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M3P brakes have stopped working

odyssey96

Member
Jan 26, 2020
53
23
Uk
Driving along, warning that vehicle hold unavailable, found brakes werent working. Pulled over and thankfully parking brake works fine. From there even pressing brake pedal couldn't change "gear" saying I need to hold the brake pedal. F'in scary as I had my baby in the back. Thank f regen also worked by the lights when the problem occurred. Tow truck on way...
 

Jez_GB

Supporting Member
Dec 28, 2019
664
595
Nottinghamshire, UK
Driving along, warning that vehicle hold unavailable, found brakes werent working. Pulled over and thankfully parking brake works fine. From there even pressing brake pedal couldn't change "gear" saying I need to hold the brake pedal. F'in scary as I had my baby in the back. Thank f regen also worked by the lights when the problem occurred. Tow truck on way...


Ooooophh! Hope you get recovered/sorted ok
 

Jason71

Active Member
May 8, 2019
2,682
2,457
Shropshire
Holy cow! Sound like “brake by wire” was faulty?

Should it happen to anyone else, pressing the parking brake switch will gradually apply the parking brake and can be used for emergency braking.
Really. I didn't know that. Does that work at any speed not just the 5mph where you can officially use it?
 

odyssey96

Member
Jan 26, 2020
53
23
Uk
Will update as soon as root cause identified currently at Tesla service center. Funnily enough I was checking franticallyfor software updates and as soon as it hits the service centre low and behold on the app it says it's downloading 2020.8.3. I was sure not to start the upgrade in case they were in the middle of some diagnostics lol. In my head I was playing pranks by opening and closing the charge cover :D
 

ilyak

Member
Mar 16, 2019
605
715
Walpole, NH
Holy cow! Sound like “brake by wire” was faulty?

Model 3 has a direct mechanical link between the brake pedal and the tandem master cylinder via Bosch iBooster. This is discussed in the leaked theory of operation document:

If the vehicle power net fails and the Hydraulic Boost failure Compensation (HBC) is also not available, the iBooster is still able to provide a deceleration through a mechanical push through which provides a direct connection between the pedal and the master cylinder.
 

ilyak

Member
Mar 16, 2019
605
715
Walpole, NH
That makes a lot more sense.
I wonder how a total brake failure came about...
ABS and ESP can reduce the braking force but not disable the brakes completely, so the only thing I can think of is a total loss of pressure in the hydraulic system. But, of course, that's not something the car can recover from by itself. Last time a report of brakes "just not working" popped up on the forum, I think the consensus was that the poster first put the car in neutral (instead of disabling autopilot) and then pressed the accelerator pedal instead of the brakes. That's one hell of a coincidence but not impossible. Curious what Tesla tells OP.
 

odyssey96

Member
Jan 26, 2020
53
23
Uk
ABS and ESP can reduce the braking force but not disable the brakes completely, so the only thing I can think of is a total loss of pressure in the hydraulic system. But, of course, that's not something the car can recover from by itself. Last time a report of brakes "just not working" popped up on the forum, I think the consensus was that the poster first put the car in neutral (instead of disabling autopilot) and then pressed the accelerator pedal instead of the brakes. That's one hell of a coincidence but not impossible. Curious what Tesla tells OP.

The brake pedal was locked in its non-pressed position. Like I think if you tried to brake a ICE car without the engine turned on, you'd have to slam your foot down. But in this case brakes didnt seem to work. Only by regen braking and driving real slowly came to a stop.

Here's the immediate previous history:

All working for 3 or so days. Car wash. Drive to CCS charger down road and drive back successfully.
This day - unlock car via mobile app and get in car and enter pin to drive. While in park, tell wife to connect her phone via BT (thought we could connect both phones simueltaneously using BT, didn't realise it disconnected mine). Brake locked in 'upper' position, when pressing it hard and trying to put car into drive, kept saying Press brake to change gear or whatever. After several times of trying reverse/drive, finally goes into drive. Thought it was a bug. Went forward slightly to then perform a reverse (I live at the end of a closed road, car facing towards end). Found I couldnt brake, 'Vehicle hold function not working' on screen. Only because of (very slow at that time) regen stopped. Crapped pants, came close to curb. Got wife/toddler out of car, locked it by walking away. Disconnected wifes phone, unlocked via app and got back in, all fine. Thought this is a serious serious bug and ill report.

Then got down the road and the 'Vehicle hold not working, use brakes when stopped' appeared again. Brake pedal locked in the upper position again. Hazards on, accelerate very slowly and try brake, cant press the pedal down. Turn into side road, come to stop, parking brake, roadside assistance.

When I posted at first, was on mobile and i was shaking slightly from how scary i found the situation given my baby in the back.

upload_2020-3-31_22-34-22.png


Here is the error above. I didnt mention the initial mobile issue as I can't see how it was a bug if the problem then occurred the second time around, just a freak coincidence imo.[/QUOTE]
 

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ilyak

Member
Mar 16, 2019
605
715
Walpole, NH
Got car back and all seems good. In case anyone wanted to know:

View attachment 528601

That's kind of a "screw off" repair. The errors above are consistent with the booster motor failing to move the pedal, but the root cause - the pedal being stuck hard in unpressed position - is rather odd. I wonder if it was something mechanical and transitory - like a small pebble or a poorly tailored floor mat getting stuck in the way of the pedal arm. If ibooster failed and no electromechanical boost was available, the car should have engaged a backup system that uses the pump that normally applies the brakes for traction control to hydraulically boost the pressure you were putting on the pedal. Again, from the theory of operation doc:

The iBooster is part of a complex system of brake safety monitoring designed to provide safe operation during various failure scenarios. If the iBooster suffers an irrecoverable fault, such as disruption of supply-power, the driver can still brake unhindered; the driver's muscular energy is transmitted through the booster to the tandem master cylinder. In addition, the ESP system detects booster failure, and generates additional brake force to aid the driver by using the hydraulic pump (normally used for stability and traction control events). The brake force is metered proportional to the measured upstream hydraulic pressure from the tandem master cylinder, to allow reasonable modulation of brake force by the driver. During this time, pumping noise and pedal pulsations will be felt through the brake pedal by the driver. This is the intended safe back-up operating mode, and allows a much greater vehicle deceleration to be achieved than by the driver’s muscular energy alone. This function is called Hydraulic Brake Boost (HBB).
During service, if any of the iBooster electrical plugs are disconnected, or the iBooster is not functional for any other reason and the brake pedal is depressed, the ESP unit will run in hydraulic brake boost-mode to compensate braking force. The hydraulic brake boost function will cease once healthy iBooster operation is re-established.

Even if this secondary hydraulic booster system failed too, you would still be able to brake. The pedal would be very stiff but still possible to press down and the force would still be transferred to the TMC by a push-through rod in the ibooster (that's the bit I quoted in my previous comment). This makes me think this wasn't an electronic issue but rather a mechanical one, with something physically preventing pedal travel. Either way, this must have been very scary, glad you were able to slow the car down and that everyone is OK.
 
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Adopado

Active Member
Aug 19, 2019
3,585
2,705
Scotland
Even if this secondary hydraulic booster system failed too, you would still be able to brake. The pedal would be very stiff but still possible to press down and the force would still be transferred to the TMC by a push-through rod in the ibooster (that's the bit I quoted in my previous comment). This makes me think this wasn't an electronic issue but rather a mechanical one, with something physically preventing pedal travel. Either way, this must have been very scary, glad you were able to slow the car down and that everyone is OK.

Yes ... there has to be an entirely mechanical emergency facility to brake! I agree that there has to have been something mechanical involved in this failure.
 
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