There should be a wire from the overboost sensor to the switchover valve on the firewall. At the moment you have no overboost cut-off of the boost signal to the ALDA. Either you knocked the wire loose or your sensor is stuck on (like the sensor was in my 81 SD) and someone left it off to allow a boost signal to get to the ALDA.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD