MBDOC, hello and thanks for the questions.

The passenger side front door lock operates normally. Only the drivers side has no resistance, turns with the key but does not unlock or lock the doors
The passenger front lock has the same minor resistance that I have historically experienced and same normal resistance as with the trunk lock. This minor resistance is approximately the same as currently felt with my W126 300SE.
The C280's passenger front door lock and trunk lock will currently open and close all the door locks. With engine off I can hear the passenger rear fender-located door lock mechanism (vac pump?) make mechanical noise.
Just the driver side door lock has no effect with the turning of the key.
The key turns without resistance with that door.

Thanks

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD