Hmmm...must be slightly different than the W123. It is most likely a problem with the vacuum valve in the passenger door if it will not operate the central locking system anymore. I would open it up and check it out.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine