Complete brake failure !!

[unbiased]

Look at the power meter in the middle tube of your dashboard. The green bar shows how much regen is used for braking.

That part I know, but how do I tell when the manual braking kicks in.

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[f1eng]

That part I know, but how do I tell when the manual braking kicks in.

At high speeds above when the power meter green bar maxes out, but you are unlikely to be looking, below that all braking is regenerative. At low speed when it goes to zero whilst still braking.

 

[WasserGKuehlt]

That part I know, but how do I tell when the manual braking kicks in.

You’re not supposed to.. (unless, you know, the brake line splits. Too soon?)

 

[bluesky]

While regen braking (not overrun recuperation) may have some stopping power, it probably cannot be triggered once all the fluid is gone. Regardless of low speed or not. It’s not that the level of regen braking is insufficient. The problem is that once the fluid is gone so is Taycan’s ability to sense the drivers braking request for blended or regen braking. Mechanical braking is disabled by lack of fluid, but unfortunately so is regen braking.

 

[Vercingetorix]

No way to run out of fluid unless both front and rear brake circuits are compromised. It cannot happen.

 

[f1eng]

While regen braking (not overrun recuperation) may have some stopping power, it probably cannot be triggered once all the fluid is gone. Regardless of low speed or not. It’s not that the level of regen braking is insufficient. The problem is that once the fluid is gone so is Taycan’s ability to sense the drivers braking request for blended or regen braking. Mechanical braking is disabled by lack of fluid, but unfortunately so is regen braking.

What makes you think that?

 

[Clive]

No way to run out of fluid unless both front and rear brake circuits are compromised. It cannot happen.

oh yes it can ……… and it did happen

i agree it shouldn’t however

brake circuits aren’t normally plumed front/ rear on road cars. they are usually split corner to corner but it’s definitely possible to loose all the fluid as that’s exactly what my car has done

 

[Murph7355]

While regen braking (not overrun recuperation) may have some stopping power, it probably cannot be triggered once all the fluid is gone. Regardless of low speed or not. It’s not that the level of regen braking is insufficient. The problem is that once the fluid is gone so is Taycan’s ability to sense the drivers braking request for blended or regen braking. Mechanical braking is disabled by lack of fluid, but unfortunately so is regen braking.

It's brake by wire. It could be set up to do whatever Porsche wants ?

 

[Scandinavian]

No way to run out of fluid unless both front and rear brake circuits are compromised. It cannot happen.

It is possible to drain the brake fluid by pumping the brake pedal several times. In old Porsche documentation (Macan ) it states that the pedal will go down very far but then mechanically engage the second circuit. However they warn that if you pump the brake pedal you might loose ll fluid?

And I guess pumping the brake pedal a few times would be a natural reaction. Question is how many times you can do it without loosing all?
Here is some text from Macan manual.

Since the Taycan has brake by wire, the strong recuperation by the motors should still work, but not a low speed of course.

Something to ask the Porsche Guru or Service Manager at Porsche Centre

 

[gnop1950]

What makes you think that?

I had the same question. I mean regen works (engine braking when you lift your foot off of the go pedal), when turned on, even if you don't touch the brakes. Hydraulic braking isn't involved when you first push the brake pedal, so far as I can tell, so why wouldn't Regen braking work? Plus the emergency brake, Park, would still work.

Regen braking by itself wouldn't bring you to a complete stop, but once you are below the speed where mechanical braking would be applied the Park button should do the rest. I'm glad I learned about that btw, it hadn't really occurred to me before this thread.

 

[DougFrisk]

Can I offer a theory as to what happened here? Having gone through the Audi J1 self study guide I think I know what happened.

They Taycan has a mostly brake by wire system. When you depress the brake on a "normal" car, a mechanical linkage compresses the fluid in the brake master cylinder, ignoring any power boost for the moment. In the Taycan, when you depress the brake pedal, you are compressing fluid in the brake pedal itself in normal normal cases there is no connection to the actual master cylinder. The braking ECU then interprets the pressure you are creating and determines how much regen and physical braking to apply based on the amount of pressure you are creating.

There is a backup in the event that the ECU fails, a direct hydraulic connection from the brake pedal to the master cylinder controlling the mechanical brakes allowing a minimal amount of braking, but only when the brake pedal is near the floor. It has to reach a threshold pressure in the pedal assembly to open the valve enabling this connection.

