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Brake stuck on during driving (!) after service

gp100dl

Member
Aug 31, 2016
46
11
UK
We just drove our 2016 Model S back from the service centre after getting the MCU 2, FSD computer and CCS charger upgrades. Most of the (~1hr) journey was fine, but just near the end the car became sluggish and we realised that the brakes (or at least one of them) appeared to be stuck on. Fortunately we were only a few minutes from home and got it back at low speed without incident.

We got "emergency braking not available" and "both pedals are pressed" warnings. The front left brake seemed hotter and to smell a bit (but not a huge amount, really). No screeching sounds or scratches on the disc.

We'll be calling Tesla first thing in the morning of course (it's late now), and not driving it anywhere, but I wonder if anyone had any knowledge of similar issues? I guess it's most likely a mechanical issue rather than electronic, and most probably somehow caused during the service (it seems very unlikely to just be a coincidence that it happened on the way back from service). It's certainly pretty concerning!
 
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Bigtanuki

Member
Dec 19, 2016
151
145
Atascadero, California
make sure that Tesla verifies the brake discs are still true and not warped too. I had some disc warpage after an annual service. I suspect the wheels may have been tightened incorrectly. After some "discussion" the front discs were replaced under warranty. My car was at about 18k miles at the time.
 
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gp100dl

Member
Aug 31, 2016
46
11
UK
Thanks for the reply. A bit of a (long) update:

I called Tesla the next morning and was told that any brake issue couldn't be related to the MCU/FSD/CCS upgrades i'd had, and so it must be a new problem, and since the car is just out of warranty I would have to get it to Tesla and have it fixed at my own cost. Since it was at home, they wouldn't arrange a tow for me (even at my cost), which was very annoying. I spent a while trying to find someone to tow it to no avail (it turns out all towing companies in the UK seem to be very busy right now, I guess everyone's realised their cars don't work after lockdown!).

When I checked the car again, the brakes were also no longer stuck on. I ended up calling the AA out, and the technician inspected the brakes and discs and did some basic diagnostics. He confirmed that it looked like the brakes had been stuck on (discolouration on the front left disc), but he couldn't see any problem with the brakes any more and suspected a software problem. We tested the brakes briefly and they seemed fine, and drove round the block and did a few emergency stops, again no problems, all felt back to normal.

Since I couldn't get the car back to Tesla until after the weekend, and the brakes seemed to be working ok, the next day I tried a short journey nearby at low speed. Everything was fine for the first five minutes, and then lo and behold suddenly lots of alerts flashed up on the dash and the brakes suddenly failed! They had very little stopping power unless I really pushed hard on the pedal, fortunately I was doing about 5 mph at the time on a small road and could pull over. I called Tesla roadside assistance, they confirmed they could see the brake issues and asked me to check under the car for any liquid. Sure enough, there was a lot of liquid coming out (like someone was pouring a can of coke out slowly, in terms of quantity and rate), which I assumed was brake fluid. This time Tesla arranged for a tow and it was taken to the service centre.

I've just had an update from the service centre, and they told me that there was a faulty brake switch which was fitted as part of the MCU/FSD/CCS upgrades, so the upgrade was actually the cause of the issue! A faulty switch makes sense for the initial symptoms (brake stuck on, car reported both pedals pressed when they weren't), and could explain the subsequent brake failure, but it doesn't explain the liquid leaking from the car. Tesla say that the brake fluid is full, they can detect no leaks and that the liquid must have been AC run-off. To me that just seems unlikely, I know that water can drip from the AC, but I haven't ever seen water running from the bottom of the car like that. It also seems unlikely that it would condense so much water over the course of a very short drive in not particularly hot or humid conditions. It was hard to tell whether the liquid was water or something else like brake fluid or coolant, since it was pouring onto a dirty road surface.

I guess it's just worth noting that a seemingly unrelated upgrade of the MCU, FSD computer and charger requires changing a brake switch, and that single switch failing can cause brakes to fail in a fairly catastrophic way.

