A strange phenomena....
The speed dial in the instrument cluster shows 120 km/h when the ignition is turned on (engine not even running).
You might think this is caused by a defective instrument, a sensor, maybe the FCU relay ...? Think again. It was caused by the PSD ecu that has a malfunction. The speed signal is used by the ABS/PSD ecu and processed internally. It also sends the signal to the RDK so that it can relate speed and pressure loss.
Replacing the ABS/PSD located on the left side wall in the driver-footwell area solved the problem.
1992 Porsche 928 GTS midnight blue