Melting fuse?

[Quicksilver]

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.

[Tom '74 911]

Originally Posted by Quicksilver

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

[Early_S_Man]

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

[Tom '74 911]

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

[911hoplite]

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.

[Jim Sims]

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.

[911hoplite]

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.

[dshepp806]

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,...

[Jim Sims]

"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

[911hoplite]

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?

[dshepp806]

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

[niner11]

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

[fred cook]

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!

[avendlerdp]

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!!!!!!!

[billybek]

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!

[Tom '74 911]

Originally Posted by niner11

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

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

[911pcars]

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

[billybek]

Originally Posted by 911pcars

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.).
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....

[911pcars]

Originally Posted by billybek

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

[911Freak]

Thanks for the formulas Sherwood! Very helpful...Brings back memories from my electrical engineering course I dropped back in the day, LOL!

wasn't my cup-of-tea...

Glad it is for other though!!