zygorepma

New guy here, I have a 1992 420 SEL and under my rear axle there is a bosch compressor of some type and i cant figure out what the hell it does...
Any ideas ?
The manual doesnt say anything about it and I called the dealership to ask them what the hell its for and the guy was like You have what on a what? He had no idea either kept teling me to bring it in.
(Sidenoteealership is 1 hr 30 min away)
It doesnt do anything cept hiss when im idling and I dotn have any idea what this thing does and whether or not the hiss is good or bad.
Any thoughts would be appreciated.

A-

Hattaresguy

First how did you get your hands on a 1992 W126? Is it a South African made car that found its way to the US? I think the W126 was made in Africa until 1994.

Anyway could it be the fuel pump? This is nothing air powered back their on a W126. The vac pump is in the spare tire well thats the only air pump that is back their.

zygorepma

I bought it in texas and had it shipped here to Boston.
From what i the guy i bought it from told me it was one of the last rolled off the line for over here. But i dont know im not the original owner and neither was he.
The title shows it as 1992 but the manuals all show 1991.
Anyway the "thing" seems to hiss only when im idling and then once its off it hisses for a couple secs and then stops.
Is located directly behind the passenger side rear wheel going inward. It mounted directly under the axle and as i said before there is no mention of this "thing" anywhere in my manuals.
also its half enclosed by a thin sheetmetal guard and i followed the lines from it and it doesnt go to the fuel tank nor the is it the vac pump.
Im stumped on it. Ill try to post some pics of it soon.

thanks for the thoughts.

electric blue

Think you hit it right on the nail. Must be the fuel pump hissing. Does the 420 have 2 pumps, I think it does. He's gonna have to open his wallet a bit to have them both replaced.

zygorepma

Hey guys, thanks for the thoughts on the "thing" under my car.
I went out and drove it to a mercedes shop and ummm oh yeah its my fuel pump.
Apparently the lines for it got knocked loose somehow (i.e new england pothole season) so they fixed it and im good to go.
Thanks for helping me out on this guys.

A-

suntubi

Yup the 420's have 2 fuel pumps. My pumps don't hiss though. There is a slight whine which is OK

braverichard

While on these fuel pumps, does anyone know if the W126 500SEL also has two fuel pumps? I can't remember from the last time I was under the rear axle.

golfboy

It's definately a fuel pump, if you crawl under there and look, there are two (twins), one mounted right above the other. Mercedes does some of this redundant pieces so that you don't get stranded.
The little metal plate is protection from road debris.

Oops... I guess I should have read the last postings on this thread... a little late huh?!?! lol.

golfboy

Yes. My father-in-law owns a 500 and it has the twin pumps too.

braverichard

Thank you!!

EuroCoupe

What year 500? Pretty sure pre '86 500's have single pump setup, at least I'm pretty sure mine did.

braverichard

1984 Euro Spec 500SEL

meowy84

Not so on the pre-'86 single setup....unless mine's an exception to the rule. I got an '84 500SEL (but says '83 on title) without the hydro-thing-a-ma-jig suspension and have the 2 canisters as well so it couldn't be anything for the suspension. I didn't realize these cars had 2 fuel pumps.

Any idea how to tell (other than crawling under there with a volt/ammeter and testing it) if one of the pumps go so I can fix it before a second one goes?

braverichard

I believe the pre-'86s also have two fuel pumps. I just don't remember since it has been a long time since I looked under my rear axle... the last time I did so was several months ago when I changed the differential fluid. Anyway, when one of the pumps goes bad, you will hear a loud buzzing sound instead of the hissing sound you are [hopefully] hearing right now.

meowy84

Buzzing vs. hissing eh? Hmmmm....Gonna have to tune my hearing aids. heh heh...

braverichard

Oh trust me, your "out of tune" ears will recognize the difference. Right now if yours is hissing (which I hope it is), then it should sound like an air leak. Buzzing sounds more like ... a shaving machine... you know, that sort of electrical device. You know what I mean. LOL

meowy84

Well if you put it that way then it's definitely hissing.

If one of the pumps does go out does the second one come on automatically or....? Unless of course they work together simultaneously.

braverichard

I think they work simultaneously

meowy84

Thanks.