Finally got around to replacing those rear differential mounts - the two that hold up the back side of the differential to the subframe, this time having them installed with the right side up; the "bear's ears" should be pointing upwards, not downwards as they had been installed on my car for years:
They were replaced by the very same worker who had installed them the wrong way up way back in 2014, at which time I pointed out to the error of his way to him. He insisted that he had installed them exactly the same way he had found the old ones; turns out he was right - the previous owner's mechanic was the real moron for having installed the earlier replacements up side down! This time, I made sure that he and his boss knew that they needed to be installed the right way up.
That pretty much took care of the brrrrrrrrrrrrrrvvvvvvvvvvvvvvvvvvrlrlrlrlrlrlrlrlr lrlrlrlrlrlrlrlr noise that comes out from under the back seat when coasting down from about 140km/h down to about 100km/h.
I also had them install the little bellows that shelters the male-female splines connecting the two halves of the driveshaft, this time using a genuine MB one, and make sure the splines are greased up and free of rust and play. This time around, I ordered several genuine MB ones to keep them handy; I can't buy these little things locally because no one has kept them in stock for years now, which unfortunately is the case for a whole lot of other W124 parts. I had a commercial-grade, non-thermoplastic boot installed in early 2017; it was gone by year's end. Since then, I had been driving the car around with the splines fully exposed to the elements! Yikes!
So... to recap this whole thread - and this whole circle of hell, for that matter:
Sources of driveline vibration, starting with the most obvious ones:
- Rubber flexible joints/flex disks/donut couplings, just like the ones on an old Lada
. These will eventually fail, no matter what. Mileage, extreme heat and/or cold, disuse, leaking lubricants, all of these things eventually will cause them to degrade and fail. The trick is to catch and replace them before they do. Treat them as tires or brake pads that need to replacing every couple of years, or as soon as they show signs that they're about to go to the great disco parlor in the sky and take you and your car with them.
- Driveshaft center carrier. Both the bearing and the carrier's rubber damper are possible - and sometimes frequent and concurrent - failure points.) It's best to just replace them together as a unit, even if the old bearing that's been in there for some time sounds and feels good. This thing lives in a hostile environment. You just never know.
Slightly less obvious sources:
- Transmission mount; this is more important than most people - including many mechanics - think! A bad transmission mount could cause the whole output flange-driveshaft junction to wobble, causing annoying and potentially destructive vibration. Note that the problem can actually get worse with a new
flex disk installed with the old, worn transmission mount left in place! Pay attention to this. Bottom line: If it's got rubber in it, it gets changed out. Period
- Having the wrong
flex disk installed at the transmission end! Here's the correct
one that needs to be behind the transmission (722.3; don't know about the situation with the 722.4 or the cars with manual gearsboxes):
The face with the writing must point backward (towards the driveshaft); the plain face forward (towards the transmission.)
... belongs only at the differential end of the driveshaft. Even though it's deceptively easy and convenient to fit at the transmission end, it would be the wrong kind of flex disk to install behind the transmission. Having a rear donut installed behind the transmission will cause a vibration that comes up around the center console, and it would be especially obvious at lower speeds in cold weather; it will recede after the donut warms up and softens up as you drive, but it will still be there if you listen hard enough. You may even feel it creeping up through the steering wheel at around 40-60km/h. On top of degrading the overall quality of your already miserable life, it will shorten the lives of both the transmission mount and the driveshaft's centering bushing (see below.)
Thing is, if you go to the parts store and ask for a front flex disk, the salesman probably will hand you a rear one, or give you two as a pair. Part of the reason why this happens is that the Mercedes-Benz EPC isn't entirely clear on which exact donut is supposed to be installed at the transmission end; at least from what I've seen on Russian sites, the EPC actually lists a single part number for a donut kit (rubber flex disk + six attachment bolts and six nuts) for both ends. It doesn't even specify whether which end of the driveshaft it's supposed to go onto, so most parts vendors just assume that it's the same kit for the front and the rear. It falls upon you, the owner of the car, to tell the salesman out right "no, give me the thicker one with the flexible bushings!" Some of them will give you "the face," but most salesmen will say, "oh, okay," and hand you the correct one.
- Driveshaft misalignment: The two halves (or three thirds, as found in six-door "Limo" W124s or, say, 350SDL W126s , for example) need to be installed together in alignment
- that is, the two/three sections that make up the driveshaft assembly need to have their =- alignment marks lined up upon assembly:
I've seen some mechanics neglect doing this, either out of sheer ignorance or a lack of integrity. What's even more confusing to people - owners, mechanics, and part vendors alike - is that the EPC lists these parts both as a complete assembly and as separate parts (examples here
.) Any Mercedes-Benz technician or parts salesman will cite this as a way to convince you that you need to buy and replace the entire driveshaft as an assembly, but of course
they'd do that, for obvious reasons; some independent mechanics and junkyard/used parts traders will tell you that you can buy each half separately, and that lining them up is enough to eliminate the possibility of them vibrating. There's just too little consensus on this.
