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Length of fusible links?

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knobbler View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote knobbler Quote  Post ReplyReply Direct Link To This Post Posted: Nov/16/2018 at 10:56pm
Alright, apologies in advance for the book I'm about to drop here.

Originally posted by Trader Trader wrote:

The rule of thumb is the fusable link is 4 gauge wire size smaller then the original conductor and the length is 6" to 12". Longer length for more amps and #8 link being 8" for automotive.
The conductor should burn out before the insulation and being a fixed length, burn out in the center.


Perfect! That was the detail I was missing that caused me to get sideways on this whole subject. See, I was not thinking at all about how length of the link could be an important factor in the physical area where a break would occur. Now all the recommendations about crimping AND soldering new links makes a lot more sense. It's not just a matter of preference regarding a general method of making a low-resistance connection to ensure an electrical fault occurs when it should. It's also to make sure the fault occurs where it should --at the center of the length of the conductor, instead of at one of the joints! Wouldn't do much good to protect the harness if the link burned up at a critical splice. . .

I had been unknowingly thinking of links kinda like resistance wire by assuming that the length was a factor because of the amount of resistance that would be had depending on the overall length of the conductor. If it was too long or too short, the resistance would change accordingly and there would be a corresponding impact on the function of the link. While I guess that could be true if I ran a link to the back bumper and back into the engine bay, or only made it long enough to attach a terminal to each end, it's a high strand-count copper wire. A couple inches either way won't alter the overall resistance to the point where it would be relevant.

Originally posted by Jmerican Jmerican wrote:

Since you are choosing fuses, my advice is this. Find the max ampacity of the wire in the circuit and protect for that. It may be tricky with old wire and unknown spec. But new wire will be rated. The ampacity will be based on gauge, insulation temp rating, length of circuit Including the ground wire side. Then de rate that for bundling in a hot engine bay. And slightly more for a fused circuit. Fuses vary with how they take overload, just like breakers.

Likely de rate by 20-25%


Thanks for bringing this up -- it helps quite a bit, particularly the advice on how much to derate by. I didn't realize that I had been getting wrapped up in factoring in the ampacity of the wire gauges, though overlooking environmental variables that are just as important. That will save me from going down a rabbit hole over figuring out how much to adjust my estimates by, for sure.

Originally posted by 6PakBee 6PakBee wrote:

NAPA publishes the length of the wire style fusible links they sell.


Thanks! Pretty handy info. I had no idea that they actually specify that in their product pages.

Originally posted by 304-dude 304-dude wrote:

You can get a similar replacement, by feel, through hobby stores, on RC Car battery wiring kits.


I never would've considered an RC shop, but that makes perfect sense, now that you mention it. If I find myself in an RC shop soon, I'll take a look at that. Even if I'm not using links at the moment, it's always good to know alternate sources for stuff like that.

Originally posted by billd billd wrote:


Note where the factory used links and where they used fuses and think about it.


Excellent advice! The Gremlin was my starting point for anything automotive related, so when I started planning the fusing for the electrical system, I didn't have the first clue what a fusible link even was.   If I hadn't taken the time to research what they are and do, and why they're present at the distribution points that they are, I easily could've ended up being one of those folks who just leave them out or put an inline blade fuse or something in their place. Not something I want to learn the hard way!

Originally posted by Jmerican Jmerican wrote:

You are very much on the right track with your assessment of the fuses you are considering.


Whew! Good deal. I didn't want to have to start rearranging things unless I had to, haha

Originally posted by Jmerican Jmerican wrote:

If you are building a higher performing charging system, lets just say there is low hanging fruit to be picked.


I'm not really going to be putting much more load on the charging system than the factory design. When all is said and done, it'll probably be a fair degree less. The changes I've made are with an eye towards longevity and serviceability.

For instance, the MEGA fuse is much easier to replace than a burned link, and if I'm not able to get a replacement, an appropriate fusible and some ring terminals are all I need to safely get me to where I can get one. The alternator type provides more current at lower RPM, which my battery will appreciate, and is also more readily available in yards and parts stores (not to mention cheaper). I've also Incorporated relays for the headlights to reduce current draw and heat in the wiring loom, while also (I suspect) extending the life of the footswitch and reducing points where a fire could occur in certain (probably unlikely) scenarios.

