These use a splined cone shape drive bushings made out of pot metal that goes over each wiper shaft and is square cut internally so it does not slip on shaft, so what happens if the wiper arm looses it's grip on bushing. In fact over time most will loosen up over time so I recommend you tighten all of the nuts on all wiper linkage since they do tend to loosen up on all wiper linkage.
Make sure you do this with engine off, key on, wiper switch in park position,,first move the arm that loosened up by tightening the nut just enough so it grabs and is no tight on shaft,,tr wipers one time to make sure wiper is going to it's normal distance,,shut off key and wipers, Once you know for sure wiper timing is correct go back and tighten all nuts another half turn.
Note: when tightening nuts hold arms from moving,,you may feel the motor trying to fight you that's normal.
Once again test to make sure all okay,,then what I do is put a drop of blue loctite on each nut shaft so the nuts won't loosen up again. If they do loose position juts do this over and re loctite and that will fix your issue.
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