ALT Fuse

DNLs1930

DNLs1930

Senior Member
I tried to search for the conversation about installing a fuse in the alternator/generator wire with no luck so starting a new thread.



I have installed a 6V alternator after a generator failure (pending rebuild) and in the process read about some installing a fuse between the Alt/Gen and the junction box on the firewall. I have the safety fuse mounted on the starter but the conversation was about the Alt continuing to feed power even after the safety fuse opened.



Thoughts? Experience?

THANKS IN ADVANCE...
 
Check out: https://www.fordbarn.com/forum/showthread.php?t=284507&highlight=accessory


There are some posts in there by Y-Blockhead showing a fuse panel install that is unique and exceptionally cheap. He also mentions installing circuit breakers on his starter motor and at the alternator. If you contact him, he will send a well-done schematic of his layout that addresses your issue. His electrical system is 12V but all the 12V components (circuit breakers) work exactly the same for 6V, which is what I also have. I just completed installation of this same setup, which works very well including the 12V version of the circuit breaker.
 
The traditional installation of a fuse for a Model A is to mount it on top of the starter. It is then electrically located between the battery and the generator (or alternator). Should the fuse blow while the engine is stopped, you will not be able to start the car because the ignition circuit is not being powered by the battery. Should the fuse blow while the engine is running, the engine will continue to run because the ignition circuit is being powered from the generator (or alternator), not the battery. The danger here is that the generator (or alternator) does not see the battery and no longer has a voltage reference. The generator (or alternator) will then run wild with the voltage raising to levels that will do damage to the generator (or alternator).

Tom Endy
 
RB_Nielsen said:
Check out: https://www.fordbarn.com/forum/showthread.php?t=284507&highlight=accessory


There are some posts in there by Y-Blockhead showing a fuse panel install that is unique and exceptionally cheap. He also mentions installing circuit breakers on his starter motor and at the alternator. If you contact him, he will send a well-done schematic of his layout that addresses your issue. His electrical system is 12V but all the 12V components (circuit breakers) work exactly the same for 6V, which is what I also have. I just completed installation of this same setup, which works very well including the 12V version of the circuit breaker.
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Thanks for the link!!! I did a quick scan and it looks pretty complete coverage of the conversation...



Tom Endy said:
The traditional installation of a fuse for a Model A is to mount it on top of the starter. It is then electrically located between the battery and the generator (or alternator). Should the fuse blow while the engine is stopped, you will not be able to start the car because the ignition circuit is not being powered by the battery. Should the fuse blow while the engine is running, the engine will continue to run because the ignition circuit is being powered from the generator (or alternator), not the battery. The danger here is that the generator (or alternator) does not see the battery and no longer has a voltage reference. The generator (or alternator) will then run wild with the voltage raising to levels that will do damage to the generator (or alternator).

Tom Endy
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My question was more while the engine is running and a short being supplied power by the alternator. I obviously need to revisit the wiring diagram to see how a single or dual fuse would affect the overall circuit.
 
My question was more while the engine is running and a short being supplied power by the alternator.
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You could put a fuse or CB in line w/the alternator output, sized according to the output rating of the alternator.
 
A generator may have a problem if separated electrically from the battery in operation but an alternator won't since it has internal voltage and current control. If the generator lost it's battery then a person would likely notice that the amp meter was pegged out but the generator could be damaged if run this way too long. A Fun Projects regulator, if the car is so equipped, would likely be fried if this happened.

If a person maintains the electrical system in good condition, there would likely not be much need to replace a fuse. Most of the risk is from a fire related to a short while the car is setting up in the garage. Rodent critters like to chew on wiring for some reason and that's generally during longer periods of setting up. A fuse will keep that from happening. It's better than the wiring or the car burning up.
 
rotorwrench said:
If a person maintains the electrical system in good condition, there would likely not be much need to replace a fuse. Most of the risk is from a fire related to a short while the car is setting up in the garage. Rodent critters like to chew on wiring for some reason and that's generally during longer periods of setting up. A fuse will keep that from happening. It's better than the wiring or the car burning up.
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My car has a battery disconnect switch installed which is always turned off when sitting static so that has that side covered or at least I think it does...



Any "wiring diagrams" available for those with fuse panels installed? My first guess is the fuses are installed in the wires coming from the switch housing but not really sure about that...
 
katy said:
You could put a fuse or CB in line w/the alternator output, sized according to the output rating of the alternator.
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My Alt is rated at 40 or 60 amps (would have to verify) so I was thinking the same size fuse as the safety fuse which is 30 amps...:confused: I think I may be in for a new harness as the wires are all pretty much the same color "grease" :D
 
DNLs1930 said:
My Alt is rated at 40 or 60 amps (would have to verify) so I was thinking the same size fuse as the safety fuse which is 30 amps...:confused: I think I may be in for a new harness as the wires are all pretty much the same color "grease" :D
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30A may be too small, after starting the engine the alternator will be putting out a lot as it is attempting to bring the battery back to full charge. You may have to experiment as what size of fuse you need there.
 
I'm not a fan of battery cut off switches that are place in the direct battery line ( in negative cable to starter or in positive cable to ground). Even though switches are rated for a particular current flow when new, they don't always stay that way as they age. Some are better than others. I like the idea of at least a 1-AWG cable between the battery and the starter for best starting current as well as at least an equivalent low resistance ground cable. The 6-volt battery is capable of a lot more amperage flow than the generator or alternator is.

