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Fuse Box removal / Fuse box leak :((

Started by Alex, 09 July 2024, 09:19 AM


Oh dear,

I was going really good, I had recently done a few odd fixes here and there, added a simple bluetooth audio dongle and whatnot. But it seems I ran my luck dry. And I mean really bloody dry haha.

My RHD SWB model has a leak. On the right hand side of the car, water makes its way down beneath the carpet, and into the rear. I narrowed this water leak to be from a small rust hole beside the fuse box. (This seems to be a common issue amongst w116s). Now the issue isn't the rust on the interior of the car. (I can fix that easy, its all pretty minor)

The issue is the stupid fucking fuse box and the narrow as fuck area the hole is in.

Ill chuck some photos in to explain.

My dilemna is.

I have a leak, water makes its way in near the fuse box. Can I fix this hole without removing the fusebox? (Wirebrush -> Fibreglass -> Paint)
Or do I have to remove the fusebox? If so, how? I need to at least remove the rust from the hole before painting right? Or is there paint I can paint over it with?

IMG_6540 (1).jpg
The hole in question, after some wirebrushing. (I may need a longer bit extender)

IMG_6541 (1).jpg
Close up.

IMG_6539 (1).jpg
The cross member attached to the fuse box on the other side of the firewall. How do I remove this or work on this?

I found this post:
Quote from: jbrasile on 10 July 2011, 02:43 PMYes... not a fun project...

After taking off all the covers under the dash you need to remove a metal bar/plate that is perpendicular to the firewall and is attached to the two lower screws that hold the fuse box to the firewall itself and to the front of the dash. Once that is out the way  untie the harnesses, undo the two top fuse box nuts and you should be able to move it out of the way for for your leak repair. I strongly recommend that you do not remove the fuse box completely since there will be a lot of wires to reconnect and you run the risk of getting something wrong.


Is this advice apt? Can I remove the (appears to be 3) bolts on the firewall side of this member, then shimmy the fusebox out a bit with the cable slack?

Cheers for any advice guys :)
Tryin to keep my head up after this absolutely shitstorm of a leak.
77' 280SE Classic White (198,000km)


Indeed this is a super common failure point on the 116 unfortunately.  As part of your fix you need to fashion a drain hole and tube at the lowest point.

There are multiple posts here on how to tackle this one on the Forum.  Do not remove the fuse box completely as it is apparently a nightmare to try and reconnect all the wires properly. Lifting it out of the way as much as you can to effect the repair is prudent.
1977 450 SEL 6.9 - Astralsilber


Miracle paint is your best friend there. No need to remove the whole fusebox, just separate it from the firewall.
Plenty of videos on that subject/product on
I've use it successfully many times in the past on w116s and others.


The W116 is more prone to  leaking on the better check there too. My theory is that it is related to what side of road the cars are parked when left outside in the rain in the streets unloved etc.


I tore apart half the car, I'm at the rust hole, just gotta patch it up. The water ingress also did a good number on the floorpan (rust-wise).

The foam mat insulation stuff is completely fucked, all soaked and gone. Any thoughts on replacements? I was thinking just some generic foam insulation is probably fine, just cut to size and cut a bit for the cables and ducts that run across it.

Anyways, heres some pics.

after a bit of grinding.

the hole from the inside


all in all, the hole is not that substantial, and the rust it created is mostly surface. What is substantial, is the fuck ton of things I had to remove to get to this stage. That fusebox is like the heart of the car, what on earth were MB thinking!??
77' 280SE Classic White (198,000km)


Hahaha yup we old time 116ers know that the engineers took the fuse box and placed it in the worst possible place, and then added no drainage to boot.  Brutal oversight making nearly all 116 rust here.
1977 450 SEL 6.9 - Astralsilber


Quote from: daantjie on 10 July 2024, 09:21 AMHahaha yup we old time 116ers know that the engineers took the fuse box and placed it in the worst possible place, and then added no drainage to boot.  Brutal oversight making nearly all 116 rust here.


On another note, tomorrow I will add some drain holes after I patch the hole, any thoughts on where they should go?
77' 280SE Classic White (198,000km)


"Best" spot must be the lowest spot of course otherwise it won't all drain out.  You need to get it right at the firewall/bulkhead where the fender dips down, right in the corner pretty much.  It's a prick of a location to work in and likely best to remove the hood completely or just unhook and remove the hood spring and hinge there, not sure how feasible this would be but it will give you more room.  You need to bring the drain hose into the wheel well so it can drain out.  Also be sure to use a large enough hose so it does not easily get plugged with crud.

"Fun" job...
1977 450 SEL 6.9 - Astralsilber