Newbie W202 Burnt fuse box

[Andyboi]

Have a 1997 W202 C180 Classic facelift M.111 with electrical fault. Wife was driving and told me she smelt burning in the boot. Had a look and the small fuse box in the boot was melted. Car still driving although has low rev problem and wants to stall. Ok when over 2000 rpm. Can anyone tell me what could have caused this and why it still runs!!
See attached picture.

 

[BlackC55]

I would guess at whatever the consumer of fuse 3, 4 or 5 is.

 

[Andyboi]

Thanks BlackC55, I had the fuel pump off and checked, runs fine. Have sourced a used fuse box and will replace when it arrives. Just can't figure out why the fuses did not blow.

 

[Dec]

 

[CabrioDave]

The box melted because of heat. Fuses themselves don't normally get hot - even high-current ones such as 50 amp.

If they are asked to carry unusually high current (because of a fault in the circuit somewhere) they blow. For a brief moment, the insides of the fuse get hot enough to melt. - but that's a tiny bit of metal, for a brief moment - not enough to affect the temperature of the holders and the plastic box. So it's not the circuit, or consumer, that's the problem.

Fuses DO get hot when the current meets a resistance at the fuse - in other words, when the fuse is not contacting properly with its holder. This might be because a fuse hasn't been pushed home fully, because the clamps are weak, or - most likely - because of some corrosion where the metals touch. As soon as a bit of corrosion begins, the point of contact starts to get warm ... and that encourages more corrosion.

If ever you find fuses getting warm, clean up their contacts.

 

[Andyboi]

Thanks CabrioDave, fully understand your comments. Must admit this is the first time I have ever looked at this fuse box and this could have been brewing for some time. Will ensure the replacement is clean before installing.

 

[Ted]

Just like to say that imvho Cabriodave is bang on here and has explained the most likely scenario better than I ever could.