Need Help understanding Fusible Links

[sprice]

I am upgrading my wiring n 68' 300. Ron Francis advises not to use Fusible Links. I am reading the wiring diagrams from the service manual and am trying to understand electric flow. What I have realized, if correct, is that electricity does not necessarily have a flow pattern. Its more of being present and will flow depending on draw. What I am trying to understand is what the purpose of the fusible link from the bulkhead is supposed to do. 12ga black from alternator to "P" in connector, to ammeters right terminal (from back), then 12ga red exits on left side to bulkhead "J", into fusible link 16ga, then to 10ga to starter relay. If its not a flow thing, what is the 16ga fusible link protecting? My understanding is that 16ga is good for 20 amps. It seems to me you have full amps from the alternator (50 or 65 amp) flowing to the ammeter gauge while being split to ignition switch and fuse block before entering the ammeter. Then what happens to the 30-50 amp charge from the alternator if there is a 20 amp fusible link coming from the alternator through "J" terminal to solenoid and battery? I think where I'm getting lost is applying fluid flow patterns.

 

[Davea Lux]

I am upgrading my wiring n 68' 300. Ron Francis advises not to use Fusible Links. I am reading the wiring diagrams from the service manual and am trying to understand electric flow. What I have realized, if correct, is that electricity does not necessarily have a flow pattern. Its more of being present and will flow depending on draw. What I am trying to understand is what the purpose of the fusible link from the bulkhead is supposed to do. 12ga black from alternator to "P" in connector, to ammeters right terminal (from back), then 12ga red exits on left side to bulkhead "J", into fusible link 16ga, then to 10ga to starter relay. If its not a flow thing, what is the 16ga fusible link protecting? My understanding is that 16ga is good for 20 amps. It seems to me you have full amps from the alternator (50 or 65 amp) flowing to the ammeter gauge while being split to ignition switch and fuse block before entering the ammeter. Then what happens to the 30-50 amp charge from the alternator if there is a 20 amp fusible link coming from the alternator through "J" terminal to solenoid and battery? I think where I'm getting lost is applying fluid flow patterns.

The purpose of the fusible link is a simple one, do not over think this. The fusible link exists to protect the wiring harness in the event there ever is a direct short to ground from one of the accessories or the wiring to them so that all the wiring in the harness between hot lead and the short does not get cooked. Think of it as a sacrificial part that protects the rest of the system. Fusible links are sized to the maximum operating load, anything more than that and they blow out to protect the rest of the wiring..

Dave

 

[Yeahrightgreer]

If you’re not going to run a fusible link I hope you’re carrying a good ‘C’ class extinguisher. I’d hate for your Chrysler to end up like this...

On a serious note. Install the link. All a fusible link is, is a fuse, in wire form, that’s inline with the rest of your wiring harness of Incase of an overload. Do not get wrapped up in what the link’s purpose is in normal conditions. Like Davea stated, it’s for moments of intense current overload.

 

[detmatt]

If you’re not going to run a fusible link I hope you’re carrying a good ‘C’ class extinguisher. I’d hate for your Chrysler to end up like this...

View attachment 357285

On a serious note. Install the link. All a fusible link is, is a fuse, in wire form, that’s inline with the rest of your wiring harness of Incase of an overload. Do not get wrapped up in what the link’s purpose is in normal conditions. Like Davea stated, it’s for moments of intense current overload.

I carry fire extinguishers in addition to keeping the fusible links in tact in both of my drivers.

 

[Big_John]

IMHO, replacing the original harness with a Ron Francis harness is not an upgrade. I've never seen an aftermarket harness that isn't some hacked up mess once it's installed.

 

[road chicken]

For a visual... electricity is like water behind a dam. Voltage is the pressure and current is the flow. The higher the damn ( voltage) the higher the pressure ( ability to jump gaps). The flow is what ever it can get away with. Without proper flow control of resistance ( like valves in the dam or the dam itself) it just rushes out and wipes out everything until it's spent. Hence the marshmallow roaster posted above.
Ok so fusible links are the fail safe incase the current flow gets out of hand. They will burn up and create an open circuit if something shorts out. They have set amp ratings and usually are sized to do so using a 2 down size. For instance if the biggest wire on the normal circuit is a 10 ga, the FL will be a 14 ga. If the biggest is a 14 ga the FL will be a 18 ga. ( Wires get larger the smaller the number)
Now I've never had one blow but if you do your kinda stuck even if you fix the problem. But it's what they had back in the day. Now, if you not worrying about factory correct, substitute a properly sized , resettable circuit breaker. Don't use a auto reset breaker like the door windows use. If the main feed shorts your going to want find out why before power gets reapplied.