Basically, I'm not trying to improve things by blindly "upgrading" things. I'm more interested in staying fairly faithful to the original design, but as it may have been done if our present options were available and if a per-unit production cost didn't have to be a deciding factor. Kinda like how I have the 4.0L head and manifolds with the 258 block. Maybe a full-on engine swap would've been "better", but to get the changes I wanted, the head swap was the cheaper, simpler, and more conservative way to go.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jmerican Quote  Post ReplyReply Direct Link To This Post Posted: Nov/20/2018 at 6:30pm
I'm hearing ya, and in that, I'm hearing alternator upgrade. I hearing faithful to the design. I hope there is no amp gauge loop. I'm hearing serviceable. So fuses. I'm hearing if cost wasn't an issue in the original. Well, it was, and voltage drop and resistance was high because of it. If you put in a fancy battery, it will take more charge, quicker. A high acceptance rate. That will demand more from the alt and cabling. You unloaded the factory harness with relays for lighting. Cool. But all the bulbs bigger? If so, more amps through alt wiring. And on. So, I don't want to overstep, and I do want to be helpful. But use big wire. And such. Then you can fuse for low resistance, keep the wire protected, etc.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote knobbler Quote  Post ReplyReply Direct Link To This Post Posted: Nov/22/2018 at 2:17am
Nah, totally fine across the board!  I've put so much into this car that I want to make sure that the work I'm doing is done right, but more importantly, that I'm doing the right work.  I figure any input is a great thing to get.

I'm still using the stock bulbs, but also have a "plug and play" HID kit in case the yellow doesn't cut it for me out on the road.  I'm thinking of fairly remote regions where there aren't a lot of people, but plenty of other stuff worth seeing as far out as possible.  Even though I expect that the OE bulbs will do me just fine, I increased the gauge of the wire to the next largest diameter.  Actually, there aren't many runs where the wire wasn't stepped up to the next gauge, ii]at least.  Even the gauge sender wires got bumped up.  The only things I can think of offhand are pigtails that would be impractical to modify (turn signal switch, for example).  In some cases, I think I even went a little overboard (00 J1127 welding cable to the starter comes to mind).

I've got a yellow top Optima battery that's been running in the Eagle (my daily), but I'm going to get it tested before I put it in the Gremlin.  The bird's RPM-related dim/bright headlight output tells me that the alternator is probably about as tired as the rest of that poor wagon.

Thanks again for your input.  It's a huge help to be able to bounce this stuff off of other actual people, instead of running in circles, burning hours researching, and going down dead ends until I forget entirely what I started with, haha


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jmerican Quote  Post ReplyReply Direct Link To This Post Posted: Nov/22/2018 at 12:41pm
Glad things are going well. The yellow top can and will accept Much much more current than an standard battery, due to its construction. To take advantage of that, and to not tax the cabling and hey, maybe a master fuse, you want to design healthy charging cables with appropriate fusing. It also sounds like your alternator design output is low at the speeds it's turning. If you're interested in low speed output, then look into a better part. In a low duty application like yours, usually none of this is a big deal. But run the yellow top down, and you may find that the rest won't keep up, or blows a fuse. Matching the designs of components is what you're after, with the removal of the penny pinching OEM. 




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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaemonForce Quote  Post ReplyReply Direct Link To This Post Posted: Nov/22/2018 at 2:29pm
These are all very good plans. I just cheat everything by measuring the amperage draw of each circuit, combination circuits, max amp draw and total draw to independently blade fuse where appropriate and guard the most important circuits with fusible links 4-6" in length at the starter. Starter itself pulls the most amperage when turning so I figure that's a great place to use as a link post for all major connections.

Nothing makes me more irate than seeing a ton of linked cables AT the battery terminals except for those battery terminals that enable that self-destructive scenario, complete with rusted bolts, corroded wires and bad insulation. Good luck if you're out of EJC#2. The starter I'm using is a factory 4.0L pulled from a yard Jeep but the screws still have blue paint on them from the rebuild job however many years ago. Made in Mexico, or as I call it: JACKPOT.

Point is, as long as you're paying attention to where your power comes from, where it goes and the first problem areas, you should be able to account for them without a bunch of engine bay fuses or link posts. I still use all the factory blade fuses in my Eagle wagon and the two circuit breakers. The only other thing is that I switched most of my relays over to minis for a better wiring profile in the engine bay. The full size box models draw an additional 10mA but they're so bulky that I would have to add a second relay pod just to fit everything and that's ugly. Here's an idea: Put the source for all of your fusible links in one primary location so that in the event of a blown link, you don't have to chase so many of them. This was the case on the FACTORY +12V side of the ignition harness and it was a cobweb headache to diagnose whenever something went wrong(usually headlamps). I'm sure the lot of you are competent enough to not have this problem anymore but from there just hunt down odds and ends like voltage drop and you're good.
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