Electronic blower regulator sounds bad. Apparently not uncommon as MB has issued an improved part. The new # is 140 821 83 51 , I believe (double check). The blower goes to highest when temp wheel turned to "MAX" and your highest circuit would seem to be out.

Edit:.... looks like joe p already addressed this issue in another thread... look there.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)