There is enough difference that I don't find myself using any parts from a W123 except for OM617 related parts on my 300SD. The cruise control seems to be the same. The computer is different, though. Same actuator and same switch. The wiring seems to be the same, too. Perhaps there is a sensor on the W123 as well as the mechanical speedo. If the differential ratio is different, the computer will be different. I believe the computers are calibrated for a certain gear ratio and sensor input. I have yet to meet an actuator unit that did not work with a W126 as well as a W123.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge