Bizzare, but true:

It could be the ignition switch (the electrical part). On my 560, it was very intermittent but I found that I could briefly get it to work by jiggling the key. Replacing it is unpleasant but the part's only $25.

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86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm