I've been working on mine in the 84sd. The new ignition assembly did not have the locking plastic key for the electrical connection so I didn't have to worry about it. But in looking at the old locking system I thought you could get the electrical connection to lock simply by turning the mechanism with a screwdriver. But to install the tumbler, it seems the mechanism has to be in the unlocked position since the tumbler must be reinstalled with the paper clip and tumbler in #1 posiition.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13