A long time ago had the same problem on my 300TD (S123). The problem was fixed by changing the multipurpose turn signal stalk that also control the wipers.

It appears that the contact on the turning switch will degrade and not engage on intermitent or slow (the speeds most often selected).

Now, being a W140, my opinion might be just one of a thousand and five options that could be happening.

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A. Rosich
CL 500, 1998
S 500 L, 1998
E 320 T, 1995 [Sadly sold ]