Ritz928

Cheers for your comment! The fact Im in the UK makes no difference because the laws here are just the same. Which makes common sense really, as the ABS is designed to reduce emergency stopping distances which will help to avoid accidents. Im quite sold on getting a GT (as Im hooked on 928's anyway) so fingers crossed the owner agrees to repair the vehicle.

Carl.

RyanPerrella

Carl,

I would assume the owner would be fine taking the car in to get a proper diagnosis. It wont cost much for the diagnosis and then you can go from there. I would try hard to find a shop that knows the 928's well though as typical porsche guys maybe unfamilliar with PSD as i believe it was only on the 928 and the 959.

Good luck

pasa

Carl
I once had an ABS fault which turned off the PSD, in the end it was just a corroded ABS fuse.
Check out the fuses first; this could be a simple fix. If the ABS/PSD fault appears a few seconds after the car is started the fault could be a power supply problem (i.e. fuse). If the problem appears once you start driving it is most likely an ABS sensor fault.( the ABS sensors are tested by the system once the car is above aprox. 7 KM/H). If you have any ABS fault the PSD will be shut down as well.
Dan 1990 928GT

Schocki

Hi Carl,
I'm in the middle of my move back to Germany and can't answer questions frequently, excuse my late reply.

All 3 system ABS, PSD and RDK rely on the ABS sensors. BTW the front wheel ABS sensor supplies the speedo signal too

Let me explain how it works: All four sensors deliver speed signals to the ABS ECU, RDK ECU and PSD ECU.
The ABS ECU compares all 4 wheel speeds simultaneously and is in stby mode if the speed of all wheel is the same, +/- a small tolerance of about 3%. The 3% are necessary to compensate for tire wear. If you're using the brake and one of the wheels starts to lock up the ABS unit becomes active and does it's work to make sure the car remains stable and steerable.
If one of you ABS sensors fails and the ABS has no idea how to deal with the situation, then the system is shut down.

The RDK uses the Sensors to compare wheel speed with the RDK sensor pulses. If you drive more than 5 km/h the system awaits RDK sensor inputs. It actually counts the pulses per wheel revolution. If one RDK sensor is open it will trigger a warning on the display. One sensor only is indicated by a steady arrow pointing at the wheel. If the arrow flashes both sensors are open.

The PSD uses 2 sensors too. The rear wheel ABS sensors and a gyro under your seat. If you accelerate hard and one wheel spins it will lock the limited slip up to 100% to make both wheels turn at the same speed. You can see this very easy if you accelerate on loose dirt. You will see one wheel spin for a couple of inches and than the PSD kicks in and it draws a nice 2 tire line behind the car
The gyro is used the counteract drifting while cornering. If you are on the Motorway and drive the car through a nice long corner and go a little faster the PSD light will come on.

What to do know:
Check the fuses as recommended.
Check the ABS relay.
The ABS ECU needs to read out.
Get in contact with John Speake he is very knowledgable about 928's.
Don't start without the proper fault analysis equipment, you're wasting your time.

Your problem sounds like a faulty ABS sensor from my point of view.
Driving the car more to cure the problem (excuse my language) is BS.

My wife's Honda had the same problem. Intermittent ABS failure while driving in the beginning. Followed by complete shut down before starting to drive.

Vilhuer

Excellent explanation Schocki. Only one small correction.

This was changed to rear wheel sensor in year or two after speedo pickup change from gearbox to ABS. I think reason was safety. When differential was used for speedo signal it was possible to say when real wheels where sliping by comparing speedo on speed car seems to move when looking through window. Sometimes in extreme winter weather its difficult to say if real wheels are sliping and speedo along with rpm is just about only indication of it. This was lost when front wheel sensor was used. Porsche probably realised it and changed speedo pickup to rear wheel sensor. Can't think of any other reason to change from one sensor to other. Without checking I can't remember which MY change was done but think it was '92 or '93 while ABS sensor use started in middle of '90 MY I think.

Ritz928

Thanks again guys!!!!!

Appreciate it!!!!

max35

"RDK is notorious for not working and in many cases wiring to control box is jumped to stop warning coming on at dash. Jump is also done to allow use of non original wheels which do not have RDK sensors installed."

Vilhauer, where does one put the jumpers on RDK systems? we are using non original mags as well.

John Veninger

Jump pins 2 and 13 on the harness connector to the RDK unit and leave unplugged