It isn't electrical fault.
1988. means it's B3. So u can get instrument cluster from Puglife and install from it speedometer clock. Only it, and not whole instrument cluster, because there is difference in tachometers. Fact that A90 have 260 kmph speedometer and A80 220 kmph speedometer [calculate miles for yourself, i'm lazy] doesn't mean anything at all, 260 kmph speedo will show the same speed when installed in place of 220 kmph speedo.
Fault is very simple - there is little plastic gear in odometer that gets damaged and stuck. I once repaired such speedo because I had other speedo which had died electrically but mechanically was ok, so I used gear from it. Otherwise just replace speedo clock or whole instrument cluster. And don't install whole instrument cluster from A90, otherwise u will ask "why my tachometer shows so high revolution count?"
