Need some advice:

At approximately 1800 RPM and with the engine under a load there is a hum that seems to come from the drive line. If the car is coasting the hum seems to go away but returns when accelerating or under load. I am thinking about taking the car in to the shop this week but thought someone may have had a similar experience.

The car is a 1984 300D Turbo with 155,000 miles. The transmission fluid was changed 15,000 miles ago and is at the exact correct level when checked. The differential fluid was changed last weekend (by me) and is at the correct level. I inspected the flex disk close to the differential and it appeared okay. Since the car is so close to the ground I could not see the center bearing. Could this be causing the hum?

Check your transmission mount.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanks for the suggestion, I will take a look at it!

Steve,

I just crawled underneath the car and looked at the transmission mount. Although it is rather old, it seemed that it was intact and no too bad. (I will replace soon, doesn't seem to difficult). I did notice something unusual, the rubber sleeve next to the the sleeve nut was out of shape. It was pulled away from the nut and bunched up on one side with wrinkles. I am thinking that this would cause the differential not to be perfectly concentric and may cause a hum? I did not feel any vibration though. What do you think?

Joe

Had the same problem and it turned out to be the flex disk on the drive shaft. Cheap fix.

I looked at both flex disks and they appear to be okay, the rubber is not cracked. Did yours have any outward appearance of being bad? I was wondering what made you aware that this was the problem? Did you repair it on your own?

Joe

Joe, Boy I hope you don't have same difficulty I have had trying to fix a vibration/hum in my drivetrain. I've replaced all rubber mounts (engine, transmission, center drive shaft, rear end), replaced flex discs, balanced main shaft and rebuilt its universal joint. Some improvement but not solved. Now replacing two rear half shafts. Good luck with your problem.

Nic
'85 300 CD at 138k miles

Nic,

Thanks for the note, your running out of things to replace! Sounds like you changed just about everything but without any success. Did you do the work yourself? Is this something I should take to the shop?

"Is this something I should take to the shop?"

You know this is the reason we get the big bucks (bg). I just have to grin at those that replace half the car with parts changing diagnosis. Not that I haven't changed a few parts to achieve a diagnosis, but my customers never pay for any more than what it takes to solve the problem. We call it the "Blue car syndrome", you know; if its a blue car then it must need a coil (bg).

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve,

Yeh, I think you have a good point. I will let the experts take a listen, maybe this Thursday. It's nice to receive the advice and experience from many folks!

Joe

Steve and Joe,

Well I'm not a nuclear scientist but neither did I fall off the truck yesterday.... I took my car to more than one place trying to determine the source of the problem but got no clear diagnosis. One of those places was the local mercedes dealer (twice). I also checked in with a number of mercedes web sites (such as this one) and found no consensus.

I paid to have most of the work done because I saw no option other than to live with the vibration....which I can't accept. I'm definately shooting in the dark and would greatly appreciate advice from anyone who has something to offer.

Nic in SC

Joe,
Car was under warranty and the dealer actually placed several microphones under the car and switched from one to the other while driving the car. The noise, in this case, was coming from the flex disk. It was totally anoying and would just "sing" at certain sustained speeds.

NIC,

I have no problem with them trying a driveshaft to verify a diagnosis. My problem is having you pay for anything more than the R&R time if it doesn't work.

BTW most of the driveline vobrations we see in 123 body diesels are in the drive axles. Recently we have found that switching the axles side to side gets rid of the vibrations. I first tried this after the Rolls Royce dealer I deal with told me to try that on a 1980 Silver Shadow. The Silver Shadow (somewhat like the MB) has changed their axle design and it would have taken a small fortune to replace the shafts (same condition with MB factory shafts - they have been redesigned). Since fixin the Rolls we have done the same trick on four different MBs (one 107 chassis and three 123 chassis). We do not do this with axles that are obviously bad.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve,

The mechanics who work on my car can not determine the exact cause of the problem and are unwilling to gamble - that leaves me basically on my own. Because I really like the car (its a nice one) I am willing to try logical options...particularly if the things I'm replacing are old and due for updates anyway (rationalizaing here).

I have read several comments about the axles that indicate they could be the culprets. You suggest that in your post. Cost will be about $300 per side using rebuilds. I can stand that. I wouldn't mind trying the switch'm idea but my labor cost would be $200 for that experiment (I don't do this work).

If this doesn't solve it I'm out of options. I just hate to give up.

Nic

Joe,
In your initial post you inquired about the center bearing. It very well can cause the noise that you describe. You can listen to it while running on the hoist w/ a stethoscope and often isolate it. Just a thought for your tech as you mentioned you were going to take it in. Hope this helps.
Dan

Steve, I like your 'Blue Car Syndrome', sort of like the
'WAG' method of diagnosis