Even less obvious sources of vibrations:
- The driveshaft's centering bushings - or, to be more exact, the rubber rings that line the insides of them. These serve two purposes: a. they keep the driveshaft's spider flanges centered to their mates on the transmission and the differential; and b. they keep all the road grime - dust, sand, water, lubricants, what have you - out of the hollow tube that is the driveshaft. You can imagine the kinds of trouble that any shit that gets trapped inside a spinning driveshaft could cause.
- Bad U-joint. This thing tends to last a very long time, but if or when it wears out, you'll know something's (belly) up! You'll hear it and feel it. Unfortunately, if you describe the issue to most mechanics, like robots, they'll default to saying "oh, we'll replace the carrier!" If you're lucky to have stumbled upon an honest enough mechanic who's worth their salt, it might occur to them to inspect the U-joint while the driveshaft is out of the car. unfortunately, too many of them don't even bother to look at it while they go ahead and replace the center carrier (which may or may not be in perfectly good working order,) ending up with the issue still being there after they've done the job and got paid for it. Make sure to check the condition of the U-joint along with
the center carrier, or during any other driveline job, for that matter!
- Differential mounts. The front one - the one up by the nose of the differential, above the input flange - won't cause any specific vibration in and of itself, but it will make a lot of noise when it goes bad, mainly just clunking around when accelerating and/or coasting. The rear ones will cause an obvious - and very annoying - vibration when coasting at higher speeds. The front one tends to fail less often than the two rear ones, but have a look at all three every once in a while. Come replacement time, make sure that the rear ones are "the right way up" (see above.)
And now for The annoyingly, irritatingly, maddeningly obscure shit that Mercedes-Benz's engineers, in their infinite wisdom, thought was a good idea to build into what would've been best kept a simple affair:
- Driveline misalignment (not to be confused with driveshaft misalignment
above): This is a job best left to a stealership, I presume, but it rarely comes up as an issue. The transmission's output flange, center carrier, and differential's input flange need to be in as close to a straight line as possible. If you find shims under the transmission mount when you're working on it for any reason, make sure you put them back under there! Also very important and often overlooked: There's a washer between the front mount of the differential and its actual housing, with varying thicknesses (the number of notches indicates thickness in millimeters,) like so:
You can just barely make out the shim/washer, right above the "tongue" that's part of the differential housing, held up by that hex bolt facing downward towards the camera. Plenty of W124s rolled off the assembly line with shims that are 3mm thick (as indicated by the three notches on each shim.) The shim in the picture is 2mm thick (two notches); A thinner shim (1mm or 2mm) will have the nose of the differential (and the spider flange) point slightly more upward; a thicker one will push it down a little. As I mentioned earlier, this rarely happens to become an issue, but it's not unheard of.
- Here's the killer: Different W124s have different ways of attaching the center carrier to the bodywork. On some W124s, the carrier bolts directly into bolt holes built into the floor panel; on others, like my E320, for example, the carrier bolts into two brackets, each of which, in turn, are secured to the floor panel with one nut and one bolt each, like so [ignore the red arrows this time]:
Note the mounting brackets between the carrier assembly and the bodywork.
Here's another W124 - no idea which model - where the bracket the carrier bolts onto is welded directly to the bodywork:
The nut screws onto a boss welded to the bodywork; the bolt screws into a welded-in bolt hole. On my car, these two bolts were missing for some reason, which was the real cause of the original vibration issue that I had started this thread about way back in 2013, and which had been plaguing me ever since I bought the car in early 2010. A young Syrian mechanic in Jeddah named Badr was the one who figured this one out, so a big thank you to him.
The most probable explanation he could come up with was that while the car was with the previous owner, some mechanic who had wanted to replace the center carrier unscrewed those bolts by mistake, thinking that they were the ones securing the carrier to the floor panel, but somehow forgot to put them back after they realized they had unscrewed the wrong bolts. Or it could just have been a simple case of not enough Loctite
. Either way, I was the one that got screwed royally; the original driveshaft, transmission, and differential - there was nothing wrong with any of them. None of that shit needed to be replaced. I ended up spending thousands trying to drive out gremlins that never wanted to leave, when I only needed to spend a few cents for a pair of bolts.
And yet, any MB fanboy will still talk your ear off about how the technology MB built into the driveline of its cars is superior to that which went into, say, a Toyota HiLux
Which brings me on to the ultimate lesson anyone would do well to learn: Never trust somebody - especially a purported "mechanic" - just because they're family. I trusted some guy who's related to me, thinking that he was qualified to clear out the original issue and smoke out every other gremlin while he's at it; he ended up taking me in for thousands, while the car ended up with even more issues than it originally had. I had to spend thousands more just to get it back to its original condition, and then make some minor changes that brought about huge improvements; it's amazing how a set of coil springs and a pair of stiffer sway bars could transform the way the car drives!
Anyway, all's well that ends well. One thing to realize, though, is that as cars get older, some of these things stop being issues to most people, especially the non-enthusiasts who see these cars as mere tools or cheap conveyances. No one in their right mind would order a brand new driveshaft from MB because the old one's U-joint has gone bad after 30 years, when the entire car is worth just as much as said driveshaft! But I know at least one person who just might be that crazy about their W124!
I also swapped in single-notch (8mm) rubber spring pads to replace the two-notch (13mm) ones I had on earlier; the improvement in handling, though small, is noticeable, but the improvement in comfort and the smoothness of the ride is a welcome change.