Hi all. Before reading this, to put things in perspective, we Australians sit on the wrong (right-hand) side of the car, in case I may confuse you, I'll try not to.
I've just had a problem develop this afternoon in relation to the windscreen wiper on my 1990 300CE.
Up to today the wiper has been working silently & faultlessly, so I have never had cause to pull it apart.
It was working fine, when suddenly the wiper arm came to a sudden halt on the left side of the windscreen, without any undue accompanying noise. I pulled over out of the traffic, turned the wiper switch off, then upon inspection I found that the tip of the wiper blade had caught itself under the edge of the windscreen trim. When I forcefully moved the wiper down slightly to free it from the trim, it immediately returned across to the park position (in front of the driver), & caught the trim on the drivers side.
I then turned the wipers on & the same thing occured. After freeing it up again I left the wipers off & returned home slowly in the rain.
Once home I removed the wiper blade assembly from the wiper arm & clicked the arm up so it was off the windscreen. I then turned the wiper on and observed how it was functioning. The arm travelled its full sweep & it did its normal out-in-out-in as per usual. It was at this point I noted that whenever the arm was at a the extreme of its travel (ie. at one side of the windscreen or the other) instead of the arm being retracted to its shortest length, it was at about 3/4 length.
Normally the arm is retracted at the end of its travel, thus keeping the tip of the wiper blade from contacting the trim. Furthermore, by turning the key on-off very quickly, & closely observing the arm throughout its arc, I found that instead of retracting when either side of the centre of the windscreen as per normal, it was out of sync, and ended up extending at the end of its travel path, instead of retracting.
I have no previous experience with these monowipers, & after doing a search I found lots of articles on noise, lubrication, transmission failures, etc., but nothing in relation to my problem.
If anyone out there has had the same dramas with their 124 I would certainly appreciate a nudge in the right direction as to how to remedy this situation.
Cheers.

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Marc Whitting
Australia