I found something
interesting on PSD yesterday. I bought new set of tyres the other day, but I
chose larger rear (rolling diameter).
I had 225/40/18 (627mm rolling diameter for front) and 285/35/18 (656mm rolling for rear) which is within 3% between front and rear.
Now, I have the same size for the front but 285/40/18 (instead of 35 aspect ratio) which is 685mm. 7% larger than the front.
I thought it should be no problem until I hit freeway above 110km/h when Warning Light comes on “Clutch-Control OFF”. I thought the PSD control unit uses the differential speed between rear wheel, but it appears to use the front wheel speed as well.
At lower speed (up to 100km/h), the speed differential between front and rear is probably not large enough to switch of the PSD.
Have you ever come across with this?
PSD: this is remarkable. No, this is new and kind of unexpected. Yes, the PSD/ABS ecu works with the ABS sensor signals, and obviously the ABS should respond to any sudden variation in rotation of any wheel announcing a loose of grip. But why the PSD would monitor the front wheels and activate the slip diff to compensate..… makes me wonder. And this happens repeatedly?
1992 Porsche 928 GTS Midnight blue
Regarding the PSD warning due to different tyre sizes front and back, it happens at around 110km/h and above when front and rear would see about 7 or 8km/h difference.
Below this speed, it tolerates and no warning sign.
According to forum, one mechanic confirms the story. When he put PSD equipped 928 S$ on dyno, warning sign comes on after a few seconds the rear wheels start to move. Also, some guys noticed the same after tyre burn out more than 8 seconds. So, there you go. PSD see front and rear speeds.
I don’t know what they did with the PSD, but it is funny that a speed difference front to rear would trigger the PSD. Maybe someone should educate me on this. But apparently it does.