i believe that the gears inside strip. mine was doing the same thing as yours. drove me crazy for a few days trying to figure out why. i stopped mine in the straight vertical position and pulled it up to it's fully extended position. it now works good, but at the apex, the blade runs over the windshield trim. i guess i pulled it past the stripped teeth. it'll get me by until i can find a used good wiper arm, since these aren't cheap.

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"the simplest answer is usually the correct answer"

1991 190e, 2.6 (helga)
91,000 mi