30A Strip Fuse for Blower? - PeachParts Mercedes-Benz Forum

Chris W. · #1 10-08-2011, 03:48 PM

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

sixto · #2 10-08-2011, 03:53 PM

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

vstech · #3 10-08-2011, 09:22 PM

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!

Billybob · #4 10-08-2011, 11:50 PM

Quote:

Originally Posted by vstech

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!

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

Billybob · #5 10-09-2011, 02:14 AM

Quote:

Originally Posted by sjh

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

Quote:

Originally Posted by sjh

It seems implausible but it may carry more than 30A before it melted.

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.

Brian Carlton · #6 10-09-2011, 10:04 AM

Quote:

Originally Posted by Billybob

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".

Chris W. · #7 10-09-2011, 10:17 AM

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

Brian Carlton · #8 10-09-2011, 10:31 AM

Quote:

Originally Posted by sjh

Ergo, the advice. Thirty (30) plus amps at 12 volts is 360+ watts. Take four (4) 100 watt light bulbs (about 10% light & 90% heat) and turn them on. Sense all the heat they put out. Concentrate all that heat in a wire, in a car, buried under a dash, etc.

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.

layback40 · #9 10-14-2011, 03:03 AM

Quote:

Originally Posted by Brian Carlton

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 can just see it now, all the members on here that are at engineering grad school are running of to their supervisors with an idea for their Phd thesis !!

scottmcphee · #10 10-09-2011, 11:39 PM

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.

Jeremy5848 · #11 10-10-2011, 11:40 AM

Quote:

Originally Posted by scottmcphee

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.

Having actually measured the W124 blower motor current, I can finally add something to this overly-long thread. The "porcupine" speed controller takes a signal from the push-button unit (PBU) and limits the current to the blower motor based on that signal. The motor current varies from 5 Amps to 28 Amps. The controller, which appears to be a Darlington pair of power transistors living on a big heat sink (the "porcupine"), acts as a variable resistor under control of the PBU, limiting the current drawn by the blower motor.

I've never been able to solder aluminum and am surprised the OP was successful -- but I guess the results speak for themselves. In any case, the rating of the repaired fuse would be no more than 30A so it should be OK until a new replacement is found. As was previously mentioned, I've also had trouble with aluminum strip fuses oxidizing, so I use a little of the paste that electricians use when joining copper and aluminum conductors.

Jeremy

Chris W. · #12 10-10-2011, 07:32 PM

Here is the fuse "as soldered". I've had the blower on high for quite a while the last couple days, no issues.

Local dealer did not have it in stock, nor did NAPA, so I'll order one...

Rgds,
Chris W.

ah-kay · #13 10-10-2011, 08:59 PM

Quote:

Originally Posted by Chris W.

Here is the fuse "as soldered". I've had the blower on high for quite a while the last couple days, no issues.

Local dealer did not have it in stock, nor did NAPA, so I'll order one...

Rgds,
Chris W.

This was what I did. I had the blower running for weeks before I replaced it. The fuse is not aluminum, it is an aluminum look alike alloy. It will solder. Go to PNP and there are plenty.

gskurka · #14 10-14-2011, 12:45 AM

FWIW, when my oxidized fuse blew, I used the "TLAR" method to fold up a suitable piece of Reynolds aluminum foil until I could get a calibrated replacement and a few spares at the dealer. I'll go to Fastlane in the future.

layback40 · #15 10-14-2011, 02:56 AM

Quote:

Originally Posted by Jeremy5848

Having actually measured the W124 blower motor current, I can finally add something to this overly-long thread. The "porcupine" speed controller takes a signal from the push-button unit (PBU) and limits the current to the blower motor based on that signal. The motor current varies from 5 Amps to 28 Amps. The controller, which appears to be a Darlington pair of power transistors living on a big heat sink (the "porcupine"), acts as a variable resistor under control of the PBU, limiting the current drawn by the blower motor.

I've never been able to solder aluminum and am surprised the OP was successful -- but I guess the results speak for themselves. In any case, the rating of the repaired fuse would be no more than 30A so it should be OK until a new replacement is found. As was previously mentioned, I've also had trouble with aluminum strip fuses oxidizing, so I use a little of the paste that electricians use when joining copper and aluminum conductors.

Jeremy

It is not that hard. Wouldnt bother with a fuse though.
Some time try it.
You need to clean the Al with a scotchbrite, not steel wool.
a good coat of Vaseline & use it as a shield. It smells & smokes a bit but it works & is quite strong. You get good electrical connection.

Many fuses are copper with a very thin silver or tin coating.