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Technical 12v fusible link

Discussion in 'The Hokey Ass Message Board' started by Katuna, Nov 12, 2016.

  1. Katuna
    Joined: Feb 25, 2005
    Posts: 1,823

    Katuna
    Member
    from Clovis,Ca.

    I'm in the process of putting the wiring back together on my father in laws coupe.

    On the 12v lead off of the hot side of the stater sol. has a resistor wire, as per the wiring diagram (EZ Wire loom. Yuck.). Is there anything special about the wire itself? Looks like a short piece of 16 ga to me.

    To make the exposed wiring under the hood I'm splicing in cloth covered wire. Makes a huge difference in the looks department. I'd like to replace the resistor wire too but if it's some super secret NASA developed lead I'll just leave it as is.


    Sent from my iPhone using The H.A.M.B. mobile app
     
  2. The wire is calibrated by length to give you a certain amount of resistance. Your not supposed to shorten are lengthen. I did that on an American auto wire for a tri five. It was too long and I shortened it. Out come was not what I wanted. On their harness for the tri five it's light brown with a white tracer. Standard GM color.
     
  3. Katuna
    Joined: Feb 25, 2005
    Posts: 1,823

    Katuna
    Member
    from Clovis,Ca.

    I miss spoke. Not a resistor wire but a fusible link.

    I knew it was a specific length but didn't know whether it's a special type of wire. Just looks like stranded copper.


    Sent from my iPhone using The H.A.M.B. mobile app
     
  4. "An electrical fusible link is a type of electrical fuse that is constructed simply with a short piece of wire typically four American wire gauge sizes smaller than the wire that is being protected."

    Resistance wire for ignition voltage drop to the coil is a specific length, but the fusible link wire to protect a circuit is just a short piece of regular wire of a smaller gauge. You can buy them if need be in the Dorman "help" section at your local auto parts store.
     

  5. Leakie
    Joined: Nov 10, 2010
    Posts: 269

    Leakie
    Member

    Basically it is designed smaller to burn up before the larger size wire starts a fire.
     
  6. Johnny Gee
    Joined: Dec 3, 2009
    Posts: 10,780

    Johnny Gee
    Member
    from Downey, Ca

    What is the wire for per the manufactures diagram ? Feeds what ? Why couldn't it be fused else where so you can accomplish what it is you want ?
     
  7. Fusible links are used because they will withstand an overcurrent for a period of time but will blow before actual damage to the protected wire occurs. A fuse is a all-or-nothing deal, it won't blow until it reaches its rating. So if you have a wire that will be undersized when a intermittent load is on, a link will 'hold' the current for that intermittent time, but will still blow if a dead short or if high current is applied for too long. A fuse sized for the maximum possible current won't give overcurrent protection at anything less than it's rating, so wire damage is very possible.

    A example would be a #14 wire with a 15 amp load but may see 30+ for a short time. Fuse it to take the highest current, you could catch the wire on fire if the load is continuous because you've exceeded the wire ampacity by 100%.
     
  8. Katuna
    Joined: Feb 25, 2005
    Posts: 1,823

    Katuna
    Member
    from Clovis,Ca.

    Cool, thanks guys. Makes perfect sense now.

    This link is on the starter sol. circuit. Makes sense because even a slow-blow fuse would give up since there's a good chance you may need to lean on the starter on occasion.

    It's good to hear since now that I have that section of the loom run this morning, the link is about 1" short. Now I'll eliminate the plastic wire and put in a nice pretty cloth wrapped piece.

    Thanks again everyone. Viva la HAMB!


    Sent from my iPhone using The H.A.M.B. mobile app
     
  9. Johnny Gee
    Joined: Dec 3, 2009
    Posts: 10,780

    Johnny Gee
    Member
    from Downey, Ca

    Crazy Steve, that's why I asked what the purpose of the exact link was for. I do scratch wiring and have no idea what one manufacture to another does as for as the endless way's of routing current and safe guard it.
     