 

[70bigblockdodge]

That is all good advice^^^^^^^
Depends on how original you would like to stay.

 

[road chicken]

As for the flow pattern..lol Chrysler had some "unique" ideas with wiring. Normally the amp meter just reads only the current going to or being drawn from the battery. The alternator output comes into the system and supplies power from a tie point under the dash. Usually a clump of 4-6 black wires that have tape where they are welded together. So not everything the alternator is putting out goes through the amp meter and to the battery. IMHO..dump the Amp meter and use a volt meter instead. Those are the number one cause of burnt mopars.

 

[cbarge]

Sprice, do not read into it too much.
I recommend following my suggestions.

As an owner of a 1968 Chrysler,and being around C bodies all my life,fusible links are your sacrificial lamb for the good of the car.
I had experienced nuclear meltdowns first hand and done many total gut and rewires of all makes of older Mopars.
The ammeter gauge is the long hair on Sampson--when cut no power!! LOL!
I bypass the bulkhead altogether and do a shunt under the hood.
Very reliable and I do use fusible links on my upgrades.
They do work and .They do appear "stock"
You can conviently use inline Maxi-Fuses but just does not look right on a 52 year old car. But that's just my opinion.

So in layman's terms do the following;
1. Run a 10 gauge wire from the battery stud back of alternator (7/16" nut) to the battery stud of (1/2" nut) at starter relay with a 14 gauge fusible link AT RELAY END. This maintains a closed loop in the ammeter circuit should the ammter shorts out.
2. Run another wire at starter relay's battery stud with a fusible link go through the firewall and connect to the POSITIVE side of ammeter gauge
3. Optional not necessary..Run another wire from the battery stud of alternator run through firewall and connect to the NEGATIVE side of ammeter gauge.
4.Optional is add a voltmeter under the dash since the ammeter gauge does not read accurately yet will still work. plug positive wire into the acc or switched side of fuse panel and black wire is grounded anywhere.
AND if the ammeter craps out, the car will still function normally.

In conclusion I am running a 78 amp alternator with my upgrades and have travelled many miles with no meltdowns. I do carry an extinguiser no matter what.
Hope this helps.
Any questions PM me

 

[Mr onetwo]

a good Mopar-centric video on the subject

 

[cbarge]

a good Mopar-centric video on the subject

Not disputing this but the videos are more geared towards the trucks.
Try to stay focused on the C body at hand,LOL!

 

[mikedrini]

I ordered a Ron Francis kit for my 64. Not only was it missing an integral wire (drawing a blank on which one, but I think it was one of the PWR wires) that ran directly to the fuse block, but when all was said and done, I ended up making my own from scratch using the original wiring diagram. Seriously, that thing was my best friend. If you can get over the initial appearance of brain surgery that is the amount of wires you have to trace with your eyes the first time you look at a wiring diagram, it became easier tackling it one wire at a time.

I tried calling their customer service line and the rep just kept telling me, "Well it should be there". Once that got me nowhere, I said eff it and built my own. If I had to do it all over again, I would just by the "guts" (21 fuse block, different wire gauges, spare fuses, soldering iron, heat shrink, etc.) and do it myself. Essentially a couple hundred dollars worth of a lesson learned for my first go-around.

 

[73Coupe]

I've been wanting to get replacement fusible links for my car to have on hand should the originals ever light up. However the connectors are very unique, I was going to start with Evans wiring to see if he has them....

 

[sprice]

I am overthinking this. I ended up redoing the harness as I wanted to upgrade some wiring from a real hack job from some previous mechanic/owner pricking every wire to find whatever and then heat cycles have made everything crumble under there. I probably used to be that hack guy in my early years but now take my time, proper wiring, crimp, heat shrink, clean, lubricate etc., make it neat, clean and safe. Cant believe how some people work on electronics in cars. The sight of aftermarket radios installed in classics urks me. I find the wiring diagrams easy to read and absolutely necessary. Thanks all.