Crack ... something to worry about?
 

I found a crack in he undercarraige of my car that concerns me. Photos attached. It doesn't "feel" weak or unstable, but I realize the car puts just a tad more pressure on this than my hand can reproduce. What is the name of this particular component and should I be concerned? For context, in case the pictures aren't clear, this is just behind the differential housing between the rear wheels.

What area is that in? could you post a more wide shot photo?

Bummer
 

Yes, I would be concerned. That's your rear axle carrier subframe (or some other more accurate MB nomenclature), and it's a stress crack. The good news is that it's not fixed to the frame (it's unboltable), the bad news is it's a PITA to remove. I think your biggest problem is the cause of the crack, which is rust. It's probably manifesting itself elsewhere, so it's time to make some tough decisions about what you want to do with the car. As a stop-gap measure, I'd probably grind out the rust, drill a hole to stop the cracking, and pull out my MIG setup to weld a reinforcing plate over that area. Long-term, that subframe should be replaced with a junkyard item, along with all new rubber mounts.

YMMV

on the plus side, it is not supporting the actual suspension i dont think. in other words while it is not something that will cause your wheel to fall off, it still appears to support your diff. and will have to be fixed. if the metal isnt too thin from rust i dont see why a good welder cant fix it in place. the trick is to get it repaired without setting anything on fire.

is it a gas or diesel car? diesel if i remember right.

so that is a plus safety wise.

tom w

I agree. And degreasing that diff before you show it to a welder will make them much more likely to want to take it on. It looks like that could be repaired pretty quickly with a MIG, I've fixed much more than that, and I'm not very experienced (at welding).

Thanks for the input guys. The whole thing makes me sad to think about ... I hate the thought that my car is falling apart. Although, strangely, there aren't many other rusty areas beneath the car (despite it being an all-season vehicle its whole life). When I had it "evaluated" by a very good body guy about 6 months ago, he said the overall condition of the undercarraige and body was quite good, aside from the rocker panel (disaster) and a couple of areas on the doors (probably due to a bad paint job ten years ago). Guess he missed this one, or it's new. :(

I'm taking the car to my indy tomorrow to have him fix my no-functional drivers window (with no AC, I need that window!), so I'll ask him if he can take care of the rear diff at the same time and look at the crack. He'll be honest about my options. I know the car is worth it to me, it's just a question of what's necessary and what's the safest option. :o

the crack doesnt look new to me.

tom w

No, I agree, but I was thinking more along the lines of that it may have been much subtler several months ago. Even then ... the rust around it would have been noticeable. You'd think the guy would have mentioned it. Then again, I had him look at the car just to give a general assessment of the body condition and how much body work I should/could/needed to do.

I'm amazed at the kinds of things professionals can miss when looking over cars. Or people, for that matter ... can't tell you how many times a doctor has missed something rather important, or seemingly obvious.

Anyhow, if my indy can give a thorough look under the vehicle, maybe he can catch anything else that's gone uncaught. I try to check things out now and then ... I'm small enough to fit under the car but the perspective ain't great, and it's kinda dark.

Answer:
 

I am guessing this is your 1987 300D, arctic white/palomino--220k miles.
Chassis 124.133
Engine 603.960 I6
Model 300D Turbo

The crack is caused by thinning metal and deflection stress from the differential.
In plain English:
Junk the sub-frame and get another.
we have several members who go to the U-Pick salvage yards, and will look for what you need.
Look through the parts forum.



Have a great day.

Yes, whunter, this is the white 87 300D ... sorry, should have specified before.

I did have my indy look at it while it was in for the window fix. He checked out the crack as well as the rear diff for anything that could be a concern. He said the crack has been there for AGES and is not something I need to immediately worry about ... he said when I go to replace the bushings (which I will have to do eventually), he can install a "brace" around the cracked area, but for now it's not going anywhere. He recommended against welding, as the metal there is too thin. He also said the rear diff is fine ... all that gunk has just built up over time but it isn't leaking. Kind of a relief ... I know this guy would tell me if there was something to be worried about ... he's been very, very good to my father and I (and my grandmother, back when she had Benzes!) and always honest.

Just thought I'd post that follow-through. :o

That diff is a piece of work! Hermetically sealed against rust, forever. What I don't understand is how could the diff not be leaking when the proof of the leak can be found all over the outside of it. Or perhaps he meant to say that it no longer leaks because the half-inch thick layer of goop has gotten to a point where no fluid can leak out anymore. I think it's safe to say that it has in fact leaked in the past since the proof is right there in front of you. I wonder if the diff oil has ever been changed. To change it however one has to be able to locate the drain and fill plugs without resorting to using an x-ray machine.

Sounds like good news. Also sounds like you have a good honest Indy. Things are good!

Ahhhh, that is a suspension part with a crack in it. I would be very worried about it! There a was post a little while back about a car that dropped the differential while going down the highway...that is bad enough.

I would be afraid the diff would drop, maybe pull the drive shaft loose allowing it to stab into the road and the car start turning flips. Had that happen in a car I was riding in about 35 years ago...it did not flip but the rear end came about 5 foot off the road, came down hard, and bent the frame, ruined the car....we doing about 35 MPH...at highway speeds it would have gotten bloody.

I would be worried. I would at least find me a WELDER and attempt to weld in a brace. Again, I would be WORRIED.

Did you look at the photo?

The crack is about 3/16" long and is in the very outside flange. It does appear to have been there for a very long time and there does not seem to be any propogation of said crack.

I would not be worried in the slightest unless there is extensive rust beneath the trailing arm that cannot be seen. Only in this case would I consider replacement.

I would wire brush the area throughly because there is some surface rust that is seriously developing there. Then I'd paint it with the POR-15 to prevent further corrosion.

There is virtually no chance of catastrophic failure from such a crack.

Clean to metal.
Weld on a little supprt.
Por 15.
Lots of jb weld (I JB everything:D
Drive on and check occasionally.