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

http://forums.pelicanparts.com/uploa...1189791783.jpg

http://forums.pelicanparts.com/uploa...1189791803.jpg

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.

http://forums.pelicanparts.com/uploa...1189799793.jpg

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

To skip past the entertainment part of the thread...

The heat is being generated right where it is melting. Where the fuse touches the fuse holder it isn't getting a good connection. It might be surface corrosion and it might be that it isn't getting a large enough contact area. I would look to see if it is contacting it all the way around the little hole in the holder.

This has obviously become the weak link in the chain. You also might want to check wires and what naught to see it something else is helping to expose this weak link.

Thanks Wayne -
For the info and for skipping past the entertainment. I'm headed out to the garage and I now have a focused search area. I'll report back w/my findings.
Tom

Tom,

Move car outdoors before trying the following, as the fumes can be toxic in an enclosed area. A respirator mask with charcoal filter and NIOSH rating would be a good idea, too!

See if you can find some heavy duty rubbing compound for automotive paint prep. work. It will be a heavy paste with some gritty feel to it when rubbed between thumb and finger. Apply a glob with Q-tip to the fuse holder and rotate an old, unmelted fuse -- any value, several times, back and forth, then remove and examine the ends of the fuse to see if darkened in a circular pattern from the cleaning action of the rubbing compound. If circle is found, rub off the compound and look for shiny, clean pattern on the fuse end. Clean both fuse holder and fuse, and apply more fresh compound and do the rotation of the fuse again for a couple of minutes, the repeat the remove, wipe and examine cycle again. When satisfied with the condition of the conical seating area for both ends of the the fuse ... clean thoroughly with BraKleen & paper towel, Q-tips, etc. ... until all compound residue is gone.

GBC ceramic bodied 'suppository' fuses are available from Littlefuse.

http://www.littelfuse.com/data/en/Data_Sheets/aftermarket-8.pdf

http://forums.pelicanparts.com/uploa...1165041837.jpg

Warren - Thanks for the top notch cleaning and re-seating procedure. If I continue to melt fuses, I will try that route. I inspected the fuse holder a bit more closely and it all looks pretty darn clean. The holder in question was a bit different color compared to the other fuses, but I'm sure this is due to the increased heat it's been seeing. There wasn't really any corrosion to speak of, but I sanded the clip and bent it up a hair so that the fuse sits a little tighter. I also snipped a bit off the end of the wire attached to the bottom terminal - there was enough slack in the wire and I figured it couldn't hurt. We'll see how things go for the next few weeks - I'll try to remember to check it periodically.

Thanks for all the tips,
Tom

I am experiencing a very similar problem on my 1980 SC. The fuse for the fuel pump has melted twice now. After the first instance, I noticed corrosion and melting of the wire were it connected to the fuse. I cleaned the corrosion and in the process noticed that the end of the wire had deteriorated such that the effective gauge of the wire had become smaller. Nonetheless, I simply wrapped electrical tape around the exposed end of the wire and replaced the fuse. Two weeks later, the new fuse melted again. I too was concerned that the fuel pump was beginning to fail and draw more current, but if the wire has deteriorated and the current is effectively running through a smaller gauge wire at that point then perhaps that is the cause of the overheating. I would like to replace the wire completely (it appears to only be about 6 inches long), but am not sure what gauge or type of wire to buy. Can I buy replacement wires from Pelican? I couldn't find them myself.

If that fails then I may need to look in to replacing the fuel pump, but I want to put of that $300 expenditure for as long as reasonably possible.

911hoplite,

The red wires on either side of your fuel pump fuse are 1.5 mm size; I doubt you can get a single piece of wire like this from Porsche. I suspect the wire between the fuse and fuel pump relay is the one you are referring to. Metric wire is hard to find in the US and the one on-line vendor I know of doesn't stock the 1.5 mm size. In terms of cross section (and current carrying capacity) 1.5 mm is between common US electrical wire gage sizes 16 and 14 (16 gage is smaller as the numbers run backwards in the US system). I would recommend you replace the degraded section of wire with red color insulation 14 gage stranded copper wire available at most auto parts stores.

Jim, thanks for the advice on the right wire gauge to buy. I haven't had a chance to buy new wire yet, but last night I decided to clip off the damaged end of the wire and run a clean end into the fuse (I also replaced the fuse). Same problem, though - car won't start. I also did the fuel pump test and with the ignition on, lifted the lever inside the airbox to see if I could hear the fuel pump wind up and nothing - no fuel pump sound. So I am begining to think that it must be the fuel pump. Does this sound right to you all?

