1982 300SD Fuse Wiring Mystery - PeachParts Mercedes-Benz Forum

bustedbenz · #1 04-02-2023, 08:19 PM

Hello, all. Very "long time, no see" but I am going to try to get back involved here again.

I'm working on a 1982 300SD (w126) and trying to clean up a mess in the screw down connections under the fuse box. Some of it is my own fault (took a few wires loose in order to get access to install the blower motor relocation kit and forgot where they go back so now I get to trace those) but I also suspect previous mechanics of getting things a little bit wrongly as well.

I may have some additional questions but my first one is this. Can someone help me understand what's going on with fuse 15 and fuse C being linked together? C is for the electric aux cooling fan for the engine and 15 is for door lights among other things. They aren't the same amp rating. But these fuses are connected together at one end of the terminals by a metal strip that has been screwed down to bridge the two circuits together. There is *nothing else* connected to the terminals that has the crossover link screwed into them.

This would "almost" make sense to me if they were using one high current capacity feed from the battery to power both circuits by screwing down to one and crossing over to the other... but neither of these has anything screwed down to it at all except that bridge. Which means that any 12v feed would be coming from the other end, which would mean that the low amperage fuse for circuit 15 would be in the circuit with the higher draw of the aux fan which makes no sense at all. It would basically be limiting the current of the aux fan to the lower of the two fuses if I'm not mistaken and that can't possibly be what was intended.

Any thoughts on what I'm seeing here, bearing in mind that tomfoolery by others working on this car in the past is a strong possibility? Why would C and 15 be bridged together at one end under the fuse box and how does this circuit actually work if it was intentional?

280EZRider · #2 04-03-2023, 01:53 AM

Bridging two circuits of different amps is pure buffoonery, something that only someone with the mentality of Marjorie Taylor Green would come up with. This can't possibly be a MBZ mistake. I would start by separating the bridged circuits and go from there. My guess is that one of the two circuits is lacking power, so the guy encharged with remedying the problem couldn't do so and simply bridged the dead circuit with one next to it.

bustedbenz · #3 04-04-2023, 10:32 AM

Thanks. That's about what I figured, but it's a very factory looking bridge. It's a metal strip fuse that fits perfectly for the screw spacing and jumps the gap from one fuse to the next; if it was a bodge job I kind of figured it would probably be more like a loose wire or something so I was just trying to consider every possibility. Does anyone with access to the service manual know if there is a page or pages that shows the wire colors for every fuse? Or do you have to look them up circuit by circuit?

I did buy the genuine FSM cds for w126 back in the day but my original CD's are lost in a box somewhere from multiple house moves and for some inexplicable reason, when I copied them onto my hard drive for safekeeping, I only did disc 1 and everything I need to sort this out is on disc 2.

I'm slowly working my way through the fuses I've got to get wired back right by looking for voltage and then applying 12v through my multimeter to check current draw (and provide a safety factor) and then seeing what circuits come online, but I'm working alone so it's kind of hard to tell what's going on sometimes. Having a color chart would really help at this point since it doesn't appear you always match color to same color on both sides of a fuse. (Demonstrated by testing... the bundle of wires for the load side of the instrument cluster has some of the same colors in it that are attached to the next-door fuse to that circuit.)

Clemson88 · #4 04-09-2023, 03:02 PM

The FMS doesn't show any bridge between fuses 15 and c, as was expected.

I want to see a photo.

bustedbenz · #5 04-11-2023, 04:34 PM

Sure thing! It's something else...
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bustedbenz · #6 04-12-2023, 03:35 PM

I have mostly figured out my mystery wires (the only one left that I don't know which terminal it belongs to is a plain black one with no color trace on it, just solid black... I'm suspicious it might be the auxiliary fan, but I can't tell, because when I put 12v directly on that wire, nothing comes on. The fan must be relay controlled and as yet I don't know which relay to jump to engage it.

But I'll worry about that later. My immediate new question is this.

Fuse 12 doesn't have a wire screwed to EITHER end of it... and yet the bottom leg of the fuse holder for 12 (the end that is mounted solidly against the plastic fuse box, not the springy end that you push back with your finger to insert the fuse) is hot with key on even with no wires attached at all.

Is that normal? Do these fuse boxes have a "power rail" in them which powers multiple circuits with one common feed from within, as opposed to every fuse having a feed wire at one end and a load wire or wires at the other?

Which ones share this common feed, if it exists? It's not the whole box, of course, because when I swipe the test light down the row, only some of them light up.

I know which load wire connects to 12 on the load side now, by testing each one until the instrument cluster came to life, but where's the supply voltage coming from with nothing connected to either end at present?

bustedbenz · #7 04-12-2023, 04:09 PM

One more question and I'm done with this project.

Does anyone know what this black wire would be for? It isn't hot with key on engine running, or with key off. So it's either a supply that only gets supplied when a relay or switch somewhere in the car is engaged... or it's a load.

I've only been working on the right hand half of the fuse box (as from standing in front of it, so driver's side = right in this sentence), so in general it should only belong to fuses 11-16, C, or D since I didn't disconnect anything to the left of those positions.

We have to consider the possibility that it was formerly a feed wire for either 15 or C and the wire got broken somewhere inaccessible and the bridge between 15 and C was someone's poor solution to the problem. But without the manual I can't be sure. Do any of these circuits have a solid black single wire?