Originally Posted by BimmerBenz

I have seen this now three times. On one particular 300D the subframe 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 subframe 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 minigrinder 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 subframe 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.