Brake failure need advise
 

I have a 2005 cayenne s bought new, now with 93K miles. The brakes failed in a parking lot. While going slow, about 7 mph, i applied the brakes to park and the PSM lights lit up and there were no brakes. Pumped it and still nothing. Had to aim for a pole and hit it instead of pedestrians and other cars.

Went to my porsche dealer and he found a yaw sensor malfunction, which he says also controls the ABS. But he would not say the problem will be fixed. Just that he will be happy to work on it the next time it happens.

I understand that parts do fail but why did the redundant system not work? Can the yaw sensor failure be the cause?

Any advice? Any help is greatly appreciated.

I just don't want to have a 5000 lb ram.

You've got to be kidding me!
I would be parking that thing in front of the dealership and demanding them get Porsche to explain what caused this and why before I took it back! I wouldn't take it out of park except to load it on the flatbed.

The NTHSA (?) has a website to document things like this.

I don't get this either. Unless they changed something, the only way the ABS fails in in a safe mode IE the brakes will still work. All abs systems are "in line" with the conventional brake system. Press the pedal it presses the master cylinder which forces fluid to press pads against the rotor. The ABS interrupts this line with valves that let a pump take over. If that pump is not working (pressurized) then the standard system moves the valves back. There is an assist system that will make it VERY hard to press the pedal if it fails. But not impossible.

It would have to be a hell of a failure to bypass all the safeties inherently designed into the system.

To be fair, I am not intimately knowledgeable about the Cayenne system, but I know a lot about ABS systems in general.

Yaw sensor for ESP/PSM shouldn't affect the brake fluid/ABS at all. If the sensor is faulty, ABS will be disabled but you should still have mechanical control of the brake pistons through your pedal.

I had this happen to be once in an old SLK. ESP/ABS was disabled but I still had brakes.

At this time I am speaking with Porsche to see what they say. Was just speaking with them when they had to drop me because of a fire drill. They said they will call back.

Thanks for all your responses. I don't get it either. The explanation from the service manager is just not coherent. I know he does not know what yaw is or what the sensor really does. But he did say repeated that it controls abs which activates and releases the brakes on cars. Could it be a fault that suddenly releases all 4 brakes? Something that occurs under special circumstances. I cannot say driving 7 mph in a flat parking lot is anything special though.

I am not sure what to really do about this. The car seems unsafe to me.

When i saw him yesterday, all he would say repeatedly to me when i questioned him was, "are you picking up your car today and how are you paying for it"

Total BS. The yaw sensor has nothing to do with this, or little to do with this at least. Know plenty of track guys that intentionally disable the yaw sensor to prevent PSM from kicking in (to avoid cooking the rear brakes). The brakes still work afterwards, otherwise I don't think it would be a very popular mod. ;)

Regardless of yaw sensor input, the brakes will still work.

If there was a true absence of any stopping power, something else is up. Master cylinder failure perhaps? Air in system would prevent hydraulic force generated by the pedal from reaching the calipers, and the pedal would just freely go to the floor.

BE455, thanks for yor input. I am at a total loss as to what the cause can be. Your help and advise is greatly appreciated.

Mechanically, the brakes seem good. It stops after the accident. And i have the car back now and it also works well. Porsche also looked at them and said mechanically they are good.

Without a ruptured brake line, I can't see how you would get a complete failure of the brake system. I'd like to know what the final outcome is, because as others have already stated neither a failure of the abs system nor the yaw sensor should ever be able to cause a failure of the braking system.

Out of curiosity, were you able to use your emergency brake? In most vehicles that is a simple cable actuated brake. Separate from the hydraulic brake system.

There are many real verified incidents of unintended accelaration that have been proven. Not sure what that is caused by. Mine seemed to be different but somewhat similar. Just want to get to the bottom of it but it is unlikely i will get much more info. Dealer has shut me off and foes not what to talk about it. Older car that i did not buy from them means they do not really want to deal with it.

I had about 4-5 second total. I was already starting to pull into the lot. People on the right and car on the left. So my first priority was to aim for the middle pole to miss what i could.Then pumped brakes, and then it hit. All seemed kind of fast even on playback.

Are you talking "unintended acceleration" or "brake pedal went to floor but nothing happened"?
2 completely different issues, at least to me.

Brakes going to the floor and nothing happened. But not a mechanical issue.

What you describe is a brake failure, I'd just like to hear the final verdict on what caused it. What you've been told so far doesn't jive.


Unintended acceleration is a different problem. Usually caused by Toyota drivers that wedge their floor mats under the gas pedal, or confused audi drivers that hit the gas pedal instead of the brake.

Provided you have a functioning brake system, the brakes will always overpower the engine and bring you to a stop. That changes when a panicked driver pumps the brakes, eventually overheating them.

While it is possible that there could be a mechanical failure leading to a stuck throttle, or a true unintended acceleration event, nearly every case in recent years have been due to driver error.

Crazy...this is no joke.
Hope you get the root cause for such failure, seems to be air in the brake line. Maybe excess humidity caused the "air" inside the brake line after a long hot drive...gasifying the water particles creating such "cushion" in your brake attempt. Did you drove for hours before stopping in the parking lot?
This phenomena can happen at any speed once the brake fluid is hot and humid...

Brakes fluids were changed and flushed 6 months ago. I do it about once a year on all my dd cars. I do not believe the brake fluid boiled. I drove about 5 miles from sunrise at the beach to the shopping center intending to have breakfast. It was in light traffic before 7 am in the morning. There was resistence when i hit and pumped the brakes. And the brake paddle rose each time. Not sure why psm came on before i lost the brakes. Perhaps i can do another brake fluod change again.

I will continue to pursue this as long as i can. The dealer is making it really hard.

PS: just a little background, I am an advanced solo DE driver with about 100 track days on many cars including open wheels, Porsches, lotus, mini, bmw and dodge vipers. And i did most of the prep on the track cars myself so i do somewhat understand wrenching. But i a still very confused by this. So your help is very valuable to me. And thanks so much to all respondents.