to resolve the issue.

This has been a problem for several years and affects both AP1 and all versions of AP since.

It may resolve the issue is some areas but my introduce the issue in other areas.

M3R | 24. Juli 2019

I have my Model 3 since last August and I observed the same thing once in awhile at the same location. I live at SoCal and driving South of 110 at the morning time, right at the Bus Station of Manchester on Express right lane, i got the speed drop from going 68mph to around 50mph and I take control every time it happens. There are no cars in front of me or around in some occasions. I wonder if it senses something there or something that trigger the sensor. So, i wonder if anyone driving in that session experience the same as thing i have.

yumei.leventhal | 24. Juli 2019

@vmulla The software (firmware) version 2019.20.4.4 was what our car had--it has since been updated to 2019.24.4.

@M3R We don't live in CA, but had the exact same issue--on a straight stretch sometimes with no cars ahead. Yet Tesla kept on telling us drivers are expected to paying attention, as if the issues were operator error. They did fix a loose/disconnected cable shortly after we got the car, so we aren't ruling out hardware issues. But I am going to try to see if this update helps a little.

@charles.a.braun I get what you are saying ... fingers crossed.

yumei.leventhal | 24. Juli 2019

In our car, the software update option was set to "standard", not "advanced". I wonder if that kept the "silent" updates from being pushed through. Any ideas?

EVRider | 24. Juli 2019

I don't think there's any real evidence for silent updates, so the software update setting shouldn't matter. Every update that affects firmware (except map updates) takes your car out of commission at least briefly, which is why you have to choose to install them.

bob | 24. Juli 2019

M3R: Boston/NH driver here. I believe the problem you describe in one area is the same problem I have. My theory is the problem Tesla has is the MAP technology. There is one spot on my drive that the same damn problem happen when no one is around. At first I thought the problem might be traffic around me and that damn phantom shadow issue, but when I had the problem happened a second time. I looked at the map and saw that I was on the edge of a border for town/county. My theory is that the map of the highway shows the highway discontinued internally due to a bug in the map and thus Tesla slows up as if you are no longer on a highway.

Techy James | 24. Juli 2019

@yumei.leventhal Standard vs Advanced update is part of 2019.16.x and later update. Unless you change it will default to the "Standard" setting. The difference in the two, with an "Advanced" setting you will get updates as soon as they are ready for your car configuration and region. The Standard will only push automatically once the version is more finalized. So the setting would be which do you prefer to get updates as soon as they are available, or wait till others have tested them out and initial kinks/bugs worked out.

As for the phantom braking and slowdowns there is plenty of topics on that. Depending on the type of road you are on and what the car picks up in surroundings on the potential causes. Having used the NoAP (part of FSD package) on both Highways and Secondary roads I can anticipate 99% of the cases where car might slowdown. Depending on the type of road there are a different set of potential causes. The one constant item I have noticed in each case, the car was reacting exactly as would an overly cautious driver might. Since I am able to predict most of these occurrences a quick press on accelerator pedal counters the unwanted action. That said here is some of the items I noticed that can lead to this braking behavior.
On Limited Access Highways:
1) Being in far right/left lane and merging traffic lane ending where a car without correction would potentially hit the car as the merged.
2) A car going slower than you merges in to your lane to pass a slower vehicle.
3) Depending on lighting conditions and sun casting shadows, a shadow in the lane ahead can cause a minor slowdown while the car determines nature of show picked up.
On Secondary non limited access roads.
1) Car exiting your lane to right or left as it turns off.
2) On coming traffic car turns across to side road crossing your path. (Even in safe manner where you wouldn't normally brake.)
3) Car pulling up to your road from a side street even as they are stopping.
4) road bends ahead an oncoming traffic appears to be potentially in lane, more common when lane indicators have break in them for side roads.

Hope these help you know what to look for. Remember the current TACC and AP/FSD features are currently in Beta testing and constantly getting updates.

yumei.leventhal | 25. Juli 2019

@techy james Of the scenarios you mentioned, only #4 (road bending) might be close to some of the situations we've encountered. For us, phantom breaking occurred repeatedly on a straight stretch on the highway with cars far ahead of us (or no car in sight). The worst was when we were changing lanes on cruise control (and AP) and the car dropped speed instead of maintaining speed, which is the safe way to change lanes on a highway. God knows what the people behind us must have thought of us -- even we knew it wasn't a good thing to change lanes, cut in and slow down.

philippem | 25. Juli 2019

While sometime it slowdown unexpectedly, often it only slowdown a few miles per hour so it is not that much dangerous. Also if someone is pushing you in the back, you can just press a bit the accelerator and wait until you move completely into the other line (and a safe distance from the car you "cut") before releasing the pedal.

