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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



Oh dear, this was quite the job. A solid 2-3 days haha.
My disassembly effort was hindered by the slow removal of the centre console, this was due to me not removing the passenger seat, which I do not recommend. (my carport is too narrow to open both doors comfortably). This made the removal of the centre console really fucking annoying. Do not do this!

Regardless, I removed the majority of the interior, seat, wheel, floor mats, covers, kick panel, accelerator, and the fuse box  ::) .

Here's the hole after some cleaning, it did prove to be larger than expected.

The floor pan was subsequently wet for a few years, and this rust shows that haha. (I grinded a small segment)

Grinded the floor pan, sprayed the pan, cleaned and all nice, then resprayed with rust converter primer stuff.

I sealed the hole with some fibreglass and sprayed over it. I then drilled a new drain hole.

To get to this stage, the main hindrance was the annoying location of the fusebox and the hole. To remove the fusebox, fuck me- remove everything basically, you can get away with keeping it all attached, just removing the crossmember between the box and the dash is a real pain in the tits.

Grinding the rust off in this tight area was done with a 300mm drill bit extender, and a drill, with a tiny wire wheel on the end. Does this work? Of course! Is it easy? Hell no!

The floorpan was all angle grinded, that was easy.

To fibreglass, I was lazy and kept the majority of the cables in the way, simply tying them off and working around them. Is it easy to do? No. But it is possible and saved a lot of further disassembly.

Fibreglass is best recommended for a decently large hole, but it is godly difficult to use in such a small space, bondo or bog could be used for smaller holes, but I can't attest to the structural rigidity of that.

Sand that fibreglass down, spray and you are all good and off to the races. Just don't forget a tiny hole for water egress! I found this hole hard to drill from the top down, so I drilled from the wheel well up (you can see where the firewall meets the inner fender easily from beneath.

Reattach the lot, ensuring not to forget the fusebox grommet! And you should be good!

I had to rebuild my floor insulation, because it was solemnly buggered.

I chose to rebuild, as being in Aus, parts are hard to get, let alone a replacement may also be in shoddy condition due to age. I also just don't really care that much, as you can't see the insulation.

I looked for generic insulation foam in multiple hardware stores, but couldn't find anything suitable. So- floor mats!

This stuff is great, piss easy to cut with a stanley knife, and cheap as dirt.

Roughly marked that up, glued to layers together for thickness (in hindsight its a bit too thick and raised the carpet up a few mm making it harder to get my feet into the pedals, not a huge deal- but I think for future followers, its best to be too thin than too thick).

And eyeballing the measurements for the handbrake cable and vent, that's my fitment. Looks pretty good in my opinion. And it is already waterproof due to the plastic nature, nice and soft, insulating, and dirt cheap!

All in all, real pain in the ass. The skills needed aren't anything crazy, I'm no bodywork wizz, but it is a lot of time to do the job. (Depending on the size of the rust damage).

Cheers gents.

P.S. Is the fusebox leak a kind of inauguration into the w116 club?
77' 280SE Classic White (198,000km)


Well done! Must have tested the patience haha
1975 280S - Australian delivered