I had a minor fender bender because my ABS "kicked in" on dry warm surface, sending a car flying forward like it was on ice. I had previous suspicions that when I brake hard the ABS kick in too early and the braking distance seemed too long. Two different Porsche specialists can't find the cause. Has anyone encountered such problem and if so - how to fix it?

 

Have you bleed the brakes lately or at all? Air can cause weird problems with ABS. Are you sure the ABS kicked in? Did you hear the buzz?

 

Well something isn't right, that is for sure. I can't understand how a working ABS working could cause the car to "fly forward". ABS simply modulates the brakes to prevent lock-up, wet or dry.

How long have you had the car? Perhaps this is one issue you should take to a mechanic to have the ABS system checked out with a hammer?

 

I have a similar problem with my brakes when the ABS kicks in. Normally, when the ABS begins to detect too much slip it should release pressure on the brakes and then quickly re-apply pressure and repeat this cycle rapidly. With my brakes the initial release is too long (perhaps .5 to 1 Secs) When the brakes release it feels like you are being launched forward. I am aware of the problem and try to avoid engaging the ABS.

It seems this problem is not uncommon - try a search on accelerometer and ABS. The following post indicated the problem may be related to the longitudinal sensor: https://rennlist.com/forums/964-forum/141266-anti-lock-brake-failure.html

Greg

 

It sounds like exactly the problem I had with my accelerometer (mentioned in the thread Greg linked to). I had the problem for 18 months and my mechanic tried many things (even a new ABS unit) before I took it somewhere else and they fixed it within one day by cleaning the accelerometer.

 

Thanks everyone for the replies!
Dave: You seem to have the same car. I called the Porsche dealer today and asked them about the accelerometer. They are saying its actually called the "Body Acceleration Sensor" and that it is located inside the center column inside the car. Could you advise if that is what you had done to the car. Thanx.

 

Have a look here.

Marc

 

Just to interject with a quick word of caution. Bear in mind that the C4 has TWO accelerometers fitted in the central tunnel. The longitudinal unit (pointing front to back) is common with the C2 and is used for the ABS system. The one behind it mounted sideways is used for the PDAS system. If you carefully lever off the lid you can GENTLY blow out any foreign matter but take care not to be too aggressive since damaging them will get you a very big bill!!!

You can get to them, BTW, by removing two screws for the inspection panel just to the right and forward of the gear lever.

Regards

Dave

 

The C2 does not use accelerometers, lateral or longitudinal. The flasher relay is located in nearly the same location (21 in the photo below) so that might be why owners are confusing the two?

 

Jason,

I apologise for confusing everybody. I was mis-interpreting something in Adrian's book which mentions that the longitudinal accelerometer is fitted "only in the C4" (on page 221) which I implied to mean that the other one was also fitted in a C2. I actually got them round the wrong way anyway.

I'll make a mental note to check-in brain before I open mouth in future.

Regards

Dave

 

Dave,

I came away thinking the exact same thing as you after reading that page.

 

The 964 Carrera 2 Tiptronic IS fitted with a lateral acceleration sensor.
Nothing to do with ABS but located in the same place as the one in C4. Only the C2, 5-speed manual is not fitted with these sensors.
Ciao,
Adrian.

 

Just curious, what does the Tiptronic use the sensor for? Is that how it "determines" that a sport shifting mode is needed? Interesting...

 

Thank you Adrian for setting our (my) mind at rest!!

Regards

Dave

 

Perhaps it is so it does not shift during cornering and upset the balance of the car. Adrian?

Marc