Melting fuse?
 

Hello -
My car has melted the plastic part of fuel pump fuse a few times this summer - so I thought I'd post a question about it. See the photos below for more detail. It's always in the same manner - the plastic at the bottom of the fuse gets hot enough to melt, thus shortening the fuse so that it doesn't fit snugly in it's socket, thus current won't run through it, thus my fuel pump won't run. Please note that it's the plastic melting - none of the fuses have ever blown. I've only had problems at start up, it's never quit while running.

It's only happened w/this particular fuse. I've looked over the socket and it's almost as if the hole at the bottom has become enlarged to the point where the fuse sits too deeply, almost allowing the plastic of a new fuse to touch - or get close to the bottom of the socket.

Is this an electrical catastrophe waiting to catch my car on fire? Hopefully that's not a stupid question w/an obvious answer. I've assumed that since none of the fuses have blown, that there's no voltage spike happening.

Thanks,
Tom

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Strange, but I found the same thing on my '74 this summer. Symptoms were that the taillights and brake lights would fail intermittently. I replaced the fuses and it hasn't happened again.

it's usually a sign that your fuel pump is on it's way out. It's drawing too much amperage.

Also likely poor connection at the fuse terminals from the extra heat, and relaxing of the springiness of the terminals.

And try to find some ceramic fuses if you can.

Funny, we sell pre-packaged replacement fuses for 911/912/356. Original replacement style like the one in your picture. Our fuses look the same as yours but the colored body is made of ceramic, not plastic. Fuses do get hot and this is a 25 AMP circuit. The true ceramic fuse will not melt like in your situation. Maybe the Chinese have pulled another fast one?
Regards,
Charles

The same thing happened to me. That part of the fuse box was old and brittle so I wired up a new "outboard" fuse w/ pigtails. It was that fuse holder that eventually started melting. I replaced the pump and all was well. This was maybe 60,000 mi ago. Now I am starting to get the same thing happening again. I moved my outboard fuse to another part on the box where I am not using the circuit. I think it was the AC, which I have removed.

A mechanic friend suggested the wiring itself could be adding excessive draw (is it resistance?) to the circuit and he suggested running new wiring directly from the battery, through a 25A fuse to the pump. I think I'll do it soon.

"The true ceramic fuse will not melt like in your situation."

No but the plastic or composite fuse holder base will melt or degrade and become brittle. The heat will also degrade the fuse holder contact. I agree with the possibility of the fuel pump beginning to pull excessive current as it ages. It could be also that the fuse holder clip/fuse contacts have just corroded and made a "run-away" electrical/thermal situation. To fix and/or help this problem: Clean the contact surfaces of both fuse cap and the fuse holder clips with fine sandpaper until they are shiny. Bend the clips so the apply good pressure against the fuse caps. Then coat the fuse caps and the contact area of the clips with a small dab of silicone grease or silicone dielectric grease. The Syl-Glide silicone grease found readily at NAPA stores works fine for this application.

I agree with Tyson (or course ;)) that there is something going on with the connections. A bad pump may want to draw more current, but it is manifesting in the heat generated on one side of the fuse, not evenly thru the fuse. You may want to make sure the wire is not corroded inside the sheath, or damaged in some other way. The connection at the fuse block is highly suspect when a fuse looks like this. Or, like Jim says, on one side of the fuse contacts.

FWIW, the old time VW fuses were all ceramic from the beginning. I don't have any idea when plastic came out. And, it doesn't matter that much anyway. You have a problem the a ceramic fuse would not necessarily have shown you.

"wiring itself could be adding excessive draw (is it resistance?)"

Increasing resistance will reduce current not increase it. In automotive wiring an increase in resistance is typically either broken wiring filaments in stranded wire or loose, corroded connections.

Decreasing resistance (which would increase current draw) is nearly impossible for wiring in automotive service conditions. A decrease in resistance is almost surely a short or insulation failure in wiring or a component such as a winding or coil.

A malfunctioning voltage regulator can also cause excessive current draw by raising the system voltage thereby pushing more current through all circuits. Voltage (in volts) = current (in amps) times resistance (in ohms).

Jim,

So would running a new wire/fuse directly between the battery and pump have no effect at all on this?

