Disaster averted & droning sound fixed! (pics)
 

For those that saw this thread:

http://www.peachparts.com/shopforum/showthread.php?t=137869

here is what I found out about my droning sound at speed (60 mph). First thing I did was replace the motor mounts. As you can see in the attached pics, not too bad for 25K miles. Pic on the left is the driver's side, pic on left, the passenger side.

Then I started checking other stuff...
 

I was thinking about replacing the engine shocks but they were pricey even w/ my MBCA discount at the dealer. I want to say $50 each roughly not including the rubber donut bushings. They compressed slow and steady using my hand so I suspect they are fine. So once I had everything buttoned-up and I was about to take the jack stands out, I was just wiping down my slow oil leaks and giving everything a once over as the folks on the forum suggested.

Boy I'm glad I did. So I'm wiping stuff down with simple green and I've worked my way back to the tranny, checking the mount near the flex disc and my hand brushes a loose bolt head laying on the back side (facing up) of the crossmember. What tha'??? It's a bolt that has sheared off. So now I'm in a panic trying to figure out if it is really old and been there for years, or something recent. I'm now going over everything closely with my light and running my hands over anything in reach of that area. BINGO! I discover that it is one of the bolts from the flex disc, and at least one more is loose (at least I can wiggle it). Thoughts of drive shaft failure has me freaking out a bit.

Luckily I have flex disc kit in one of my cabinets in the garage. At first I was only going to do the bolts that were bad, but I end up doing all six, torquing them down per the Factory Service Manual (what a ***** to do on your back!) and I add a drop of blue locktite just in case to the nuts.

From what I can diagnose, one of the three bolts shears on the triangle (for those who know how a flex disc goes together, think two triangles offset into a six point star). That sheared bolt then stresses the two remaining bolts and though they do not fail, have stretched considerably from fatigue (see pic). The other three bolts in the opposite triangle look to be fine, but I replace them anyway.

So, after spending way too long on this project, my road test reveals no vibration or droning. Even the shifter knob feels perfect (no vibration). Ahhh, luxury ride restored and disaster averted.

Moral of the story - listen to your Mercedes - she's trying to tell you something!

PICS - two stretched bolts far left, sheared bolt, on right - other bolts fine

most unusual failure!!
 

First time I've heard of those bolts failing. The elongation is a real troublesome aspect--indicating extreme tension placed on those bolts (they ARE strong--yield strength of that steel is about 60,000 psi/in2)

Why were they stretched? Way overtorqued last time? An air impact wrench set on max and 150# air? Seen it happen at NTB on wheel installation

Yep it happens, the flex discs need to be checked on these cars. I know of several drive shaft failures that happened because of a broken flex disc.

You have got to love TX only their will 20 year old steel bolts look that good!:D

And the NW :D

No salt around here.

Those bolts really held up to a lot of stress...think about what sort of forces you are talking about there...espically in 1st gear on takeoff.

Impressive. Glad you caught it in time :D

Granted most of the flex discs that seem to fail are on gas cars. M117's seem to do a number on them.

Yeah you guys that live in a salt free environment are spoiled!:D Up hear a massive breaker bar, liquid wrench, a butane tourch, and a hammer are needed for suspension work.:rolleyes:

Yep I just tore down an aircooled VW engine using 1/4 drive socket/rachet stuff.

Even though oregon is known for its ability to rust an object the moment you set it on the table, we don't have that nasty corrosive road salt. :)

Actually...
 

I replaced the front flex disc 25K miles ago... so, I might be the one to blame if something was done wrong. I definately didn't use air tools, and I always try to go by the FSM (Factory Manual), but who know.

So that's why the bolts look so "new" - TX car plus only 25K miles.

this is one
 

time stretch bolts didnt work!

kidding.

tom w

actually, to me it looks like those bolts have the area under the head worn away, rather than stretched. Anyway, glad you found that, and thanks for posting that for other members, as that's an easy yet important thing to check and fix.

you are right
 

it looks like wear.

tom w