I will replace the wire first since that is cheap enough but am feeling like I am going to need to spring for the fuel pump. Are there any other test I can run to help determine what the problem may be? I would hate for it to be something simple and cheap and spend $300 bones on a fuel pump I don't need.

Reworking the fuse box was the FIRST order of business upon buying my '89 almost a year ago. (Uh oh,..here comes the EE...) No upgrade mods (just yet),..just a detailed rework off everything copper under that plastic cover...I felt better afterwards, as this can be quite a problematic area, if left unattended. Inanycase,..point is:

I noticed one thing I was doing (initially) that was not good for the fuse's integrity. Looking downward at the fusebox, one sees all fuses (typically) displayed with their metal strips looking back at you, so as to easily see what's go,..what's nogo. I tended to install replacement fuses with their strips being outward. Upon having to bend the fuse end caps BACK onto the body on a few fuses, I realized that this procedure was applying pressure to the fuse cap IN THE WRONG DIRECTION. From then on, I always install them with the strip to the rear,..THEN rotate them so that I can see the strip from above. Any installation springing pressure is applied IN LINE with the fuse cap's dend (not against it).

Now I know many of you are probably saying OF COURSE, I ALWAYS DO THAT! I just wonder if they're any who don't,...how many fuse caps have been bent back on the body LOOSELY, at that? Not to mention possibly creasing the fuse element? Can't be good on connectivity....just something to think about.

Check current draw on the existing fuel pump circuit...then replace with a fresh copper (fused) path in place and check it's current draw...differences? Could quantify what's happening...

Sand the hell out of that fuse block, retension/coat ...clean/retension wire connection points at fuse block?,..looks like you've got these down.

Best of luck,...

"I haven't had a chance to buy new wire yet, but last night I decided to clip off the damaged end of the wire and run a clean end into the fuse (I also replaced the fuse). Same problem, though - car won't start. I also did the fuel pump test and with the ignition on, lifted the lever inside the airbox to see if I could hear the fuel pump wind up and nothing - no fuel pump sound. So I am begining to think that it must be the fuel pump. Does this sound right to you all?

I will replace the wire first since that is cheap enough but am feeling like I am going to need to spring for the fuel pump. Are there any other test I can run to help determine what the problem may be? I would hate for it to be something simple and cheap and spend $300 bones on a fuel pump I don't need."

911hoplite,

I doubt the problem is the wire. I'd next check the fuel pump relay (the red round one) for correct functioning. Try reseating it in the ruber base. Then jumper around the relay and see if the fuel pump runs.

Cheers, Jim

I agree. I also doubt it is the wire.

It has a new relay (I replaced it about a month ago when the trouble started.) Initially, replacing the relay solved the start problems, but now that the fuse has burned twice I am begining to think that something else caused first the relay to fail (1.5 months ago) and now the fuses (starting 2 weeks ago). I will try jumping the relay to see if that is the problem, but could this succession of problems point to a failing pump? When the pump begins to fail does it draw more current and then overheat the relay and/or fuses?

Along with the Relay check/changeout.,..do inspect the relay socket side of the equation.

Tom, I just read this thread and I was wondering if you have A/C and if it was running while these fuel pump issues occured? Last summer I had a few fuel pump fuses get warm enough to melt a little while my air was running. With the air off it was never an issue.

Testing the fuel pump current draw could tell you if the pump is even an issue. Guessing that your relatively new pump is not the problem but stranger things have happend. Chuck

Melting fuses..............
 

On my SC, the power for fuse #16 (fuel pump relay fuse) comes via a jumper wire from the + side of fuse #11. If the wire(s) bringing power to the fuel pump relay fuse had developed a lot of corrosion and therefore a high resistance it will cause that end of the fuse holder to get very hot. The (new) fuel pump on my car pulls 10 amps when running. That should not cause the kind of heat that the melted fuse was exposed to. You could try splicing in a separate fuse holder and see if it experiences the same amount of heat. If you look closely at the side of the fuse holder that was connected to the most melted end of the fuse, you will most likely find the culprit.

Good luck!

It's a bad connection at the fuse. I have had this problem on my Mercedes A/C fuse as well. An aggressive cleaning with a small wire brush and or sandpaper should take care of the hot spot.

I like the plastic fuses because they melt before the fuse box does. They are also LIGHTER!!!!!!!

Erasers work very well for cleaning corrosion on contacts and you can shape the ones on the end of a pencil to fit in some tight spots...
Easy clean up too, just blow!

Chuck -
I don't have AC. Thanks for the thought though. Since I posted, I did some minor cleaning and things seem to have been better - at least the last time I checked, which is pretty regularly now, it all looked normal. Hopefully, that's all that was needed.