This is the tube that likely split. The ECU would see no pressure and you've lost the physical connection resulting in a total brake failure. It also explains the brake fluid on the floor inside the vehicle. (I've had a brake line split and cause this kind of total loss of braking and was only able to avoid rear ending a stopped bus by driving into a snow filled ditch.)

I'm not particularly worried about this happening to our Taycan though. There are billions of brake tubes and hoses in the world and a one in a million manufacturing defect is just that. Until another Taycan or Audi GT suffers this same issue I'll consider this a freak occurrence. If it happens a couple of more times, well, then I'll want a recall.

The practical takeaway as posted upthread is to know that the park button will initiate an emergency stop if you press and hold it.

 

[daveo4EV]

I had the same question. I mean regen works (engine braking when you lift your foot off of the go pedal), when turned on, even if you don't touch the brakes. Hydraulic braking isn't involved when you first push the brake pedal, so far as I can tell, so why wouldn't Regen braking work? Plus the emergency brake, Park, would still work.

Regen braking by itself wouldn't bring you to a complete stop, but once you are below the speed where mechanical braking would be applied the Park button should do the rest. I'm glad I learned about that btw, it hadn't really occurred to me before this thread.

Taycan does not have lift off regen - it's all blended with the brake pedal - only Porsche engineers could tell us if regen would be invoked with a "dead brake pedal"…

the amount of regen from lifting off the accelerator is trivial and no where near the maximum amount of regen available from the brake pedal.

 

[gnop1950]

Taycan does not have lift off regen - it's all blended with the brake pedal - only Porsche engineers could tell us if regen would be invoked with a "dead brake pedal"…

the amount of regen from lifting off the accelerator is trivial and no where near the maximum amount of regen available from the brake pedal.

You are contradicting yourself. The Taycan does have lift off regen if you choose to use it, as you said above, albeit a small amount. I find when I'm going slow it is often enough to stop the car.

I'm not sure why the brake pedal wouldn't work as normal until the point where the mechanical/hydraulic brakes kick in. If I were to guess I'd think that regen worked normally until the point where the hydraulics should have kicked in, but no way to know for sure without testing.

Now, if some brave soul wanted to test it they could drain their brake fluid and try it out Personally I'll wait and see if someone can find a reference or Porsche chimes in with an answer. In the mean time I'll keep the parking brake in mind if something should happen.

This is the first I've heard of this type of "total" failure so I'm thinking it is an outlier. I'm glad that no one was hurt.

 

[daveo4EV]

You are contradicting yourself. The Taycan does have lift off regen if you choose to use it, as you said above, albeit a small amount. I find when I'm going slow it is often enough to stop the car.

I'm not sure why the brake pedal wouldn't work as normal until the point where the mechanical/hydraulic brakes kick in. If I were to guess I'd think that regen worked normally until the point where the hydraulics should have kicked in, but no way to know for sure without testing.

Now, if some brave soul wanted to test it they could drain their brake fluid and try it out Personally I'll wait and see if someone can find a reference or Porsche chimes in with an answer. In the mean time I'll keep the parking brake in mind if something should happen.

This is the first I've heard of this type of "total" failure so I'm thinking it is an outlier. I'm glad that no one was hurt.

the blending of regen and friction is software controlled - there is no fixed point of cross over between the two - it’s constantly variable based on conditions

one such example is there is no regen after overnight to clean the rotors

as i said only porsche engineering can say for sure if the regen blending is active with a dead brake pedal

 

[gnop1950]

the blending of regen and friction is software controlled - there is no fixed point of cross over between the two - it’s constantly variable based on conditions

one such example is there is no regen after overnight to clean the rotors

as i said only porsche engineering can say for sure if the regen blending is active with a dead brake pedal

I agree, for the most part and I would like to see Porsche answer, or find an answer to what happens to the brakes/pedal when the fluid is gone. One would hope that the software would just use regen braking until the car is slowed enough for using the Park emergency brake. Unless you can feather, push/release/push, the Parking brake at speed.

However, the minimal regen that simulates engine braking seems to always engage (if turned on). At least my power meter always shows regen occurring, even first thing in the morning, when I lift my foot off the go pedal.

But good discussion and I've learned at least one new thing that will be useful should a similar emergency happen to me in the future.

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