  10. FishFry
    Joined: Oct 27, 2022
    Posts: 237

    FishFry
    Member

    After 40+ years of wrenching on cars, I just learned about fusible links a few weeks ago.
    Probably because it wasn't really a thing here in Germany, and the US cars I use to work on are mostly pre war to early 50s.

    Actually I don't really get the concept.
    I mean - yeah I understand what it does and how it works, but it still doesn't make much sense to me, so I have a few questions.


    1. is it just a regular wire but smaller gauge, or is it some special "fusible" material as the name implies?

    2. we talk a lot about how important it is to have the right wire gauge (and rather going a big bigger, to be on the safe side etc.) for - let's say a charging circuit - right?

    So now that I have all my big fat wire in place, and then I ruin all the benefits of that 8 gauge wire by building a 12 gauge bottleneck (basically making a 12 gauge connection out of my 8 gauge connection) with the goal that it catches fire under my hood, where there probably would be no fire in the first place without it, but with a fuse or breaker instead?

    Also that stuff is pretty expensive, compared to a fuse, and once it blows it's hard to diagnose and replace on the road (compared to a fuse/breaker).

    And yeah, I understand that a fuse would snap at a certain point, while it takes the fusible link a while to catch fire. But still, it sounds like a very odd concept to me, more like saving money in the production by cutting corners.

    Frank
     
    Last edited: Nov 19, 2022
    Blues4U and firstinsteele like this.
  11. '29 Gizmo
    Joined: Nov 6, 2022
    Posts: 152

    '29 Gizmo
    Member
    from UK

    There are better solutions now but it should be upstream of the fuse box if it has one. I have replaced mine with a resettable circuit breaker that was for marine use. At least that way i wont get stuck if it pops for any reason.
     

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  12. FishFry
    Joined: Oct 27, 2022
    Posts: 237

    FishFry
    Member

    Hmmm...

    Would't all those fuses in the box pop first, long before that breaker switch reacts?

    Frank
     
  13. Phil P
    Joined: Jan 1, 2018
    Posts: 427

    Phil P
    ALLIANCE MEMBER

    The breaker would be there to protect the wire to the fuse block

    Phil
     
  14. My harness kit came with a 50 amp maxi-fuse. I would use those over anything else. Fusible links were always a nightmare for me.
     
    vtx1800 likes this.
  15. It is a cost-saving measure. The OEMs build millions of harnesses every year, even small savings add up quickly. Reducing wire sizes to the bare minimum for adequate operation is standard procedure as it saves them money. That's not the only reason however; lower weight, ease of installation, less bulk also enter into it. What they build isn't the best solution, it's an adequate solution. The aftermarket duplicates this, with a few more shortcuts added.

    As to the construction of fusible links, there are two types. The cheaper ones are simple smaller-gauge wire covered with insulation that will contain any melting. The better ones also use wire, but also have a solder-like coating applied that helps dissipate any heat generated when overloaded and then solidify again after the load is removed.

    Automotive electrical systems are the only ones that have no regulations for protection of property or life safety. Commercial vehicles, aviation and marine all have standards that must be met for legal operation.
     
  16. I worked with a guy who bought a '78 Datsun new. It was loaded with fusible links. He managed to get a reference book at the library that showed the location of them. I was shocked that there were around 12 of the evil buggers.
     
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  17. Hemi Joel
    Joined: May 4, 2007
    Posts: 1,402

    Hemi Joel
    Member
    from Minnesota

    I have thought along those lines as well. It seems to be a conundrum. We use the proper gauge wire to avoid voltage drop. And then throw in a little fusible link. Wouldn't that create voltage drop? But the same train of thought also applies to a fuse. Look at the tiny little filament in that thing. Wouldn't that cause even more of a voltage drop than a fusible link? But of course this is all just thinking. I have never actually tested for voltage drop through a fuse or a fusible link with a load and a meter.
     
  18. G-son
    Joined: Dec 19, 2012
    Posts: 1,098

    G-son
    Member
    from Sweden

    Measuring voltage drop across fuses is actually a pretty good and simple method to find which fuses are carrying any significant current, it's just millivolts so nothing to worry about but a good indicator when you need to know if anything is going through the fuse. One way to narrow down the search area when a modern car drains the battery overnight, the problem will be in one of the circuits fed through a fuse carrying current.

