Hot fuse

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IDave

IDave

Joined
Jan 3, 2003
Threads
86
Messages
7,184
Location
McCall, ID
So, I have an issue that is different from Zetasig's (not to get it confused). My Headlight fuse keeps melting out. It is NOT the fuse blade itself that melts, it is the solder that attaches the blade to the metal cup at the fuse end. Turning on the headlamps causes the fuses to get hot: eventually hot enough to blister the end of my finger after 3-5 seconds. The solder drips out from between the glass and the metal cup end of the fuse, and of course, then I lose my lights. The metal blade itself does not burn through. The only items on that fuse are the dash lights and the running lights and headlights. All of the wires seem to be intact, and everything works normally. There is no current flow across the fuse if the switch is off, and the fuse only heats up when the switch is on. Over a week or so, the fuse will eventually go. The plastic plug that goes to the headlight switch has melted a little in the past, but there is no direct contact between the various contacts that I can see, and I have replaced the switch. I had the Hella Halogens SOR sells on there, but replacing them with OEM lights hasn't changed the hot fuse end.

What next?
 
sounds like a poor ground maybe....make triple sure the ground wires from the lights are solid.

a 71 FJ40 just "might" have a few rusty ones....maybe....
 
IDave, you may have some junk fuses, the weak link SHOULD be the fuse wire itself. I've had that happen and just replaced the fuse. I also have some that have dripped solder, but are OK. If your wattage divided by 12V is = or< 20 amps, you should be ok.
Ed :)
 
I would bet that something got hot a while back with the harness and that there are wires touching each other that should not be.. Look at the high-low beam switch too, make sure that something is not jacked with that too, as it is in a great spot on the earlier trucks to collect corrosion and create lighting issues...


Good luck!

-Steve
 
I will bet that if you open up the harness, you will find something melted together causeing you issues...


Good luck!

-Steve
 
I took apart the high-low switch. Cleaned it up. Everything looked ok, and it seems to be working well. I coated the exposed contacts with liquid electrical tape (in place of the boot, which is irreplaceable), with no improvement.

I have the same harness system Zetasig has, with the two trunks coming out of the opposite sides of the firewall. No corresponding wiring diagram that I have run across yet, so the idea of pulling apart the harnesses is a daunting one. I am afraid it might be inevitable.

But can someone explain why the fuses get so hot? And why the blade survives, if it is too much current? Is it a resistance thing at the fuse box itself? I have been through at least 5 boxes of fuses from various sources, so I can't think it is just a bad batch of fuses.
 
Resistance causes heat....

I have a hard time believing that the fues block itself can be causing the problem....or that it is a 'bad batch of fuses..

Even the hanes diaper has a decent early year wiring diagram...at least my diapers do... ;-)

Good luck!

-Steve
 
Paged ya. :D
 
Dave, there is a short somewhere in your wiring. I'd have too say that it's probably not in the switch, Poser hit it right, you have a couple wires touching somewhere in your loom. I'm surprised the fuse don't blow before the solder melts. But, then again, I have my XM radio plugged into the light plug in the g-box, and sometimes when I hit the brake peddle the XM receiver shuts off...wires suck.
 
Well, I tried feeling for a hot wire in the loom with the lights on like Steve suggested. But it will probably take removing the heater to get up under the dash to get at it all. Anyway, I didn't find anything warm. I also checked the back of the fuse box for corrosion. No luck there, either.

I guess I'll be unwrapping the electrical tape from all of the looms and looking. Gloomy prospect.
 
The hot fuzes I have seen are due to too much contact resistance at the fuze connection. I would get in there and scrub the contact surfaces in the fuze box with a wire brush or emery board (whichever is appropriate).

You can't be pulling too much current (i.e. a short somewhere else downstream in the harness), or you would burn the fuze "element".

But you can be generating a lot of heat by applying resistance across the several amps that the headlights are drawing. My bet is bad contacts in the fuze box.

Rocky
 
Someone else (electrical engineer brother-in-law of a friend) suggested that, and I did file the fuse contacts. The fuse box is only a year or two old. ???
 
[quote author=Rocky_LC link=board=1;threadid=16574;start=msg159640#msg159640 date=1084939267]
The hot fuzes I have seen are due to too much contact resistance at the fuze connection.
But you can be generating a lot of heat by applying resistance across the several amps that the headlights are drawing. My bet is bad contacts in the fuze box.

Rocky
[/quote]

I would agree. is the fuse box one of the old style ones wiht the screws holding the wires on, or one of the newer black plastic ones?(you can get these out of toyota cars in the JY for cheap)

My guess is that somehow oyu have a lot of resistance in the connection. what does the wire connector look like? If it is dirty, take a shot glass of elec. contact cleaner (or muriatic acid-if you carefully neutralize it after the cleaning) and dip the offending wire connector in it to clean of the corrosion, Check for detriorated wiring and replace if needed. I had the same(more or less) issue on my fj45, where between the fuse and wire connector I was somehow going from 12V to 6.4V.

-Brett
 
It is the old style spring/clamp fuse box. As I said, it is relatively new, and there was no corrosion apparent. I did file the wire connector, just to get things cleaner, and I took a small file to the fuse contacts as well.

Still hot.
 
So, I went out and got a better multimeter (with a 10 amp range) and tried to do some isolation.

Maybe someone can help me interpret this:

Resistance at the fuse box is OK, I think: .5 ohm across terminal to fuse clip on either side, and 0 on the fuse.

With the headlight switch off, there was no current draw.

I unplugged the headlights, taillights and front running lights, and left the 2 little dash bulbs attached.

Dash bulbs only, the flow was 3.5 amps across the fuse terminals. Same when I added any one or more of taillights, front running lights or combinations.

Adding the right headlight only drew 6.4 amps.

Adding the left headlight only drew 6.6 amps.

Adding both headlights drew 10 amps.

Throwing in all lights drew 12 or so (but beyond the 10 amp range, so?).

Now, here's the interesting thing, I think. If I bypass the switch and directly connect the dash lights (with my multimeter/ammeter), at the switch plug, the draw drops to .5 amps. (remember 3.5 with switch attached).

Where'd the other 3 amps go? If I bypass the switch to the running lights or the headlights leads (lights unplugged) there is no draw.

Have I learned something useful?
 
I had the same problem with my 70 40 head light fuse solder melting. What I figure my problem was(after screaming down the highway and loosing all lights with no street lights) the replacement fuse holder I got from napa and some dirt got in behind the holder(no idea how that happened) and it must have arced to the metal on the tub and melted the fuse. It got worse and worse til I put a little rubber pad between the holder and the tub. I guess that when there was a draw it gave enough amps for it to arc but when it was off it was fine. Only experience I have had and only thing I could think of and it has been DD 4 years with the same fuse now.
 
Eternal: I did check for that: didn't find any grounding on the fuse box.

Steve: I have 2 switches, both are used. The draw from the isolated headlights was 7 amps bypassing the switch. One switch drew 10 amps, the other 13 amps (!) in the same setting.

I'll go check the resistance in the switches now.
 
So the resistance was all over the map with both switches, although it would settle in to around .5 ohm. Then I cleaned the contacts on both switches, and resistance seems more stable at .3 ohm.
 
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