I believe the ACC and alternator can be "disconnected" temporarily during heavy accelerations to maximize performance. So yes, under light accelerations that might be the case. I can't imagine that any S80 should have trouble getting past 170 km/h though, doesnt sound right. What if you do a kickdown at, say, 150 km/h or lower? Does it stop at 170 km/h all of a sudden or what exactly happens?