It will help if it bypasses bad connections or broken wire filaments in the existing circuit.
Also if a larger gage (more copper cross-section) wire is used, it will heat less (than the original circuit) if the pump is drawing higher currents due to degradation of the pump.

Hot Fuse.............
 

Check the connections on the fuse block for corrosion. To remove the wire, loosen the screw and pull the wire out. You will most likely find that the end of the wire has turned a cruddy green (corrosion) which creates a high resistance and thus heat. clean the wire and the socket. You can use a .22 caliber brass brush to clean the crud out of the hole in the mounting stud. If the fuse socket has been heated too much, simply wire in an in line fuse holder (connected to the two wires on the fuel pump relay fuse). You may find that the clinching screw for the wire(s) may not come out. If that is the case, you will have to cut the wires and strip the insullation back to connect an external fuse holder. Or, you could just replace the entire fuse holder block with an ATO style fuse holder.

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Thanks everyone for the discussion. My fuel pump is, in fact, new. I replaced the old one early this summer - maybe 2-3,000 mi. ago. I can figure the exact milage (not that it's really important) and also get the exact part number of the new pump. Is it possible that the new pump has a higher amperage pull than the old one and that's what is causing the problem? I know the part numbers weren't the same, but the shop I bought it from assured me that it would work (Porsche shop).

Being that my car is a '74, old wiring and fuse block is a given.

Thanks,
Tom

Fred - I like the idea of a new fuse block entirely - maybe this winter. I think I've got some threads bookmarked from your update. Generally speaking, the fuse block is fairly clean and free of corrosion. It seems to be in good shape. I will check it more throughly tonight and at a minimum give it a good cleaning.

Tom

Jiim,
Thanks for the explanation. I'm going to try it w/ heavier gauge wire.

Tom74,

Since you have the pn maybe you can check the amp draw w/ the manufacturer compared w/ the original. I'm not trying to contradict your source, but let's face it, there are a lot of people out there who say a thing will work that maybe doesn't really work all that well, especially in an old car w/ old wires, connections, etc.

Heck no, contradict away! I will do some PN research this weekend and see if I come up w/anything.

Thanks,
Tom

Charles: It's bad form to slyly pitch your own product when the same (or similar) product is sold by our host (part number 003481803-M44).

Scott

+ Fred. The pointy euro fuses are famous for this because the tiny contact area only needs very slight corrosion to increase the resistance to the point of overheating. While the ceramic fuses won't melt, when they get hot the fuse box can be damaged. The high resistance can also cause component failure, especially motors (like the one in your new pump). Changing to a modern style fuse arrangement (I think Hargett sell a kit) in one of the best things you can do for any older german car.
regards,
Phil

ScotB, Just trying to qualify my experience with fuses. Sorry you felt it necessary to comment and accuse someone just trying to help of having "Bad Form". Next time I guess I will keep my 20+ years of automotive electrical experience to myself.
Regards,
Charles

Charles: You could have just as easily left out the sales pitch and gone with the second part of your post, which said, "Fuses do get hot and this is a 25 AMP circuit. The true ceramic fuse will not melt like in your situation. Maybe the Chinese have pulled another fast one?" This would have been useful information from someone with 20+ years of experience.

The post in this thread follows a pattern I've noticed in many of your posts -- that they usually contain some reference to what you sell at your business. Sometimes it's just a sales pitch, and sometimes it's combined with useful information (like in this thread).

Here are some samples of your sales pitches:

Here's one where you pitch Blaupunkt radios: http://forums.pelicanparts.com/porsche-911-technical-forum/366964-becker-radio.html

Here's one where you pitch dash caps: http://forums.pelicanparts.com/porsche-911-technical-forum/359553-dash-cover-options-1989-a.html

Here's one where you pitch battery chargers: http://forums.pelicanparts.com/porsche-911-technical-forum/354382-carrera-battery-problems.html

Here's one where you pitch antennas: http://forums.pelicanparts.com/porsche-911-technical-forum/350835-what-wrong-power-antenna-81-sc.html

So, if you're going to help, then help, and leave the sale pitches for eBay (where you appear to be selling products as woodyscustomshop of Costa Mesa).

Busted! http://forums.pelicanparts.com/support/smileys/wat3.gif