The 3.5 Coupe used vacuum-operated seat-back locks that release the seat when the engine is off; these frequently fail or get disconnected, and originals were often swapped for simpler 250SE manual-adjust seats. Owners describe practical fixes ranging from repairing a plugged vacuum line to substituting electric solenoids.
Vacuum seat locks on the 3.5 Coupe
Mercedes-Benz W111 Coupe / Cabriolet · mechanical