    I assume a fusible link will have a bigger voltage drop. The link contains far more metal to melt than a small fuse, so you need more energy to melt it, and you get that extra heat energy by having a bigger voltage drop across the link.
     
  19. FishFry
    Joined: Oct 27, 2022
    Posts: 237

    FishFry
    Member

    That's probably the reason why the former owner of my 41 replaced the only fuse in the whole car (in the light switch) with a bolt :D

    But yeah - you have a good point there. Looks like I will start measuring fuses for voltage drop next week.


    ...or I ask google: https://www.motor-talk.de/forum/aktion/Attachment.html?attachmentId=721311

    Frank
     
    Last edited: Nov 20, 2022
  20. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,218

    Elcohaulic

    Pontiacs had a self resetting circuit breaker built into the light switch so the light come back on momentary so your not completely screwed. They also had one on the power windows so you could still get the windows up or down.
    It was built into this junction block on the innerfender.
     
  21. Fuses, links and circuit breakers all have drops across them, but voltage drop is primarily caused by distance. The short circuit lengths internally in these present a minimal drop, probably no more than the drops caused by their connections.
     
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  22. PotvinV8
    Joined: Mar 30, 2009
    Posts: 328

    PotvinV8
    Member

    Whether it's a fuse, fusible link, or breaker, it needs to be sized for the circuit it's protecting. I'm assuming the wire with the fusible link is providing 12V to the fuse panel as that's usually how they're set up (as opposed to coming directly from the battery). Add up the loads in your vehicle and you can replace that fusible link with the correctly sized fuse or circuit breaker.
     
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  23. FishFry
    Joined: Oct 27, 2022
    Posts: 237

    FishFry
    Member

    My concern was not about the voltage drop per se, but - let's say for an alternator - a 10 gauge wire is recommended - putting a fusible link at 14 gauge in there, wouldn't that bottleneck not make the whole connection a 14 gauge?

    So would it not be more logical to use a 6 gauge wire fore the alternator and a 10 gauge link instead?

    Frank
     
  24. G-son
    Joined: Dec 19, 2012
    Posts: 1,098

    G-son
    Member
    from Sweden

    No, that would make a little section of the circuit 14 gauge, and that is what you need if you want something to fail before the rest is damaged during an overload.
     
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  25. A fusible link is more than a "section of smaller gauge wire"; the links are an actual part and manufactured with special heat/fire proof insulation.
     
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  26. G-son
    Joined: Dec 19, 2012
    Posts: 1,098

    G-son
    Member
    from Sweden

    Yes, absolutely. But still, the point is that they're supposed to be a weak link in the chain.
     
  27. I had one go bad after I had a shorted positive battery cable on an OT Chevy wagon. Right behind and under the distributor on the firewall. I cut out the bad link... wire feels like a worn out rubber band... and spliced in a new wire and fuse holder.
     
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  28. FishFry
    Joined: Oct 27, 2022
    Posts: 237

    FishFry
    Member

    But what good would my 10 gauge wire do, if the whole connection can only handle the load of a 14 gauge, before it goes up in flames?

    If you want a 10 gauge performance, than also your weakest link must be able to handle 10 gauge performance - so the rest should be 6 gauge.

    I don't know - I feel the whole concept is somehow backwards.
    Anyway, I don't have fusible links, and I probably never put them on my cars. I put fuses where I think it makes sense, and keep a hand full of spares in the glove box - just in case.

    Frank
     
    Last edited: Nov 21, 2022
  29. One of the factors that affect resistance is wire length. A short piece of smaller gauge wiring can handle more current than a long run of the same gauge. That’s why the length of the fusible link is important.
     
    Blues4U likes this.
  30. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 4,258

    2OLD2FAST
    Member
    from illinois

    Wonder how many pre 60's cars drove how many miles with no protection between battery & power distribution ?
     
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