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Old 03-07-2017, 09:46 PM   #1
71CHEVYSHORTBED402
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12 AWG red in cab - What are these?

Fusible links? How do they come apart or do they? How do they install? The clear ones are on a smaller black wire in-between the 12 AWG red. It's attached before the red wire is soldered onto three additional red wires wires. One routes to the alternator, one to the voltage regulator ,and the other to the junction block.

Before the junction block there's a what-U-call-it. This is also on a smaller black wire.....Thank you.



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Old 03-07-2017, 11:04 PM   #2
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Re: 12 AWG red in cab - What are these?

Looking at the diagram below, yeah these are probably fusible links. Safe to say you cut the bad ones off and crimp the new on?

My wiring wasn't the same as this diagram. Differences are:

1. The diagram doesn't show the fusible link described by the first image above, for red. Again, this link is between the firewall block and where the wires are soldered as described above.

2. The diagram doesn't show the fusible link described by the second image above, for red. Again, this link is at the end of the red wire and attaches at the junction block.

3. The diagram appears to show a fusible link between the junction block and the positive terminal on the battery. Mine was just a wire.

Any input appreciated...........

http://farm4.static.flickr.com/3645/...c2a8064d_o.jpg
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Old 03-07-2017, 11:36 PM   #3
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Re: 12 AWG red in cab - What are these?

Yes, those are fusible links. Well, technically those clear and black round things are just the insulation the factory used where they joined the fusible link to the wire it protects. The fusible link itself is the short section of smaller gauge wire (with special insulation).

Unless your wiring has been subject to a short circuit or has other damage there generally isn't a need to replace the fusible links. But in the event that one does need to be replaced, yes, you just cut them off and crimp/solder a new one in place. Be sure to use the same gauge fusible link as the original and try to keep the length about the same (no more than 9" long).

As for the differences in your wiring vs. the diagram, I have noticed that GM made a few subtle changes to the wiring over the 67-72 year range. So even though that diagram you found is labeled 67-72, some of the details may only be correct for the earlier years. For example, the early years had that fusible link located between the battery + and the junction block. Later, the fusible link was moved to the end of the red wire attached to the junction block and a regular wire goes from the junction to the battery + terminal (like the setup you have).
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Old 03-08-2017, 01:31 AM   #4
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Re: 12 AWG red in cab - What are these?

Quote:
Originally Posted by ray_mcavoy View Post
Yes, those are fusible links. Well, technically those clear and black round things are just the insulation the factory used where they joined the fusible link to the wire it protects. The fusible link itself is the short section of smaller gauge wire (with special insulation).

Unless your wiring has been subject to a short circuit or has other damage there generally isn't a need to replace the fusible links. But in the event that one does need to be replaced, yes, you just cut them off and crimp/solder a new one in place. Be sure to use the same gauge fusible link as the original and try to keep the length about the same (no more than 9" long).

As for the differences in your wiring vs. the diagram, I have noticed that GM made a few subtle changes to the wiring over the 67-72 year range. So even though that diagram you found is labeled 67-72, some of the details may only be correct for the earlier years. For example, the early years had that fusible link located between the battery + and the junction block. Later, the fusible link was moved to the end of the red wire attached to the junction block and a regular wire goes from the junction to the battery + terminal (like the setup you have).
Great info. thank you. Let me ask, what's the difference between a fusible link and an in-line fuse? One reason I ask is I have two other whatUcallits described below. These units are rubber boot insulated and pull apart. In fact, I pulled one apart and it sounded like I may have broken the gizmo inside. No harm done, I'm getting new wires. If needed I'll add pictures.

One rubber boot dude for the black wire from the firewall block to the junction block. The unit is near the junction block.

One rubber boot dude for the black & white striped wire from the firewall block. The end of this wire is soldered where the cluster of red wires are soldered as described in post one. The unit is near where it's soldered.
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Old 03-08-2017, 04:58 PM   #5
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Re: 12 AWG red in cab - What are these?

Quote:
Originally Posted by 71CHEVYSHORTBED402 View Post
Great info. thank you. Let me ask, what's the difference between a fusible link and an in-line fuse? One reason I ask is I have two other whatUcallits described below. These units are rubber boot insulated and pull apart. In fact, I pulled one apart and it sounded like I may have broken the gizmo inside. No harm done, I'm getting new wires. If needed I'll add pictures.

One rubber boot dude for the black wire from the firewall block to the junction block. The unit is near the junction block.

One rubber boot dude for the black & white striped wire from the firewall block. The end of this wire is soldered where the cluster of red wires are soldered as described in post one. The unit is near where it's soldered.
You're welcome! A fusible link is basically just a short section of smaller gauge wire that (in the event of a short to ground or major overload) will act as a fuse to protect the rest of the wire in the circuit from melting. The fusible link wire has special insulation that is designed to stay intact even when the wire inside burns out. So a quick & easy way to test a fusible link is to give it a gentle tug to see if it feels stretchy (if it does, the wire inside is burned out). The insulation will often partially melt and give off lots of smoke when a fusible link burns out so they should never be used inside the passenger compartment of a vehicle.

One of the biggest differences between a fusible link and an inline fuse is that a fusible link acts like a really slow blow fuse. In other words a fusible link will stand up to short term overloads but still offer protection in the event of a major problem. That can be an advantage in situations where you don't want a fuse to blow due to a temporary surge in current (such as when an electric motor is started). Fusible links also tend to be a bit more compact than a fuse in an inline holder (especially for larger sizes). And not having a fuse holder cuts down on potential loose contact & corrosion issues.