In many other situation, just quickly press the accelerator a bit to tell the car you don't want to slow down. For example, that can happen when road is moved during construction and the GPS data is not update.

Effectively, the most annoying slowdown is when you want change lane as soon as possible because someone is pushing you. For a mysterious reason, the car tend to slow down if you "cut" too close in front of a car (almost like if the software think the car is in front of you instead of realizing that it is behind you)

bryan | 26. Juli 2019

Latest update seems to have helped lessen the ghost breaking .

CaliforniaMaki808 | 26. Juli 2019

I had the same problem. No vehicles were in front of me and it tried to slow down as if there was traffic. The guy behind probably thought I was brake checking him/her. My first words were, WTF, out loud.

MauiDave | 26. Juli 2019

I've had my Model 3 for a couple of months now and have noticed the same problem, however I have noticed a pattern that makes sense. It happens in a slight curve (or a heavier curve, for that matter) when there's a car in the next lane slightly ahead of me. I think the M3 sees part of that car in the next lane, since it's sort of still pointing at it and thinks its in front of it in it's lane.

Ngm98 | 27. Juli 2019

I have autopilot and it has done that and what I have noticed is that the speed will reset to the speed limit posted in that zone. Ex: AP/TACC ser to 70 and suddenly based on the navigation map the new speed is 65. It will set down to posted speed limit.

wayne.ranchuk | 27. Juli 2019

I did not have any AP issues until the last update 2 days ago. now he same road I have driven on several times before with no issues but now it is happening a lot. no cars arround me. the other issue I found yesterday was it tried to make a lane change with a car right beside me. I hope they get it fixed soon

FISHEV | 27. Juli 2019

Crazy thing is Tesla lists the Adaptive Cruise as Beta product in the manual. It certainly acts like. Most dangerous I've found is on interstate with tight curves it will perceive cars in their lane as in your lane and brake hard, very dangerous as on one, including you is expecting it. Well I do now and keep foot on accelerator.

All of which takes the utility and safety away from adaptive cruise.

As you see here, it's a common complaint but there is no indication Tesla views it that way or plans to try and fix it. Focused on "Full Self Driving" which is probably the cause of the problem in the first place. The FSD requires an overly cautious adaptive cruise and that's what we have.

rwa | 27. Juli 2019

Wait a minute. There's two different scenarios here and you haven't really told us which one it is.

When speed drops, does it drop and *stay* dropped? Does the max speed value drop?

Or does the car "stab" at the brakes momentarily, and then you return to the previous speed?

FISHEV | 27. Juli 2019

"Or does the car "stab" at the brakes momentarily, and then you return to the previous speed?"@rwa

I think what happens is Tesla takes foot off accelerator and aggressive regen kicks. They are not coordinated and that's what causes the problem passing left with clear lane where it drops 2-3 miles an hour and then picks up. With the problem seeing cars in their lane as in your lane, that feels like regen and brakes. Likely the most dangerous case of adaptive cruise not being a good implementation.

brodemy | 28. Juli 2019

I have experienced the sharp breaking when in autopilot while crossing the SF Bay Bridge at a specific location, regardless of the traffic conditions. As others have pointed out, its sensors/camera see something that consistently causes this reaction. If anybody out there drives the bay bridge to SF regularly, get into the far left lane while going through the Treasure Island tunnel (westbound), and see if it happens to you.

yumei.leventhal | 06. September 2019

@FISHEV Your theory about "aggressive regen" not coordinated with adaptive cruise control seems to make most sense. @rwa: Yes, the car picks up speed shortly after, but that one or two second experience feels harrowing when one is in mid-traffic (scary even when there is no other car around).

I'm back on this thread because of what seems to be a close call yesterday with the cruise control. Will update you all once we get the car checked.

