Without seeing the actual arms it's hard to say if they need to be replaced. That being said I have the original arms and had problems with replacement wipers falling off. The replacments that I purchased had an adapter to mount on the MB hook style arms that rarely mounted severely. Two years ago I went to Bosch wipers and have not had a problem as the wipers were made for the hook type arms. I have not noticed a problem with the wipers. This also goes for my Civic which has the hook style wiper arms. I also replace the wipers twice a year.

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Fred Johnson
1977 240D
1983 300SD