A master cut off switch on the system buss wire that connects to the starter switch is a good idea since the load is lower in the buss line. A person can put a 20-amp fuse there too to protect the buss from shorts. Fuses are to protect the conductor wires as a general rule. A person can use in line fuses for each component if they want. More modern vehicles use fusible links to protect the alternator side of the buss. It was not a common thing to use fuses in the power generation line for the older cars. The fuse could blow if a battery has a problem that causes undue charging rates so they just didn't do it.
 
rotorwrench said:
More modern vehicles use fusible links to protect the alternator side of the buss. It was not a common thing to use fuses in the power generation line for the older cars. The fuse could blow if a battery has a problem that causes undue charging rates so they just didn't do it.
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50214324053_a9799f2504_z.jpg
 
You ought to see what happens to a buss wire from a 70-amp alternator when it shorts to ground. This was an 8-gauge wire on a 24-volt helicopter. The wire shorted to ground and burned withing 3 seconds. The whole wire from the alternator to the instrument panel went up. It ruined every wire that was bundled close to is as well. This stuff rarely happens but in this case the helicopter was dated to the early 70s, had spent most of its existence out in the open, and it had an antiquated type of wire that has been obsolete for many years. It was in fact the same type of wire that brought a 747 down due to a short inside the center fuel tank some time ago.

I don't much care for fusible links either but they were used by manufacturers for some time. Ford didn't start using them till the 60s.
 
If you want to run a fuse between the alternator and battery, it needs to be larger than the max output of the alternator. The wiring will need to be sized accordingly as well. When I converted my A to 12v, I installed a Powermaster PowerGEN, which is rated at 90 amps max. First trip around the block, I must have replaced the 30amp fuse six or seven times before I finally got it to limp back home. I swapped the alternator wire to the lug on the fuse holder where the battery wire attaches and problem solved. The alternator was sending more than 30 amps through the fuse, blowing it.

Someone mentioned battery disconnects and amperage draw. Just install it in the ground side and you're good to go.
 
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PotvinV8 said:
If you want to run a fuse between the alternator and battery, it needs to be larger than the max output of the alternator. The wiring will need to be sized accordingly as well. When I converted my A to 12v, I installed a Powermaster PowerGEN, which is rated at 90 amps max. First trip around the block, I must have replaced the 50amp fuse six or seven times before I finally got it to limp back home. I swapped the alternator wire to the lug on the fuse holder where the battery wire attaches and problem solved. The alternator was sending more than 50 amps through the fuse, blowing it.
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I don't quite understand how you have it wired. Is the alternator output bypassing the amp gauge and going directly to the battery?

The reason I ask is because I have a PowerGen and have never had a problem with fuses/circuit breaker. Wired as stock except 12v neg. ground and 30a circuit breaker at starter.

In other words I have a 30a circuit breaker between my Powergen and battery and no problem.
 
I wired the alternator directly to the battery terminal on the starter using 8ga wire. Stock harness otherwise. Originally, I attached the 8ga alt wire to the same side as the yellow wire on the fuse and that's when I had issues.

50216783083_dd0a06b8a9_k.jpgIMG_4414 by PotvinV8, on Flickr
 
PotvinV8 said:
If you want to run a fuse between the alternator and battery, it needs to be larger than the max output of the alternator. The wiring will need to be sized accordingly as well. When I converted my A to 12v, I installed a Powermaster PowerGEN, which is rated at 90 amps max. First trip around the block, I must have replaced the 50amp fuse six or seven times before I finally got it to limp back home. I swapped the alternator wire to the lug on the fuse holder where the battery wire attaches and problem solved. The alternator was sending more than 50 amps through the fuse, blowing it.

Someone mentioned battery disconnects and amperage draw. Just install it in the ground side and you're good to go.
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Burnt 50 Amp fuses? WOW that seems very excessive... Also if the fuse is rated higher than the output how can it protect anything? It would seem the fuse would do nothing because the rating of the fuse exceeds the maximum amp output.



I have a positive ground set up with two ground wires, one to the frame near the battery and one that runs to the transmission housing so the disconnect was installed by my uncle (previous caretaker) between the battery and starter. I get the concern with amperage flow through the switch and smaller wire (if used) but I have the larger sized wire running from the disconnect to the starter. Works good as far as I can tell with no issues.
 
katy said:
30A may be too small, after starting the engine the alternator will be putting out a lot as it is attempting to bring the battery back to full charge. You may have to experiment as what size of fuse you need there.
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I do see the amperage increase for a short time :D get it..:)

I maintain the battery with a charger so the alt amp increase is only a few seconds to maybe a minute.


My observation is there are not many owners here that have a fuse in the alt/gen circuit so my thought is this is not really a problem.
 
There will always be a surge immediately after starting, to replenish the battery after the strong discharge. Shouldn't be more than 10 amps or so.

The starter mounted fuse is to protect the battery from a stuck cutout, unnecessary with an alternator.

The only fuses that need to be considered are horn and lighting circuits, the two yellow wires going toward the lighting switch. 10A for horn, 20A for lighting. Fusing the coil is generally a bad idea; instead rely on good looms, properly secured.

If you add accessories they should also be fused, with proper wire size.
 
Also if the fuse is rated higher than the output how can it protect anything? It would seem the fuse would do nothing because the rating of the fuse exceeds the maximum amp output.
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Alternators and generators can put out a lot more than they're rated for. The rating is to protect the manufacturer.
 
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