Those two black rubber boot things you found are inline fuse holders that hold the fuses that protect the ammeter (battery gauge) circuit. They use SFE-4 fuses and replacements are still readily available if you broke one. This application uses fuses instead of fusible links since the current is so small. A fusible link wire small enough to burn out at 4 amps would be way too tiny and delicate to be practical.
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Old 03-09-2017, 06:06 PM   #6
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Re: 12 AWG red in cab - What are these?

Don't ask me why the thread title says in cab when.......................Yikes.


You're welcome! A fusible link is basically just a short section of smaller gauge wire that (in the event of a short to ground or major overload) will act as a fuse to protect the rest of the wire in the circuit from melting. I see why it's a smaller wire. Good to know.

The fusible link wire has special insulation that is designed to stay intact even when the wire inside burns out. So a quick & easy way to test a fusible link is to give it a gentle tug to see if it feels stretchy (if it does, the wire inside is burned out). Also good to know.

The insulation will often partially melt and give off lots of smoke when a fusible link burns out so they should never be used inside the passenger compartment of a vehicle. Same

One of the biggest differences between a fusible link and an inline fuse is that a fusible link acts like a really slow blow fuse. In other words a fusible link will stand up to short term overloads but still offer protection in the event of a major problem. Makes sense.

That can be an advantage in situations where you don't want a fuse to blow due to a temporary surge in current (such as when an electric motor is started). Quite an education, thank you.

Fusible links also tend to be a bit more compact than a fuse in an inline holder (especially for larger sizes). And not having a fuse holder cuts down on potential loose contact & corrosion issues. Agree.

Those two black rubber boot things you found are inline fuse holders that hold the fuses that protect the ammeter (battery gauge) circuit. They use SFE-4 fuses and replacements are still readily available if you broke one. This application uses fuses instead of fusible links since the current is so small. A fusible link wire small enough to burn out at 4 amps would be way too tiny and delicate to be practical. If I got it right, the circuits with an inline fuse are:

The BLACK/WHITE striped circuit runs from #1 on the cluster to the fuse panel, then to where it's soldered with the 12 AWG reds.

The BLACK circuit runs from #12 on the cluster to the fuse panel and off to the junction block.
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Old 03-09-2017, 07:21 PM   #7
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Re: 12 AWG red in cab - What are these?

Quote:
Originally Posted by 71CHEVYSHORTBED402 View Post
If I got it right, the circuits with an inline fuse are:

The BLACK/WHITE striped circuit runs from #1 on the cluster to the fuse panel, then to where it's soldered with the 12 AWG reds.

The BLACK circuit runs from #12 on the cluster to the fuse panel and off to the junction block.
That is correct. Although these wires do not actually connect to any fuses in the fuse panel ... they simply go through the bulkhead connector assembly (which is sort of part of the fuse panel assembly). The 4 amp inline fuses are located out in the engine compartment (close to the battery/alternator ends of these wires) so they can protect the full length of these wires.

The way the black/white and black wires are connected to the junction block and 12 awg red wires serves to place the ammeter (battery gauge) in parallel with the 12 awg red wire that runs from the soldered splice over to the junction block. So in addition to serving as the battery charge wire, it also serves as a shunt for the ammeter. The majority of the battery charge or discharge current flows through that red shunt wire with only a small fraction taking the parallel path through the meter assembly (and it's black/white & black connecting wires). One can also think of it in terms of the meter responding to the small voltage drop that is present across the red shunt wire due to current flowing through that wire which (like all wires) has some resistance.

In any case, the current that flows through the battery gauge (and it's black/white & black wires) is quite small ... it only takes about 1.5 Amps for the meter to read full scale. So this circuit can safely be protected with those little 4 Amp fuses in the rubber holders.
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Old 03-10-2017, 08:09 PM   #8
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Re: 12 AWG red in cab - What are these?

Quote:
Originally Posted by ray_mcavoy View Post
That is correct. Although these wires do not actually connect to any fuses in the fuse panel ... they simply go through the bulkhead connector assembly (which is sort of part of the fuse panel assembly). The 4 amp inline fuses are located out in the engine compartment (close to the battery/alternator ends of these wires) so they can protect the full length of these wires.

The way the black/white and black wires are connected to the junction block and 12 awg red wires serves to place the ammeter (battery gauge) in parallel with the 12 awg red wire that runs from the soldered splice over to the junction block. So in addition to serving as the battery charge wire, it also serves as a shunt for the ammeter. The majority of the battery charge or discharge current flows through that red shunt wire with only a small fraction taking the parallel path through the meter assembly (and it's black/white & black connecting wires). One can also think of it in terms of the meter responding to the small voltage drop that is present across the red shunt wire due to current flowing through that wire which (like all wires) has some resistance.

In any case, the current that flows through the battery gauge (and it's black/white & black wires) is quite small ... it only takes about 1.5 Amps for the meter to read full scale. So this circuit can safely be protected with those little 4 Amp fuses in the rubber holders.
Shunt, I had to look that one up. Great information. Other than that, I read where the "black" wire from #12 on the cluster to the junction block may also be brown and black. Apparently you've seen plenty of black. I'm color challenged, but my wife says it's black so I say black.
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Old 03-10-2017, 08:21 PM   #9
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Re: 12 AWG red in cab - What are these?

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Other than that, I read where the "black" wire from #12 on the cluster to the junction block may also be brown and black. Apparently you've seen plenty of black.
Yeah, black wire is all I have ever seen for this circuit on 67-72 trucks (and in the wiring diagrams).

There is a brown wire that goes to the voltage regulator (and in trucks with the basic warning light cluster it connects to pin #8 for the GEN light). I think I have seen some posts where folks have been converting from warning lights to gauges and were wondering how this brown wire gets connected to the ammeter. Short answer to that is that it doesn't. It's a separate circuit.
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