I have had a couple of episodes in this arena and here's what I learned. A few years ago my ignition got a little sticky. Finally reached a point that the key would not turn. Did the vibrator number and got it out and replaced the tumbler with a genuine MB part. All was well until a couple of months ago. One day my key would not turn at all. No slow death, just worked fine and then totally stuck. After much jiggling and wiggling I finally got the key unstuck and surmised that the steering lock must be the problem, since the tumbler was relatively new and supposedly good quality. I read in this forum (somewhere) about the trick of jamming the steering lock in the open position, rather than disassembling the dash / column. A machinist friend of mine fashioned a small keyway, inserted it into the switch in such a way that the steering lock is permanently jammed in the unlock position and I've had trouble free starting since that day. Hope this helps.

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John Schroader
bio burnin' 83 300D, '83 300 SD, '79 240D
"I've never met a man who was good at making excuses who was good at anything else" Ben Franklin
"You cannot permanently help a man by doing for him what he could and should do for himself" Abraham Lincoln