Cracked frame. W126 owner alert.
 

I have seen this now three times. On one particular 300SD the sub-frame actually broke completely off of the frame on one side at both mounts! Although the car was able to limp home it was a rather unexpected, catastrophic failure for the owner.

In my case I was servicing the front brakes yesterday and noticed the undercoating bubbling near the rear sub-frame mount. I figured I would scrape all the loose bits of and reapply undercoating to the area.

When I removed the loose coating I was disturbed to see a rather extensive crack. See photograph. I had used a wire wheel on a mini grinder to expose the crack as seen in the photograph. I also cleaned up the mount horn to see if there were any cracks there.

On the two previous cars the cracks I saw were actually at the bottom of this horn where the horizontal 1/4 inch steel plate, that the sub-frame mount bushing bolts to, attaches to the thinner frame material of the vertical sections of the horn. This is why I used the wire wheel to expose the steel here as well.

I welded up the crack and reinforced the mount horn then gusseted the horn to the frame at the 1/4 inch plate. Although the left side was not cracked as far as I could tell I also did the same procedure there. Second photo is of preventative maintenance repair to driver's side (identical to repair on right except no welded crack).

Although there was some obvious pocking from corrosion at the crack it was nowhere near serious enough to cause the issue. Indeed the car that completely failed had pristine steel when compared to mine.
I have no idea why this is failing on these cars but three is now cause for concern. All were SD's. I only observed and did not repair the others.

I would like to say that anyone with a W126 should pay close attention to this area any time you are servicing brakes or removing a front tire for any reason. If you see bubbling, cracked, or loose undercoating remove it to expose the steel underneath. Pay particular attention to the bottom of the frame horn and the area just forward of the horn where it attaches to the frame.

I think I have a crank in mine now that I see the pics. But I blew it off not thinking much. I will look again for sure!

30 years of rough road can do that.....we just had the rear trailing arm fail on one of our SDs.....cracked right across the bottom and bent at a severe angle. Car will likely be retired at this point. :o

I posted a few days ago about my SD... the crossmember cracked where it attaches to the outer frame...

High milage fatigue? Cars with very high total milage have always raised questions in my mind. That also is a very large and heavy car.

The 300d 123s are not reporting many cracking issues that I am aware of. Your repair looks adaquate and a good ideal to brace up the otherside at the same time as you did.

Rust can get this particular location as well. We have an identical model low milage gas parts car here with a mint interior. It has corrosion failed on the drivers side at the same location. Euro lights and a great engine and transmission. We could not pass it up for 100.00.

Could be. Not sure on the other cars I saw that failed/cracked but I have 334,000km on mine.

i have seen it on an early '80's 280S. it cracks where the rear part of the front subframe is attaches to the frame rail of the unit body.

W123 and W126 sub frames are configured differently... The W126, of course is bigger and heavier. I've helped change out he rear sub frames which were badly corroded. Of course, southern cars have a better chance of surviving longer due to the lack of salt that they encounter. Northern cars should be reviewed periodically to ensure they are not "falling apart".

What's a "crank?"

If you have owned three out of three W126 chassis MBs where this has happened at mileage/kilometers, that would just about confirm that it was an eventuality in these cars. At least in your part of the Continent. I think every W126 owner should take note, and check for cracks.

Structural cracking would seem to be widespread on W126es, it is looking like.

Well....most of them are a minimum of 20 years old with most being much older at this point....there's no telling what 300k of environment and vibration will do to welds....I doubt MB intended for people to be driving these cars for 30-40 years and 500,000 miles.

mine has 422k and the odo stopped working about 3 years ago :eek:

I know I have seen some fishy looking rust in that area. But rust is my SD's major problem. There is a section under the right rocker, behind the front wheel, where I can actually stick my arm up to mid bicep in a hole between the rocker and floor.. kinda hard to describe... it's got rust real bad.

I hope to sell it in the spring :o
I may have to rob it for parts and send it to scrap.

That looks like a spot where I was tempted to jack up the car, but decided not to because of uncertainty. Maybe some shade tree mechanics were lifting the car at that spot which led to failure.

Answer
 

This issue is common in Michigan salt.

Who has the most rust and still drives
http://www.peachparts.com/shopforum/bodywork-repair-paint-tools-tips-tricks/86054-who-has-most-rust-still-drives.html





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None of the cars I saw nor mine were cracked at welds. All cracked at the seam of the horn to the frame (horn is spot welded), at the vertical junction of the horn as it bends to meet the frame, or at the base where the 1/4 inch plate sits in the horn just above the mount.

I copied this thread so it's in both spots on the forum. I may add it to one of the stickies. ..

So we post only the 126 problem and repair on this thread?

Whunters thread on the rust was excellent it had quite a few eye openers.
it also contained some 126 problems and repairs on it.
I don't know if you can transfer those to this thread or maybe highlight
those posts with a link. I think it would be helpful.

I say this because my wife's 81 SD has bubbling in all the places shown here.
We have been driving it not even worried about the rust.(cars originally from Alaska)

Its going to make me take a closer look. Especially around the structural areas.

Excellent find THANKS Shayne !!

what type welding?

115 VAC flux core MIG... it's all I have at home.

I lost one to this also. Not comfortable welding it with my little welder, so I just found another car, and transplanted, and away we went.... 400K and moving still...

None of ours have such cracks. Only rust related failure so far was a rear trailing arm on one of our SD's.....it is looking like we might retire it to a parts car due to overall rust/body flex. :eek: