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Old 08-08-2009, 08:21 PM
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Join Date: Aug 2009
Posts: 5
new 300d, mystery noise


I'm new to this forum, but I've recently acquired a 1983 Mercedes 300D TurboDiesel. It's got 180k on it, and seems in great shape for the most part.

I've had it for two days or so and recently I've noticed an infrequent noise at 50 or 60 mph. It sounds like a dull fast banging noise coming from under the drivers seat. Kind of like a helicopter, if you will. It lasts for 30 seconds or so and usually dramatically changing speed will make it go away. It's only happened twice in 300+miles of driving.

I've done my homework (a little) and I have some theories:

1. The carrier bearing/rubber mount on the driveshaft. I inspected this and it seems intact and with minimal slop.

2. the flex-discs. I inspected them and they seem intact and without cracking.

3. somewhere in the rear end, being telegraphed up the driveshaft? cv-joint wear or differential troubles?

4. something non-driveline related. Exhaust banging? Motor mounts? I'm fairly certain that the motor mounts are tired, based on the shake I get at idle.

Can anybody shed some light on this? Would the likely suspects of the carrier bearing or flex-discs be the culprit even though they look fine? I would love a few educated opinions before I start throwing money at this thing.


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Old 08-09-2009, 12:28 AM
Wodnek's Avatar
Vintage Mercedes Junkie
Join Date: Jun 2004
Location: Southeast Wisconsin
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I still think it is your center support. There is a bearing in it that can go bad.
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Old 08-09-2009, 10:19 AM
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Join Date: May 2006
Location: South Louisiana
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When my carrier support went out on the previous car, I had a sort of pharumping noise whenever the trans shifted at higher speeds. The rubber part of the carrier had come apart, allowing slop in that part of the shaft. It would flop with any change in rpms. Can't remember if you can see that with it on the car, but it would be worth peering up at with a light to see if its ripped open. If the driveshaft has to be dropped for any of it, I'd replace both discs, support, and carrier bearing as a preventative repair, so you'll like never have to do it again. Its on my to-do list in the next couple months with my daily driver. Make sure the car is supported off the ground and well chocked so it doesn't roll on top of you and squash you.
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Old 08-09-2009, 01:30 PM
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Join Date: Mar 2009
Location: Philadelphia
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The best way to diagnose your problem is to crawl under the car and inspect everything. The driveshaft support bearing, and flex discs would be my first thought, but you should also check the axle boots and joints.
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Old 08-09-2009, 02:09 PM
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Marine Engineer (retired)
Join Date: Aug 2006
Location: West Virginia
Posts: 1,268
Arrow I have an idea....

I don't know this "for certain" but before I changed my guide rod support "ball joint" on the passenger side I would get an unusual clunk on occasion from the passenger floorboard these are mounted directly in front of the passenger and driver footwell....and depending on wheel balance, and/or shock absorber wear.....I would think that you might get a noise like you describe.

I might be way off base here, but from the noise that my car made on the other side (sits by exhaust header) it sounds plausible to me.

Also of note: The rear sway bar links can also make an odd noise....found that accidentally while tightening the car up in cornering.


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Old 08-09-2009, 02:20 PM
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All above-listed possibilities are valid. According to your discription, I vote for the axle boots. A chunk of rubber flailing about would definitely cause the noise you mention.

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Old 08-09-2009, 02:23 PM
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Look in at flywheel to see if flex plate is cracked,also check crank pulley bolt.
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Old 08-13-2009, 02:36 PM
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Here is an update, after my second week of 300D ownership.

All the driveline components I've inspected appear to pass. Flex discs look pretty good, center carrier mount is intact (i'm not sure how to assess the bearing, but there isn't much play), CV boots are intact, etc.

I think that the culprit was the slightly shady aftermarket straight pipe exhaust. I noticed that it was off of one of it's little mounting hooks. Once I put it back on, I haven't heard the rapid thunk thunk thunk again. I think it must have gotten a little off line and made some contact with the drive shaft somewhere.

Is there some terrible malfunction that I could be oblivious to? Some monster lurking in the transmission or something? I do have some transmission fluid drips-- my theory is a bad pan gasket and potential bad B2 piston o-ring. Any other thoughts?

Thanks for all the input,
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Old 08-13-2009, 04:08 PM
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Location: Nova Scotia, Canada.
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I would add some oil to the constant velocity joints on the rear axel. It sounds like one of them may be warming up and clunking. If this is the case you will seldom if ever get the noise on short runs. Not enough time to warm one with low oil lubrication up.

Check the joint boots for integrity and any evidence of oil leaking externally. It will not hurt to remove a single use clamp. Use a very large syringe to inject some heavy oil and apply a normal clamp to close the boot again...

These intermittent thumps are common on all types of cars with cv joints. Right now it sounds like you are just getting a warning to lube one of them. When the problem becomes constant it is then too late. The joint is shot. Remember they may still be the original axles and have not seen any attention since new as they are usually very long lasting.

I have personally done this on non mercedes cars I owned and got another 50k miles or more. I state more because I either stopped driving them about then or sold the cars and they were still quiet. You have to catch the fairly dry joint early though. The price of procastination is replacement. Unless you locate something else pretty fast thats what I would do.

Now for the interesting senario. If axels are removed and properly drained then relubed with the right quantity of lubricant. Then installed on the opposite sides you then almost have a totally new wear area in those joints to start on.

The same rule applies to getting a used axle. Get one from the opposite side of the scrap car. If you follow this procceedure a used axel is in most cases far superior to a new chinese fabricated one. Remember in most cases an old mercedes diesel is not scrapped because a rear axle cv joint has gone south.

Usually when a fairly dry cv joint starts thumping it does not stop by just changing the speed a little. You usually have to stop and let it cool down to stop the thumping. On that basis it could have just been your exhaust pipe. Yet that pipe thumping should have still happened on shorter trips as well. If it comes back even once again though remember this post. All cv joints do not always follow the most common behaviours in my opinion.

Last edited by barry123400; 08-13-2009 at 04:18 PM.
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Old 08-13-2009, 04:18 PM
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Join Date: Aug 2009
Posts: 5
Thanks Barry. That seems like some good information. What correlates for me is that the noise did happen ONLY on long trips, so the CV theory makes sense.

Is it legitimate to simply switch the half-shafts and then the joints would have a new lease on life? Are they symmetrical? I'm interested in hearing more about this.

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Old 08-14-2009, 02:25 PM
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Location: Nova Scotia, Canada.
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My understanding is yes a basically new unused area where the balls load against the cage other than when backing up is put into use. Thats with an axel rotation from side to side, This has to be done before much if any real damage has occured. Permanent damage is indicated by constant thumping.

If you take an older cv joint apart you might actually by examination see the basically totally unworn areas. As for symetry any axel can go on either side. There are no left and right axels for these cars and many others. Those joints are omini directional.

I do not have a background of cases where this has been commonly done. At the same time I canot take any credit as I am aware it has been done by many people over the years.

The problem is the chinese axels that are cheap enough are questionable. The so called rebuilts also make me wonder if rebuilt with oversize balls and the pockets or grooves machined to match. How is the factory hardness restored to the ground areas if penatrated during the grinding process? Then there are the brand new axels from a dealer. With taxes possibly 500.00 apiece. These are of course the best alternative in my mind yet very pricey.

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