Does anyone have any insight into whether AC run-off is plausible for all the liquid that was leaking? I guess if the brake fluid reservoir really is still full and they can find no leaks, it can't really be brake fluid. I guess there are other coolants and liquids in the car though.

Tesla are doing another round of checks and I hope to pick the car up tomorrow. It sounds like they won't charge me since the problem was caused during the upgrade process.
 
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Silicon Desert

Active Member
Oct 1, 2018
3,482
3,330
Sparks Nevada / GF 1
Good summary. And there you go..... the SC sometimes says "this can't be caused by the things we did", yet I never believe that statement until I prove it out. Sure it could be a coincidental new issue, but in my lifetime I more often find that "cause and effect" happens more often than a "coincidence" :) Nice story. Glad you got the brake issue resolved. Not sure about the liquid issue.
 
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gp100dl

Member
Aug 31, 2016
46
11
UK
Yes it's not clear to me either how a faulty switch caused all these issues. It makes sense that there's some kind of switch/sensor to detect that the brake pedal is pressed, and it makes a lot of sense that that was broken since the car was not detecting it being pressed at various points and at other points thought it was pressed when it wasn't.

But I don't see how that would cause the brakes to lock on (although it being parking brake related is a good suggestion), or how it would cause the brakes to fail.

The more I think about it, I wonder if the servo assistance for the brakes failed (do brakes still work that way?!) When the failure happened I could get some braking effect, just not much, and the Model S is a very heavy car...
 

gp100dl

Member
Aug 31, 2016
46
11
UK
It does sound very similar to other brake booster failures, eg. Power braking assist reduced. I also got the juddering effect (a bit like Abs kicking in). There may have been a message about power assistance not being available, I'm not sure in the chaos!

I'll make sure the SC fully check the brake booster.
 

gp100dl

Member
Aug 31, 2016
46
11
UK
SC say that only the brake light switch is at fault, and that they've fully checked the ibooster and found no problems with it, or any leaks. I've asked them to explain how a break light switch could possibly be on the critical path for power assisted braking, since that just doesn't sound right, and what the exact part is that failed. I don't want to be fobbed off with a partial fix and then have it fail again!
 

gp100dl

Member
Aug 31, 2016
46
11
UK
Just as an update, we got a more satisfying response from Tesla in Friday: they are going to have their Master Tech look at the car and discuss with the original engineer on Monday, then give me a call to explain the issue. They also requested authorisation for another 20 miles of test driving to make sure there are no issues. Hopefully this will clear things up!
 

gp100dl

Member
Aug 31, 2016
46
11
UK
Quick final update: I spoke to the Master Tech and he explained a few things (hopefully I've got it roughly right):
  • The brake pedal switch was replaced during the MCU2 tuner upgrade (it's a single use part that needs to be replaced if removed)
  • The replacement was defective and they've now replaced it and tested the new one
  • The switch is actually involved in the decision to apply the brakes, it's not purely for the brake lights and detecting the pedal press for non-braking reasons (they call it a brake light switch but it's not really accurate, the brake lights are triggered from the pressure sensor). Since the car needs to apply the brakes itself, they can't use the pressure sensor alone.
  • Because it's involved in important decisions, the switch is actually two switches, acting as a failsafe.
  • Usually only one switch fails or they both fail together. Our case was very unusual in that the two switches failed in different states. Presumably the failures were also intermittent.
  • This is a situation that's not well handled in the control logic, and it causes the odd behaviour we saw, which might be different depending on whether the brake pedal is pressed or not. This is why we saw an issue where the brakes locked on in one case, and power assistance was disabled in another.
  • He's escalated the problem to engineering for review since it seems unusual and perhaps could be handled better.
I haven't fully thought through the logic of what the control system might do here, but it makes some sense to me that such a double failure would make it hard to distinguish the right course of action. I guess this is a pretty rare case. We have the car back now and I'm fairly happy with the explanation, in that it seems likely the switch is actually the cause of the problem, not just something unrelated leaving a deeper issue uninvestigated.
 
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