What causes a burnt fusible link

[Catfish28]

So I've searched everywhere and found a mind numbing about of info on mud concerning fusible links. But no one really explained what causes one to fail. I saw a couple of folks ask but no answer. And what would cause a harness to overheat and burn up in the first place? Thanks in advance.

 

[73FJ40]

A short circuit to ground. The fusible link is intended to melt and disconnect (it's made of thinner wire than the rest of the circuit) before the entire circuit overheats, melts, and destroys the rest of the bundled wires.

Imagine what would happen if you connected a wire directly from the positive battery terminal to the negative. (don't try it)

 

[Catfish28]

Thanks for the response. I understand that. But what in an fj40 would cause it to fail. Regulator or igniter not grounded a possibility?

 

[Living in the Past]

Your looking for a simple answer, there isn't one. Just so many things in different places that could cause this. The fuseable is close the battery/source of power to protect the entire system.

 

[pithicus]

could be a short in the alternator.

 

[reddingcruiser]

So many potential problem areas, compounded by the probability a PO left a time bomb somewhere.

Was there something specific that was operating when this happened? Heater on? Radio on? Lights on? Anything that can limit it to a specific circuit?

 

[reddingcruiser]

Check for dead shorts, bare wires, especially where wires pass through the sheet metal or across metal edges. I learned the hard way NOT to assume a grommet is doing it's job, especially if it's dried out and cracking.

 

[pithicus]

to test the alternator, get a probe test light . disconnect the main charge terminal from the alt. Keep the negative on battery connected to truck. Disconnect the positive from battery, Attach probe test light clip to battery positive. Touch probe pin to alt charge terminal. If it lights up it could indicate a short in the alt.
As others have said, it could be lots of things.

 

[Catfish28]

Thanks all. This is the last bit off a rebuild. Haven't cranked the motor yet. Lights and a variety of other things are working and I'm trying to get other stuff going. Ground on the igniter didn't test good.
Looks like I've got a lot to learn about wiring on an fj40. Something to do this weekend.

 

[Huroneymoose]

Should all vehicles have a fusible link? Are fusible links on the positive side near the battery?

 

[thebigredrocker]

Can we assume this project came to you with a melted wires? If so, I'm sure many others would be curious which wires. It's frustrating to have electrical issues but very rewarding to correct them. Coolerman is a great guy to purchase OEM compatible harness supplies from if you're in need.

 

[Solace in Solitude]

Looks like I've got a lot to learn about wiring on an fj40. Something to do this weekend.

It will likely take longer than a weekend to "learn about wiring on an fj40"

I hooked up my battery backwards... Melted the fusible link and also melted my harness... The fusible link didn't save my harness...

But, to be fair, the fusible link is connected to the positive battery terminal, with the presumption that a sane person will connect the positive terminal to the positive battery post... NOT to the negative battery post.

When I connected the negative terminal to the positive post, I energized the frame and subsequently melted the harness.

Add that to your list of scenarios, albeit rare (I hope), for melting the fusible link.

If the battery is connected properly, the fusible link is sacrificial... Assuming it's sized properly, it should melt before the harness is damaged.

If/when the fusible link melts, it indicates the potential for a damaging short... You need to address that short (find and fix) before replacing the fusible link.

@Coolerman sells fusible links and terminals/connectors... Or, you can make your own.

 

[RAYJON]

Fuseable links can also fail with age and heat, you can check resistance to ground on the circuit, if it is grounded, start isolating parts of the system... basic electrical trouble shooting.

on the cheep, find a mid 90's chevy truck , on the firewall there is a power block with 3-4 fuseable links. Or my local napa sell fuseable link wire in a small roll in various sizes.. toyota also sells the fuseable link pack for specific models.

 

[bj40green]

Is the fusible link OEM or a diy project from the PO?
Do you have extra accessoires hooked up to the harness instead of powered by an in-line fuse from the battery?
The FL (Fusible Link) protects the OEM harness. If you add fog lights, fridge or any other stuff to the harness (after the FL) you'll exceed the max current of the FL.

Rudi

 

[Catfish28]

Lots of questions.
The harness is oem with no add ons. I removed it as part of the project and checked everything on a piece of plywood. Now trying to get it back up and running and learning lots in the process.