Thanks a lot for the input, I appreciate it.

Tom

May I suggest dispensing with the workaround re-wiring jobs? If the fuel pump is drawing excessive current, repair or replace it instead of adding a fat fix-it wire in the electrical system. With this type of repair philosophy, pretty soon the system will be a jungle of make-shift bypass wires, and usually with no wiring diagram for a tech or the next owner to decipher.

And btw, if excessive resistance occurs in a normally-working circuit (e.g. dirty switch contacts), heat isn't generated. The added resistance creates a voltage drop and thus less current flow through the circuit (dim bulb, slow or no-go motor, etc.).

However, if the load malfunctions and creates excessive current flow, the portion of the circuit that could overheat is the part that doesn't have adequate current capacity (thinner wire, small contact area, etc.). A properly-sized fuse would prevent that. Instead of compensating by increasing the size of the source wire, repair the cause of the malfunction.

The deformed fuse could be a symptom of an oversized fuse allowing too much current flow and thus overheating the smallish fuse contact area. A properly-sized fuse should open the malfunctioning circuit and not allow the fuse body to overheat.

Replacing the cheapie plastic-bodied fuses with ceramic types is a good idea.

Sherwood

While I agree with most of your post, I don't agree with the theory that you have less current flow through a circuit with added resistance.
When you change one of the variables in a circuit (V,I or R) one of the others have to change as well. Add resistance and you will almost always get an increase in amperage and a drop in voltage.
There is always heat generated at the site of resistance. It is at this site that "work" is being done. That heat is a killer to electrical systems in a car.. If that work is being done before it gets to the device that is being supplied, there is less energy available to do the work. The load still wants to do its job with a lower voltage available, the loads resistance hasn't changed (although the circuits resistance has) so the load demands more current to try to do its job...
I could be wrong, but that is the way I see it....

Bill,
I'll concede the fact that a small amount of heat is generated at each resistance point in a circuit. That's why I compared two circuits: a "normal" circuit and a circuit drawing an excessive amount of current.

Using Ohms Law we can see the relationship between voltage, resistance and current. This relationship hasn't changed since Georg Ohm thought this up:

V/R = I or
Voltage ÷ Resistance = Current

Example A:
V=12 ÷ R = 2 ohms, then I = 6 amps

Example B:
V=12 ÷ R = 4 ohms, then I = 3 amps

The voltage source in a vehicle is pretty much constant at 12-14 volts. It doesn't change according to the load since 12 volt devices are designed to operate at 12 volts and require, in most cases, a fixed amount of current for normal operation. Circuit loads like tail, IP, domelight and headlamp bulbs, window defrosters, window regulators, electric seat motors, etc. require fixed amounts of electricity to operate properly. Devices such as radio amplifiers and variable speed motors can have variable electrical requirements.

If the normal circuit resistance in Example A is 2 ohms and a dirty switch adds another 1 ohm resistance, then: V=12 ÷ R=2+1, then I = 4 amps (current decreases, not increases). . In addition, circuit voltage drops anytime circuit resistance is encountered. Normally, the only time voltage drops to zero is after the load (it's used up operating the load). However, a dirty switch that creates resistance will reduce the voltage available to the rest of the circuit. Ever wonder why a bulb is dimmer than normal?

Assuming circuit components are properly chosen to support the designed electrical load, there should be no heat-related damage during normal operation unless one has designed for Lucas.


To beat a dead horse in this post, let's see what happens when overall circuit resistance decreases. This can happen when the load, in this case, the post topic - is a malfunctioning electric fuel pump (constant speed motor).

If Example B, normal circuit resistance = 4 ohms, thus: 12 V ÷ 4 ohms, then I = 3 amps

If a short develops in a the pump motor, resistance decreases (current takes a short cut to ground). Thus: 12 ÷ 3 ohms, then I = 4 amps

Thus, as circuit resistance decreases, current increases, and more current creates more heat. If circuit components aren't designed for the added current flow, they can overheat. If the fuse-to-fuse holder connection has the lowest current-carrying capacity, it will overheat first. But it doesn't overheat because of resistance. In this case, it overheats because the load has increased current demands on the circuit and the fuse holder doesn't have the capacity to carry the increased current.

Another Porsche-specific example is when owners install high wattage headlamps. The headlight switch, not designed to support the increased current flow, soon fails. The solution is to install high current capacity relays to relieve the switch of the main current carrying capacity. However, don't do this to compensate for a failing fuel pump.

Hope this helps and wasn't too wordy
Sherwood