Does anyone have the part number for this? I plan on stopping by the dealer and asking if they have one in stock. The parts guy is so young he's probably never heard of one...

BTW, I just soldered mine as a temporary fix, mine had a mechanical break. It had broken in the middle and my son suggested trying to solder it, so we did. Burnished the broken ends put a drop of flux on each side and went for it. Worked like a charm.

Rgds,
Chris W.
'95 E300D, 351K

__________________
Objects in closer are mirror than they appear.

000 545 20 34. $0.67 at FastLane. All 124s use that fuse so I'm sure it's a stocked item. Be ready to spend $7.50 at the dealer.

How does a charm work?

Sixto
87 300D

while I'm sure the dab of solder worked to get the blower going, you have caused a possible fire problem with your car... I'd not use the blower until you have the correct fuse in place!

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Wouldn't the original parts of the fuse still function as designed? There would be no increased risk of fire, the remaining parts of the fuse would simply function as the weakest link in the chain and melt through at the current overload as originally specified. The cross sectional area of the fuse's metal and the metalic properties determine the current rating of the fuse, as long as some part of the original cross sectional area of fuse material exists in series with the current flow the circuit and load are protected.

In fact the solder may actually melt at some lower current threshold and act as a fuse with a reduced rating

Quote:

Originally Posted by sjh Only way you'd know how his newly constructed fuse worked was by testing.

I'm going to assume that you are familiar with the fuse in question, and know that it is essentially a flat strip of a particular alloy with particular cross sectional dimensions the result of which it acts as an over current safety device that melts when a current above which is rated flows through it. Seen here:

http://catalog.peachparts.com/ShopByVehicle.epc?q=1990-Mercedes--Benz-300d-Body--Electrical&yearid=1990%40%401990&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6178%3AED%7C10000095%3AMBC%7C1501%40%40300D&catid=241959%40%40Body+Electrical&subcatid=241971@@Fuse&mode=PD

In this case the metal strip fractured in or about its middle according to the OP, and it was soldered at that fracture point to restore both the mechanical and electrical integrity of the metal strip.

In the event that the fuse was subjected to a current flow above which it was originally rated the original un-soldered metallic strip on either side of the solder joint would melt the same as it would if it was in original condition thereby creating an open circuit.

I would postulate that it is inexplicable rather than implausible, as to how given the circumstances the fuse in this case could possibly carry more current after the repair as described, as it would have before the repair. I would be very interested in hearing about and understanding the metallurgical theory that would cause the alloy in question act differently, all other circumstances being equal.

Here is a postulation of a possible outcome that is not in agreement with your conclusion:

The one variable for which you are ignoring is time. A fuse does not melt instantly upon application of the rated current load, or a load that is slightly above the rated current. The fuse must gradually heat to the point where it will melt the alloy. This will always occur directly in the middle of the fuse, because the edges of the fuse are, effectively, a heat sink.

When one solders the fuse in the middle, the remaining strip will be closer to the heat sink, and, therefore will melt somewhat later than the original fuse with the same current applied.

Note that the assumption is that the solder joint melts at a higher temperature than the strip fuse. If incorrect, the entire discussion becomes moot.

Note, additionally, that the delay in the time for melting of the remaining strip is probably insignificant with regard to the protection required for the device, however, it is noted to correct your conclusion of "inexplicable".

I suspected when I posted that this discussion might ensue. My logic was akin to Billybob's, i.e. that the fuse would still function as intended because there is plenty of fuse of the original cross section remaining.

My break was mechanical, so I decided to "risk" the safety issue until I could go to the dealer early next week. For the record the break was just about dead center. I also know that my blower motor is in reasonably good shape from having R&R'd it not that long ago, so not likely to seize - admittedly one never knows however.

To test Mr. Carlton's theory I should buy several fuses and test them vs a control fuse after artificially breaking or cutting the fuse and soldering. Except that I don't have a handy way to put a controlled amount of current through the fuse...

Rgds,
Chris W.
'95 E300D, 351K

__________________
Objects in closer are mirror than they appear.

The heat is only generated in a light bulb due to the very thin wire and the resulting very high temperature due to the current in the wire.

A 60W bulb has only 1/2 amp, and yet it generates significant heat.

A fuse accepts 30A while remaining below the temperature threshold required to melt it.

Two completely different concepts.

The postulation is not presented with any thought or request for confirmation..............

