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  #1  
Old 05-19-2019, 05:43 PM
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w123 whine/moan at 40-50 MPH

I recently got my 300CD out of being parked in my garage, and took it for a drive. I noticed that it runs smooth and quiet, idles nice, etc.

But when driving on the road, at ~40-50, I get a whine or moan sound from under the car.

It doesnt come on gradually that I can tell, rather it comes on abruptly at the 40 or so point, and then is not noticeable (dont think its merely due to engine/road noise) around 50.

Letting off of the throttle makes the sound go away, so it may be related to the drivetrain. More throttle doesnt change it, it just continues on until I go a bit faster and then it disappears.

My thought is that perhaps from all the sitting, whatever minor oscilatory/vibratory modes that may exist for the driveshaft, perhaps has a slight motion and perhaps there's a lack of lubrication in there.

My gut is that it is NOT the diff, because Ive had a bad, whiny RWD diff before and the location and sound differs. Also, the CV axles are OE Mercedes, installed before I parked the car in my garage. So Im pretty positive theyre OK.

Any suggestions?

Thanks!

Any thoughts on what it might be or how to best test?
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Current Diesels:
1981 240D (73K)
1982 300CD (162k)
1991 350SD (113k)

Past Diesels:
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1985 300D (233K)
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  #2  
Old 05-19-2019, 05:56 PM
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Perhaps the center support and or bearing or u-joint got tight from sitting. Look at the u-joint cup/seals extra close for any sign of rust.

Does the car sag in the rear at all? Wondering about a collapsed diff mount which may be made from unobtainum IIRC


Sounds silly but what does it cost? Back it up quite some distance and then do slow full turn figure eights forwards and backwards. It may free something.

Agree probably not diff from sitting.
Good luck!!!
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  #3  
Old 05-20-2019, 12:15 AM
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IIRC it’s the TD diff mount that is different somehow...

The car has always had slight sag to my eye, but it’s not definite that it’s truly the case...

You would t see it today when I took this pic:


Good idea with the backwards driving! First ai need to fix a stuck caliper!
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Current Diesels:
1981 240D (73K)
1982 300CD (162k)
1991 350SD (113k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #4  
Old 05-20-2019, 12:18 AM
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Speedometer cable?
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  #5  
Old 05-20-2019, 04:11 AM
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In your other thread you show a serious over heating problem on one of the brakes - I'd fix that first and then see if there's still some weird **** going on at 40 - 50 mph
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Don't leave that there - I'll take it to bits!
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  #6  
Old 05-20-2019, 07:43 AM
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As suggested take care of the caliper first. A sticking overheated brake with a slightly warped rotor may be the problem, especially if it is a rear caliper.

Good luck!!!
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  #7  
Old 05-20-2019, 07:50 AM
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Quote:
Originally Posted by BWhitmore View Post
Speedometer cable?
That did give an atrocious scream once in cold weather when I first had the car... changed. Donít think thatís it... though routing the new cable was a challenge to avoid bouncing and whatnot.
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (162k)
1991 350SD (113k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #8  
Old 05-20-2019, 07:51 AM
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Yes I will be taking care of the caliper first. Will do it as I have free time, which isn’t very soon/often...
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (162k)
1991 350SD (113k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #9  
Old 07-01-2019, 12:42 AM
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To follow up, drained the fluid. There were no metallic chunks or shards, but there was a good deal of metallic sludge in there.

The grit in the pan here was from me not cleaning it well last time.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (162k)
1991 350SD (113k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #10  
Old 07-01-2019, 09:36 AM
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Temporarily put some 75W-140 gear oil in it the Diff to see if it quiets it down.
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