M3 | 07. September 2019

If the road classification changes (not road or speed limit) it can affect the max speed over limit (drop to +5 or increase substantially). I’ve seen both sides of this.
It’s extra special when the speed limit drops a lot (going into a town) and then the road classification moves to highway (while still in the town).
On AP with a 50mph target in a 45 then... Yikes auto accel towards a 75mph target in a 45mph zone!

FISHEV | 07. September 2019

“Your theory about "aggressive regen" not coordinated with adaptive cruise control seems to make most sense.“yumei.leventhal

What doesn’t make sense is why Tesla’s adaptive cruise would take its “foot” off the accelerator when passing. Car should at least maintain steady speed or acceleration if it had been slowing for traffic. I think the basic safeties (adaptive cruise, blind side indicators, lane keeping) have been sacrificed to the Full Self Driving deity. The FSD is given priority and it acts too cautiously to the point of being dangerous (the phantom braking, the inconsistent lane keeping).

Shwood | 08. September 2019

I've noted the same problem. I DON'T think it is an issue with the adaptive portion of the cruise control (i.e. the car sensing an object and slowing down), as mine also abruptly will drop the maximum speed on the cruise control setting. This generally seems to happen randomly, but I've also noted one location on the interstate where it happens reliably every time, regardless of traffic, time, etc. Since the cruise control relies in part on sensing speed limit signs and GPS data, I suspect the drop in maximum speed is somehow programmed in at that GPS location ... even thought there is no speed limit sign or change in speed limit. I'd hoped the Tesla database would "learn" this after making manual corrections at the same location on numerous occasions...

yumei.leventhal | 16. Oktober 2019

Update: I posted this on a different thread, but I thought this may also help people who encounter speed drops in cruise control. Our car went to a Tesla service center for the second time. Both times they fixed some "discrepancies in the wiring mechanisms" in the "B-Pillar camera" assembly, but apparently the first time the repair wasn't done right. Driving back (60+ miles) after the second repair, we didn't encounter the weird sudden speed drop on cruise control at all.

An unexpected outcome was that we now can see cars moving into the blindspots! We got a loaner car (a Model S) while our Model 3 was being repaired. I noticed that the Model S's console shows cars in the blindspot in adjacent lanes whereas my Model 3 didn't. Some people seem to think we weren't swiping up/down correctly to see the whole image. Well, after we picked up the car from the service center, the blindspots are no longer blind--as soon as a car approached close enough from either sides, it shows up on our screen (despite the rain today). No swipes needed. The images of cars nearby no longer do the ghostly dance. So the issues was with the hardware as we suspected. Hopefully other cars are perfect and require no such fix. But if yours happened to be less than perfect, you may want to ask Tesla to check it out.

FISHEV | 16. Oktober 2019

Mine goes in for service for similar issues on 29th. But they did check them out previously, twice, road crew one time and in the shop the second time, and stated that everything was "within normal limits".

Be interesting to see what happens. I was always able to see people in the blind spot, just not approaching the blind spot or nosing into it as good BSI's can detect. So doubt there is any malfunction in my case. Directly behind the car is always going to be a bit of blind spot due to the just short range back up sensors.

lbowroom | 16. Oktober 2019

Rear Camera, not sensors. You can’t be this dumb

g.clarke | 17. Oktober 2019

I've had my model 3 for 3 weeks and have experienced issues with phantom braking in Traffic Aware Cruise Control mode. The most recent was on a country road with trees hanging overhead and shadows on the road. Also it was windy, so the shadows were moving around.

I'll raise the issue with Tesla when I take it in to fix some paintwork defects next week.

rhuete | 27. Dezember 2019

I experienced the "phantom braking" while driving on cruise control on the fast lane with my wife yesterday and happened with no car ahead of us at all !! My wife immediately reacted looking to our back worried that a car could had been behind us and hit us ... good thing that there was none.
It repeated it again after a few miles so I decided not to use the cruise control again.
Very worried about this and I hope Tesla pay attention to this condition.
Couldn't come out with an answer to this set up but I think maybe related with the GPS ????

howard | 27. Dezember 2019

It repeated it again after a few miles so I decided not to use the cruise control again.
Very worried about this and I hope Tesla pay attention to this condition.

@ rhuete, I have owned mine over a year now and I still don’t use TACC either except briefly with each software release. It is getting a bit better but still not safe to me. Sadly it is not a priority to Tesla. Even sadder is the fact that I am in AZ for a couple of weeks driving my other EV which has flawless adaptive cruise.