1985 300SD rust on frame - PeachParts Mercedes-Benz Forum

SecretlyKarlMarx · #1 04-18-2016, 01:22 PM

A few weeks ago I bought a 1985 300sd for $900. I looked underneath and I didn't see any rust at all. Mechanically the car runs completely fine. So I bought it and later I was looking underneath and I scrapped some of that undercoating off. The frame is full of rust holes. I don't think it's bad enough that it will just fall apart while driving, but if I get in a wreck with it I know it will just snap without crumppling. I don't want to risk myself or anyone who may be my passenger. So, this need to be fixed. I'm on a very small budget and I would like to find a cheap fix I can do myself. If I can avoid welding steel on I would like to. There's some holes near the body mounts that are pretty bad. I saw somewhere else that using fiberglass mesh will make a good structure. Has anyone else fixed rust holes on their frame?

BillGrissom · #2 04-18-2016, 04:09 PM

Not sure exactly which areas we are looking. I doubt the car would just fall apart. The whole assembly w/ floors and roof adds structure. More likely, high-stress attachment points would detach first, like at the lower control arm pivots or K-frame attachments. In general, if the rust is fwd of the front wheels or aft of the rear wheels, it is not very important structurally, until you are in a severe accident. The front just holds the radiator, headlamps, and fenders.

I don't think fiberglass would help much. for one thing, it won't bond securely to rusted steel. Some special cars had a structure of fiberglass epoxy over steel that turned out as horror stories, with the steel underneath rusting out. Instead of protecting, I understand that fiberglass held moisture against the steel. I saw on a show like Wheeler Dealers. Perhaps TYR, Delorean, or such.

panZZer · #3 04-18-2016, 04:19 PM

There is no frame on a benz.
they are unitized construction.

You might say channel structure on the bottom of the "Box" has rusted thru,

and if there is that much showing there is plenty more festering.
cars like this are commonly called winter rats up yonder in yankee land.

Zacharias · #4 04-18-2016, 05:02 PM

Fiberglass patches, POR with POR mesh, are not suitable repairs for the areas you indicate.

Assuming there is no state/provincial underbody inspection where you live, you can continue to drive the car that way for a while.

Unless you know someone with a welder who (a) knows that they are doing and (b) works VERY cheap in their spare time, the rust I see in those pics is not worth trying to fix. Even if (a) and (b) exist for you, I'm not sure it would be worth the trouble.

These are very old cars now. They turn up with rot even from fairly "dry" states.

Hate to further pee on your parade, but if the car is that far gone in some places, then you should have a look at what is happening with the floors. These cars came with a very thick undercoat from the factory. People have literally started scraping and found that was pretty much all that was holding their seat in place.

dude99 · #5 04-18-2016, 05:29 PM

I agree, If I were you I would look for a cheap gasser w126 and throw the diesel into it. The gasser ones (at least by me) go for really cheap, are almost always low mileage, and usually have better options\ are in better condition than the diesels. Not to mention you then also get the 2.47 gasser rear end which is sweet.

panZZer · #6 04-18-2016, 05:51 PM

look on the bright side, its got maybe $200 in aluminum you can scrounge ,if you love disassembling everything. the rest of the heap is worth ....$120

or-the motor and tranz might go for 6 -700$

So one of your little mistakes, we have all done it.
Welcome to the old benz world, I would turn around and rethink before getting another, ( and I live in a part of the US where there are lots of forsaken solid ones)

BillGrissom · #7 04-19-2016, 03:49 PM

I would just keep driving it. Probably worst that would happen is that it would sag down and drag on the road, which would be the time to scrap it. Primary safety concerns are that the steering linkages are solid and the brakes work.

I doubt one could quantify any loss of "crumple zone" safety from that rust. It would take many controlled tests and the rusted cars might even come out ahead. But, an extremely rusted car could be compromised. Look for the crash video by a safety group pretending to support the claim that new cars are safer. They run an offset head-on crash between a ~1990's sedan and a ~1961 sedan. The newer car slices right thru as half of the old sedan disintegrates in a cloud of red rust. Seems opposite the "crumple zone" idea. Even appears the old sedan didn't have an engine or frame rails. Can everyone say, "staged"?

leathermang · #8 04-19-2016, 03:57 PM

I agree with all the posts...including Bill's First post, but not his last....
Some things can not be fixed... particularly with your ' low budget' criteria...
this is one of the main failings of unitized construction....
and MB did not do a great job of Rust proofing their cars... even building in some particularly bad places which , if not watched carefully, encourage rusting... the water drain at the back side of the hood, the area behind the battery holder, etc...
Pull the good stuff.. send the rest somewhere to be recycled....
Safety first.