2002 Carrera (Facelift model) - ABS Problem help needed!

[msaif]

Hi everyone, I have a 100,000 km Carrera 2002 model. It's had new discs and pads fitted in the last 9 months (less than 8,000 km ago), new Pirelli Tyres all round about a year ago (less than 10,000 km ago). The service is bang up to date, including the replacement of the brake fluid about 8,000 km ago.

Problem is that whenever I brake the ABS activates, unless I brake so softly that is. It's not in an emergency braking scenario where you expect the ABS to activate. My wife's stock saloon has better brakes. It's heart beating stuff as I feel the car will not stop in enough time. The feeling is a pulsating judder in the brake paddle. The dealership read the fault codes and confirmed there were no ABS errors.

I've had previous Porsches and am familiar with the progressive feel, non over served brakes of the 996. This is different and I do not have confidence in an otherwise perfect car.

The climate here in Kuwait is dusty and hot, very hot. But even in Winter when there's so sandstorms the brakes are not right. Any help would be greatly appreciated.

Edited October 8, 2011 by msaif

[xmac]

A couple of things I can think of:

There is that there is a pressure sensor on the ABS manifold itself that may be incorrectly measuring pressure. The same Bosch module is used for some BMW's (and other cars) and that sensor has been known to fail. I have had failed sensors or modules that did not report errors to the OBD system so it would not surprise me that there is something that failed but is not being reported.

I would start with this pressure sensor and in parallel have the wheel speed sensors removed and cleaned. The wheel sensors pick up lots of ferrous material and can send the ABS module incorrect information. Here is a generic cleaning procedure for removing and cleaning a very dirty sensor but the main thing is to pull each one out from each wheel hub and removed the junk that is on them: http://autorepair.about.com/od/regularmaintenance/ss/abs_sens_cln.htm

[X911CarreraX]

I had a similar problem when I had two different pairs of tires on the car (front tires were the same brand and the rear tires were another brand). My ABS was kicking on early, but with some decent amount of pressure, and it all went away when I got new tires that match the front set. Not sure if this applies to you, but something not associated with the ABS sensor could be at fault too.

[wvicary]

I would first be sure all the wheel speed sensors are reading the same speed. Corrosion can effect a sensor or the trigger wheel and give a different wheel speed then the other wheels and early ABS activation.

[msaif]

Hi everyone and thanks. The tyres are all official Porsche rated tyres and match front to rear so that's not the cause. The car's going in for an oil change soon at the local official porsche garage so I'll ask them to change the pressure and wheel speed sensors all round. I'm hopeful this will cure the fault, if not then at least I'll have peace of mind that the car has had new parts fitted as I plan on keeping the car into old age. Thanks again everyone, and I'll post the results in a month or so.

[Loren]

Hi everyone and thanks. The tyres are all official Porsche rated tyres and match front to rear so that's not the cause. The car's going in for an oil change soon at the local official porsche garage so I'll ask them to change the pressure and wheel speed sensors all round. I'm hopeful this will cure the fault, if not then at least I'll have peace of mind that the car has had new parts fitted as I plan on keeping the car into old age. Thanks again everyone, and I'll post the results in a month or so.

There is no need to change out everything. A good Porsche shop will have a PST2 or PIWIS tester and they should be able to run tests to diagnose the problem to the exact cause. JMHO.

[xmac]

Loren, he mentioned that the dealership checked and found no fault codes. I agree not to replace parts either without more investigation. I noticed that I cannot find a part# for the pressure sensor. Does that mean you have to replace the whole hydraulic assembly ($1800-3000) if the pressure sensor dies?

Msaif, I read your post again and noticed you did not tell us when the problem started. Is it recently or after one of the services you mention were done to the car?

[Loren]

There are ABS and PSM tests in the PST2 and PIWIS testers -- that is what I suggest be run.

Just checking fault codes is not a complete diagnosis.

[wvicary]

When the wheel speed sensors fault in the above manner they will not set a fault code. The tech needs to read the wheel speed and inspect the sensor that is reading too high or too low compared to the other sensors. Throwing parts at the vehicle isn't the correct route to take.

[msaif]

Hi everyone, thanks again for your feedback and guidance. I had the wheel speed sensors removed and cleaned, and the official Porsche garage plugged in the testing device (not sure which one) and took the car for a test drive whilst they checked each corner. The ABS still kicks in. Perhaps it is because my last car was a 996 C4S and it had the turbo set-up, plus it was in the UK so no sand and bigger discs. Maybe it is normal for sandy climates?