50 cents will get you a new one at NAPA (Napa part no. ATM N171252) - when my 30A blower fuse blew out, dealer wanted about 7 dollars and tax (no way) - NAPA usually has it, got a spare and a spare 80A too. I keep a selection of fuses in a little box which i stash in the middle console.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Quote:

Originally Posted by Brian Carlton Here is a postulation of a possible outcome that is not in agreement with your conclusion:

Perhaps you should reread the previous posts then attempt to formulate some cogent response to the issue at hand rather than the "theoretical time differential' straw man argument you've barely managed to qualify.

The issue related to "inexplicability" was put forth here

The argument that the fuse material offset “directly in the middle of the fuse” will melt at some time greater than material “directly in the middle” while in theory is not an impossibility, any difference would be infinitesimally small and in practice completely insignificant as any such difference would easily be within the fuse’s operational tolerances +/- %.

More importantly this “straw man argument” does nothing to explain how the original fuse modified by reconnecting its two pieces with solder could be capable of carrying a greater current load than it would have originally been capable of carrying which was the “implausible” claim.

The arguments that the soldered fuse is any more of a fire hazard or any less capable of operating as a over current interrupter are ridiculous and only demonstrate a complete lack of understanding of basic electrical principles. If anything it is most likely that the solder material has a lower melting point than the fuse alloy.

My contribution to this thread was to dispute a moderator’s opinion that as a result of the fuse modification “you have caused a possible fire problem with your car”. , and the opinions of anyone else who mistakenly agreed with this incorrectness.

Brian chimed in and offered his “contribution” for reasons best known to him!

With regard to the concerns of any “heat” produced.

First the fuse is located on the wheel well in its holder not in/under the dash. In fact the 30A strip fuse for the ACC blower is design upgrade that was developed after the original fuse scheme was discovered to be inadequate to the task early in the 124 chassis evolution. The heat produced as a byproduct of work is produced almost exclusively at the electric motor and at its controller located underneath it, both cooled by the flow of air when the blower is running. There is very little if any measureable heat produced within the wiring connecting the loads to their power source. The overall resistance of properly specified wire is very low for any given load demand.

A fuse operates by “failing” in series, it is one of the most simplistic and therefore dependable circuit elements, damn near fail-safe if properly implemented, other unrelated causes of short circuiting not withstanding.

Perhaps you should read your own post and try to have some understanding of your own writings before you ignore a valid and absolutely accurate refutation of your own misleading info:

Although you might think, in your small mind, that your contribution was to simply refute the moderator's conclusion, what you actually put down on the forum was completely different.

Quote:

Originally Posted by sjh What I'm saying is extremely simple and it's becoming WAY to complicated.

You're right it is extremely simple, but I don't think that anyone is arguing that under the extremely unlikely "if the fuse doesn't blow" scenario there is not a potential for bad things to occur. Or that bad things can happen in “unfused” circuits.

The original issue raised by the moderator and it seemed supported by your earlier statement that the repaired fuse was potentially less likely to blow and protect the circuit than the original unrepaired fuse.

Your original statement seemed to imply that the repaired fuse in this case had some greater propensity to not blow than as originally designed. I've offered my explanation as to why that position is mistaken, that explanation has not been refuted (despite Brian's tantrum) and you have not offered any explanation to support your original position and it seems you've in fact wisely reversed your position as to what would likely happen.

These aluminum strip fuses oxidize quite a bit over time (that white powdery chalk coating) so the material left carrying current gets thin and effectively lowers the rated value of the fuse. When the rating gets to about what is needed by the blower motor the strip will heat, and on/off cycles will stress and crack aluminum, eventually causing failure. Even when your fan motor is fine and not drawing too much current and is operating normally, the fuse can "blow" this way.

On a related note I'd think operating the motor on any setting might draw the same amount current draw, so operating on low with a tender fuse is not inherently more safe. As a DC motor some dummy load in the circuit is needed to shunt power away from the motor to make it spin slower. The total load on the circuit (fuse) might be the same at any setting. Never measured the theory on this fan motor tho.

__________________
Cheers!
Scott McPhee

1987 300D

I am by no means an electronic expert, however, I doubt any manufacturer of fuse strips would authorize soldering a broken fuse to handle the same current as the undamaged one.

it's generally the entire fuse element that "handles" the current it's rated for.
adding anything to the fuse, especially if it's broken in the center will certainly alter it's capabilities to open at a certain rating.
if a fuse failed, even if it LOOKS like a vibration break, it's best to install the correct fuse, not mend the broken one. it may have failed as a break due to near limit draw, and vibration from driving caused the break... I'd not chance burning down my car with a mended fuse.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!