Last year I cleaned the innards of my W126 (1982 300SD) fuel sender unit. It worked fine for a while, then started sticking at 1/4 tank again. Lo and behold, there's enough new crud on the center rod to block the float, even though the rest of the device is still clean.

The center rod is stainless steel (I think). When I scraped the crud last year and polished the rod with emery cloth, there were some discolored and pitted areas. It appears those areas have generated new crud consisting mainly of rust (soaked with diesel, of course). And I thought stainless isn't supposed to rust.

A little online research yielded some information about "passivation" of stainless steel. Apparently, if stainless steel is physically abraded, the protective surface layer of chromium or nickel oxides is removed, exposing iron which can then rust. The problem can be alleviated by "passivating" the metal with an acid such as citric acid.

Any metallurgists on this board who can corroborate this and provide further guidance? I don't want to clean the rod again, only to have it rust and block the float again in a few months.

Thanks.

pjc

I'm not a metalurgist, to any real extent. There are two main types of stailess steel, the 300 series and the 400 series. Some say that the 400 series is the 'true' stainless steel. Try a magnet on the part. If the magnet will stick to the part, it is probably 300 series. I think the 300 series is less resistant to corrosion. I did work as a gunsmith for a few years. There was some flooding in the part of the country (US), during that time. One guy brought in a SS revolver, that was under water for a month. It was corroded or at least stained. It didn't look good. I didn't want to spend time hand polishing the thing, and then only get warranty rate. Or probably get a denied warranty. I sent the revolver to the factory. THe customer, insisted that since it was stainless it shouldn't rust. I let the factory make the call. They polished it up, for free. They didn't warranty the wood grips though. The customer was happy. No skin off of my nose. I can check tommarow, if no one comes up with any definative anwers.
Tom

Check it with a magnet. 300 series SS is non magnetic. 300 series can look like it is rusting under a few conditions and will corrode through in salt water with a little stray current and in contact with something else that cuts off the oxygen that could get around it. Passivation could be done but it is not worth the trouble in this case (complete dis assembly), assuming that it is some sort of Stainless. Passivation removes the iron on the surface, works well on 15-5PH, 17-4PH and 18-8PH Knives and kitchen ware.

Make a new one, if you go to the trouble to take it apart go to the welding shop and get a piece of 1/8 316L welding rod and a 3mm die to put the threads on with (4-40 will also work)

Good luck getting it apart but I think that it can be done.

Later
Robert

Stainles steel rusts.

GM used 409 stainless for their exhausts, of course it rusts, so they aluminized the surface to make it shiny silver. Looks good on the brochure.

300 series stainless is the "non-magnetic" stainless, I believe that surgical tools are usually 304 stainless, high-end boat railings etc. are 316. It still has iron in it or it'd be called er, nickel. Depending on the environment, it will corrode. Surface treatments are the best way to prevent it if you are so equipped.

__________________

Gone to the dark side

- Jeff

Don't waste your time fixing the corrosion.

The fuel sender from any 1981-1991 gasoline or diesel W126 will work, without issue.

Thanks, all. I appreciate the advice.

To answer the early question: A magnet sticks to the rod. Strongly.

whunter: I hear 'ya. Trouble is, a used one might end up with the same problem. If I can do something to help the existing unit, I'd like to try that first. But I'm not sure I want to go so far as to have a new rod welded in. (The center rod is the easy part; the resistance wires look like a PITA.)

The use of 316 is more related to avoiding fatigue cracking caused by hydrogen or chloride embrittlement.
If you are interested, Chemical Engineers Handbook, Perry et al or The Handbook of Chemistry & Physics, C.R.C. have more information.
You may just have a faulty rod. The fact that the problem occurs in the same place makes one think that maybe there is something unusual in the metallurgical structure of the rod. Best just do as Roy suggests, Get another sender or rod!!

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

But I'm not sure I want to go so far as to have a new rod welded in.



I was offering the 316L welding rod as a source for a known material that would be easy to obtain and thread to replace the existing rod. Not to weld it in. It would also be difficult to weld to the plastic insulator. It can be done but it would be tedious.

Yeah, I spoke too fast when I said "welded." The top IS plastic, after all.

What I really meant is that I'm hoping to fix the problem without replacing the rod (and wires). If it comes to that, it might be better to replace the entire unit.

A pic & pull one from a junk yard will be fine.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Has anyone ever tried using a bronze rod? Or, would that cause too much electrolytic action?

__________________
Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

Funny you should ask.
I work for a supplier of stainless steel products and we do employ a
metallurgist.

Our products are 304L/316L tube, fittings, and valves.

But you already have the answer - if a magnet sticks it is
most likely not Stainless Steel. And the emery to the surface may have
removed the protective coating.

But under certain curcumstances SS can be magnetic and
the rust proof part is actually a very thin coating on the outside
that can easily be altered with emery cloth.

Interestingly, the part that makes SS SS is an incredibly small fraction
of the overall material.

Anyway - I agree, get a replacement.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

The gasoline units do not seem to have this issue = grab a used one.
My last one from the salvage yard was $5.00 ...

You are correct, the wires are a PITA...

for what it worth, even non-magnetic stainless will rust, any machining operation done to the stainless leaves a surface that will rust. for true non-rusting stainless you need to acid dip it, its called pickling in industry. we did a lot of this at GE before the EPA wars.

__________________

1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)