Why is my headlight fuse melting?

bp m1009

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So this weekend my truck decided to start melting the 30a headlight fuse. The weird thing is that it melts the plastic but doesnt pop the fuse. It had a 20a fuse in there so i thought that originally this was the problem. Then i noticed that there was some metal pieces in the terminals so i used a pick and seemed to have gotten it all out. I replaced with the correct 30a fuse and it still does the same thing. Next i unscrewed the fuse block connection on the firewall cleaned out the gunk and then applied di-electric grease to the connector. still same result. any suggestions?
 

K9Vic

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Are they Harbor Freight Fuses ? they have a fuse recall listed on there recall notices page.
Yeah, I buy allot of tools at HF, but I will never use their fuses.

To the OP, did you search?
This has been covered multiple times and the solution is the headlight relay harness from LMC truck.
 

bp m1009

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no they are not HF fuses, i have used multiple brands of fuses with the same result. There has to be a way to fix it without going to the LMC relay mod. It worked in stock form for 28 years
 

bp m1009

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i did search before posting and from what i found is this sort of thing usually stems from poor/loose contact with the fuse. i did mess around with the prongs on the fuse block side to try and make them hold the fuse tighter because they were loose, but it didnt help. Is there anyway to replace just that terminal in the fuse block?
 

bp m1009

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i probably will do the relay mod, since i can get the LMC one from a buddy for free. but wanted to be able to fix it before that so i can drive with my lights on for more than 5 mins. seeing as i wont be able to get the relay kit for a week or so.
 

cpf240

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It usually comes down to dirty contacts in various places and 25+ year old wiring that may have corrosion inside the insulation. The system wasn't that great to begin with, as most of the time "good enough" is the penny-pinchers phrase of the day. You could take the time to clean every connection, terminal, ground, headlight switch, etc and still have the problem. If all the harnesses, switches, fuse box, etc were replaced, it might work fine for another 25+ years.

Even when these trucks were new(er) it was not unheard of for the headlight switch to get warm or even hot. A relay system should have been used in the first place.

I don't know if you can rebuild the fuse box, but if a man built, a man should be able to rebuild it. The question is is it worth the time and investment? Sadly, the fuse box is not available as a single part, it is hard-wired to its harness.
 

Warthog

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So this weekend my truck decided to start melting the 30a headlight fuse. The weird thing is that it melts the plastic but doesnt pop the fuse. It had a 20a fuse in there so i thought that originally this was the problem. Then i noticed that there was some metal pieces in the terminals so i used a pick and seemed to have gotten it all out. I replaced with the correct 30a fuse and it still does the same thing. Next i unscrewed the fuse block connection on the firewall cleaned out the gunk and then applied di-electric grease to the connector. still same result. any suggestions?
Read the headlight sticky. While it doesn't solve your problem it does explain why.
 

swbradley1

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If the plastic is melting that means that heat is being generated AT the fuse. That means there is a slight resistance in the contact surface of the fuse when it is plugged in. There should be close to zero Ohms of resistance across the connector when the fuse is plugged in.

THESE NUMBERS ARE EXAMPLES TO SHOW THE HEAT ACROSS A RESISTANCE. (I know they may not match the CUCV.)

Say a normal contact has .1 Ohms resistance.
P=I^2 x R = 9 Watts at say 30A when the lights are on = no problem

Say the same contact has 5 Ohms.

30Ax30A x 5 Ohms = 4,500Watts = problem

Your mileage may vary but it is because there is heat being generated at the fuse. I just watched this happen with my own two eyes 10 days ago working on wife's car. Headlight not working. Corrosion had increased the resistance of the headlight bulb connector and with the lights on I watched the wire that was showing start glowing red as it melted the connector. New bulb and cut the connector off and put two new lugs on fixed it.
 

doghead

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Does your horn function properly?

Just for fun, try removing your 15 amp horn fuse and see if your 30 amp fuse still melts with your lights on.(I have a theory)


If you think you did not solve the loose fuse connection, maybe you could solder in wires and use a new external(out of the fuse box) fuse . I'm thinking about using a 30 amp inline fuse holder, with the wires soldered into the original fuse terminals/sockets.


Also, you could try measuring the resistance in all the headlight wires to see if you have a bad one.
 

doghead

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It might be intersting to use an IR gun, to check or monitor this heat issue.
 

prairie

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If you think you did not solve the loose fuse connection, maybe you could solder in wires and use a new external(out of the fuse box) fuse . I'm thinking about using a 30 amp inline fuse holder, with the wires soldered into the original fuse terminals/sockets.
Why not just do the headlight relay modification, which solves the immediate problem, and fixes and upgrades an obvious design flaw? Anybody who knows much about automotive electrical wiring will tell you that the CUCV headlight circuit is a poor design and set up for inevitable failure.
I ended up replacing my whole fuse block and doing the headlight relay modification. That not only fixed my melted headlight fuse problems, but cured a few other electrical gremlins, that up to then we had not been able to fix.
 

doghead

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Doing that relay conversion does not repair or pinpoint the original problem. If it is a bad contact in the fuse socket or in the firewall/harness assemby, your still using that bad curcuitry, and simply releiving most of it's load.

My comment about the horn fuse, is along a whole other line of thought(that Anybody who knows much about automotive electrical wiring can figure out).
 

bp m1009

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Does your horn function properly?

Just for fun, try removing your 15 amp horn fuse and see if your 30 amp fuse still melts with your lights on.(I have a theory)


If you think you did not solve the loose fuse connection, maybe you could solder in wires and use a new external(out of the fuse box) fuse . I'm thinking about using a 30 amp inline fuse holder, with the wires soldered into the original fuse terminals/sockets.


Also, you could try measuring the resistance in all the headlight wires to see if you have a bad one.
No my horn doesnt work :(
 

doghead

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Is your horn fuse out? If not, remove it and see if you still have the problem with the fuse getting hot.
 
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