Negative INFINITY Camber: a new reason that causes this
 

I recently took my car to mr. mb motors in tarzana to have some work done on it. As it was on the hoist, I peek under the car and notice a crack in my left rear lca. it was like, 2" long. I showed it to the mechanic and he recommended I have it welded up or replaced as soon as possible. I figured I had lots of time to get this to a muffler shop and get it re-welded. Well, I was wrong. I arrived at the junkyard this morning for half price day sale, got out of my car, and noticed the wheel has crazy camber to it. I peek under the lca and that crack grew, in like 1,000 miles, to all the way across the lca--from the point where it's stamp welded together from the factory on one end, all the way under the depression where the spring and shock sit, to the other stamp weld on the other side. I google mapst my way to the nearest muffler shop, and as a temporary measure, got him to weld and reinforce the crack. It took him about half an hour of arc welding to get it patched up. You guys think it's safe to drive like this till the new LCA arrives?

That depends on how good the welds are. If the welds are very good, the control arm might be stronger than it was when it left the factory (assuming it wasn't heat treated). If the welds are less good, then it won't last as long. I would keep driving on it, but inspect it very regularly.

On the W123s the Metal on the Rear Trailing Arm might not even be a full 1/16" thick.

Oh wow. I wasn't aware. THis is clearly a major fail point after a while then. Unless you consider an LCA which lasts for 30 years a success. I guess I would. I just hate to have to pay my mech for this labor. Looks like a pain to remove this LCA. I would bet he wants two hours' worth of labor. I'm tempted to buy a pair of new rear shocks while I'm at it, but I don't feel like paying for 4 hours' worth of labor. I'd do it myself, but it looks harder to do than the one on my 190e. That one was cake compared to this. I'm going to take the advice and keep driving it for now. He welded it all the way, plus welded two rectangular spare peices of metal as braces. My big fear is that as I drive on the freeway one day, BOOM!

I would change it as soon as possible. As mentioned above it is pretty thin and derives its strength from the shape rather than brute thickness. That is why it works so well and gives a nice ride.

X2

I'd measure the lifetime of that fix in hours not days

If you want to replace a trailing arm I'd most certainly remove the subframe to make it easier for yourself. It might sound like much more work but the fixings for the trailing arms are a bugger to reach.

There's loads of information in this thread (largely thanks to Charmalu and dieseldan)

http://www.peachparts.com/shopforum/diesel-discussion/276507-rear-subframe-removal-bushing-replacement.html

That was a Pic of one of My own driver side Lower Control Arms. It had rust Holes in it which is actully typical.
In My case I hit a 3 Inch high Traffic Island and finished it off.
I reaplaced it with a used Control Arm.
I did drop unbolt the drivers side of the Sub Frame to get it out.

I don't think you actually need to remove the whole Subframe.

My Passanger Side also had 2 rust Holes. I Welded those over when I Welded the Rear Body Cross Member. However, I have another used Control Arm for that side.

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when replacing the 'trailing arm' (what I call the lower control arm--is it the same thing?), should i also replace the metal bracket that holds the pin like bolt, as well as that pin like bolt itself? what about the subframe bushings? is the subframe bushing easy to replace if you loosen the subframe enough to remove the lca?

Do you agree with stretch that the weld job is a much more temporary fix than i think? Maybe I should post some pics of this fix so you guys can see what the weld looks like

I would have just pulled one from the yard you were in and swapped it while I was there... of course it requires some big wrenches to pull...

Not a bad idea - but the problem that we have is that we can't see inside the hollow trailing arm to see if there is more rust in there.

User JamesDean has done loads of work on his trailing arms - worth a search

Lower control arm is used to describe a similar(ish) part on the front suspension. The trailing arms trail (behind) from their mounts on the sub frame.

Pin-like bolt? Metal bracket? Are you talking about the bits on the sub frame here =>

http://www.peachparts.com/shopforum/...gone-wrong.jpg

I think even the FSM says to loosen the sub frame on one side (and perhaps the diff mount too?) - support it - and then tackle the trailing arm fixings.

Removal of the whole sub frame is most certainly excessive if you look at what the book says...

...but it kind of doesn't say how you should be careful of older sub frame mounts. To lower the sub frame enough to get to the trailing arm fixings you need to drop it quite a way and you'll twist the sub frame mounts in doing so. This kind of "abuse" could finish them off. It might not; but then again trailing arms sren't expected to rust in the FSM are they?

My advice is kind of doctored to suit an old car - and as I said previously it is way easier to fix the trailing arms in position with the sub frame removed.

On the one I welded I Just welded one small rust hole and a tiny crack that was up against the actual casting. So the areas I welded were not in stressed areas.

Since yours was badly cracked I would say it is ready to give out on you any minuite.

When I welded up the cracked Crossmember on the Body. I welded up the crack and then I welded two 1/8" thick Plates over the Crack to spread the stress over a large area of the Plate.

Answer
 

A factory NEW Genuine Mercedes-Benz
LEFT trailing arm is
MB# 1263503205
$3053.50 each

This is a common issue in Michigan.
Welding is not safe or durable.
I would replace it with a good used unit.

http://www.peachparts.com/shopforum/body-repair-restoration/220623-1985-300sd-rr-trailing-arm-rust-pictures.html

http://www.peachparts.com/shopforum/diesel-discussion/276377-rear-trailing-arm-broke.html

http://www.peachparts.com/shopforum/1933120-post2.html

http://www.peachparts.com/shopforum/diesel-discussion/119396-rust-battle-434-rear-control-arm.html

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http://www.peachparts.com/shopforum/diesel-discussion/111052-trailing-arm-bushing-w126-1985-300sd-diy.html

PeachPartsWiki: Replacing the Rear Shocks & Springs

http://www.peachparts.com/shopforum/diesel-discussion/74439-sub-frame-trailing-arm-bushings.html#post467465

http://www.peachparts.com/shopforum/diesel-discussion/288805-w126-complete-rear-suspension-rebuild-diy.html

http://www.peachparts.com/shopforum/diesel-discussion/112850-rear-sub-frame-bushings-w126-1985-300sd-diy.html#post796976

http://www.peachparts.com/shopforum/diesel-discussion/266324-w123-wagon-rear-trailing-arm-bushings-r-r.html

http://www.peachparts.com/shopforum/diesel-discussion/111127-steering-squirrelly-w126.html#post778649

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