Everyone things this stuff is 'So Simple',
But it's actually Evolution of about 130 or so years, and it's MUCH more complicated than most think...
The 'Resistor' in the altenator circuit 'Excite' wire is a stop-gap to get the engine to shut off when minimizing the wiring from ignition switch to engine bay.
Running everything off one ignition circuit in the switch (Gauges, ignition, alternator, ect.) if anything in ANY of that wiring goes wrong, your ignition might not work.... Or work correctly.
A Diode is a VERY simple way to replace that 'Iffy' and trouble prone resistor wire.
(Corrodes badly, rots terminals, breaks easily with vibration since it's a solid core, ect.)
The diode cost next to nothing, eats nothing, and works well without ANY back-feeding into the ignition system,
Doesn't heat up like the resistor wire does,
Doesn't break with vibration since it's connected to flexible strand wire on both ends, ect.
The fuse in the system is there because several generations of vehicles burned to the ground.
A solution had to be found, and a fuse was a good way to protect the wire to the generator/alternator.
When hooked to... Or very close to, the battery, they had explosions...
The way to avoid that was to move the fuse away from the battery, so it wound up at the starter relay/positive battery cable so it still had a direct, large gauge cable connection to the battery positive.
Now, battery doesn't blow up and vehicle doesn't burn down.
Fusible Links blow much 'Slower' than regular fuses, so it allows the alternator to put out a BUNCH of current without popping a regular fuse,
But it still protects the heavy wire against a 'Dead Short'...
You CAN blow the fusible link by grounding a wrench out when taking the wire off the back of the alternator!
Some people have a 'Bad' alternator, go to change it, can't figure out why the 'New' alternator won't charge...
Mostly because they grounded out the 'BAT' terminal on the back when removing the alternator and created another 'Issue' by blowing the fusible link.
What I usually do is...
Use a battery 'Load' tester on the 'BAT' terminal of the alternator and 'Load' the fusible link.
If it puts good current through to the tester, then the fusible link/alternator heavy wire is working correctly,
And the 'Issue' is elsewhere...
I don't know how many 'Cooked' alternators I've seen in over 30 years of doing this because the alternator couldn't get a solid 'Ground'...
If you are going to produce a bunch of current (Some alternators will produce 140 amps all day long!), then you MUST have a good 'Ground' connection back to the battery.
Rust in bolts/bolt holes, loose bolts, painted brackets, rusted brackets, Rusted bolts on 'Ground' cables, corroded or loose 'Ground' cables, corroded battery terminals or cables will ALL Kill and alternator,
And all that resitance adds up in the system to REDUCE the current your alternator produces, but can't get through to the battery/vehicle loads.
(Load = Short for 'ELECTRICAL LOAD', Anything that uses electricity on your vehicle.
The starter being the largest FACTORY load,
An electric Winch being the largest added electrical 'Load' usually.)
If you want the charging system to work 'Correctly',
You need a DEDICATED 'Ground' wire to the alternator at least as large as the 'Positive' or 'BAT' terminal wire.
In most cases, that's 10 Ga. or larger.
'Ga.' refers to 'American Wire Gauge' or just 'Wire Gauge'.
'Ga. is an abbrivation of the word 'Gage' or 'Gauge', either spelling is acceptable.
The American Wire Gaged standard is based DIRECTLY off the Brown & Sharp scale from 130 Years ago, and I've listed the original Brown & Sharp tables below.
Wire gauges are kind of strange,
The LARGER the number, the SMALLER the wire! Dead backwards of what people think it is!
A 16 Ga. wire is smaller than a 10 Ga. wire.
And when you get up in SIZE (Smaller Numbers) to about 6 Ga. we call them 'Cables' instead of 'Wires'.
( your battery 'CABLE' is probably a 6 Ga. or 4 Ga. 'Wire', but we call them 'Cables' after about 8 Ga.)
Now, as the wire size increases, it can be a '0' (Zero) gauge,
When they get larger, we call them 2/0 ('00', Pronounced 'Two Ought') and even larger would be a 4/0 ('0000') Gauge...
This FURTHER confuses things!
This is a copy of the 'Brown & Sharp' scale, and it tells you more information about wire sizes than you probably ever want to know!
The SMALLER the number, the LARGER the wire and the more amperage it will carry.
This also tells you resistance per foot (long runs of DC wiring reduce the current delivered, so you UP size the wiring on long runs).
Direct Current (DC) is VERY hard to push through wires very far,
Works GREAT for automotive purposes, which is why we are still using it 150 years after the first car makers started using it,
But it can be confusing until you get a grip on the terms and quirks of how it's measured/gaged, transfered from battery to 'Load' and produced in the generator and transfered to the battery.
This table is for CONTINUOUS USE,
When it says that a 10 Ga. wire will only handle 32.5 Amps (like the wire between your alternator to your battery),
That means you can run 32.5 AMPS CONTINUOUSLY through that wire without it heating up.
You can force MUCH MORE amperage through that 10 Ga. wire for a short period (Which your alternator does quite often), but the wire WILL heat up because of internal resistance in the wire.
The more you try and force through it, the faster the wire will heat up, and the higher that heat will go with higher amperages...
Most guys with 80 Amp alternators and a 10 Ga. wire have the delivery wire SERIOUSLY undersized if they think the alternator cranks 80 amps all the time!
This resistance/heat up of the wire is why you use a MUCH larger wire for the starter than you do for some little relay,
And why you use even LARGER cables for a winch than you do a starter, because a winch usually pulls harder/longer than a starter does...
Welding usually only has to handle between 40 and 100 amps,
And starters usually pull between 75 and 300 amps,
Where your winch will often pull 700 Amps or more!
Welding takes SMALLER gauge wire to do 'Correctly' than a starter,
And a winch will take MUCH larger cables to supply the electric motor correctly since it's loaded even harder than welding!
(People don't realize you can weld off of two batteries pretty easily, even through they are trying to pull 700 AMPS through 6 Ga. or 4 Ga. wires on a winch!
Does't make sense to me, but I see it all the time, winches with 6 Ga or 4 Ga. wire,
And 0 Ga. or 2/0 Ga. welding cables!)
Some of the newer vehicles have reduced load starters, so they only have a 10 Ga. or 8 Ga. wire feeding them...
Try that with a Jeep, and you will be changing a LOT of starters since you aren't delivering enough current to the starter and burning them up!
ALWAYS BETTER TO GO A WIRE SIZE OR TWO UP AND DELIVER A GOOD SOLID SUPPLY TO THE LOADS,
Than to be popping fuses, staring fires, melting insulation, ect.
You supply your 'Loads' with larger gauge 'POSITIVE' supply,
You MUST add a reasonable size 'Ground' or 'NEGATIVE' return wire to the battery for the 'Load' to function correctly!
THE CIRCUIT MUST
CLOSE BACK TO THE BATTERY!
If it doesn't 'Close', then it's not a 'Circuit', it's an 'Open' and your stuff won't work correctly!
Hope this helps.