Fuses, Electrical System

[Bob Bidonde]

This is my analysis of fusing the electrical power supplys in the Model A's electrical system, and I am posting it for a fact finding check.

Many Model A's have a fuse in the battery power wire connected to the terminal box as shown in the picture. If a short circuit occurs, this fuse will open and disconnect the battery from the terminal box. This fuse protects the electrical system providing the generator or alternator is not running when the short circuit occurs. However, if the engine is running and the generator or alternator is supplying current to the terminal box when a short circuit blows the fuse in the battery power wire, there is going to be smoke because the generator or alternator will have a current output to the short circuit!


To protect the electrical system while the engine is running, it is also necessary to put a fuse in the generator or alternator wire going to the terminal box in addition to the fuse in battery wire. The maximum current output of a stock 6V generator is 22 Amps. An alternator will spike to 30+ Amps just after starting the engine. So the fuse in the generator or alternator wire to the terminal box needs to be 2.5 times the current rating.

[Jim Brierley]

I don't think so, but can't prove it. I think when the fuse blows, everything is protected?

[alexiskai]

This was discussed a few years ago:

https://www.fordbarn.com/forum/showt...8097&showall=1

The hypothesis suggested by Badpuppy was that, if a short occurred while driving, the current from the generator would divert into the short, because it would have lower resistance than other elements, specifically the coil. Without power to the coil, the car dies, which shuts off the generator.

[Keith True]

I can see the theory behind it,nut I don't really see a need for it.If that fuse blows at speed,the engine will sound like a drive by shooting.It will not stay running at high revs with 30 volts going to the coil.You can get home at low speeds though,the generator isn't putting out a high enough voltage to do much damage.I've blown that fuse maybe a dozen times over the years.Mostly no apparent reason.Brake light switches caused the problem 3 or 4 times,and defective new headlight switches a couple more.The internet stories over the years have gotten folks to thinking the generator will instantly self destruct if the battery is taken out of the circuit,that's just not true.It has to rev up enough to generate the high voltages,and long enough to heat up.If it was a known common problem for A's I would be looking into doing it on mine,and customers cars.

[jrelliott]

If the fuse blows while driving, like shown in the picture above, the engine will die. Have experienced it first hand when a tack shorted out wire to inside light before going to the ON/OFF switch. Took some time to find as was hidden under molding around the windshield.

[katy]

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Easy enough to check/test, with engine running, remove the fuse.

[Jim/GA]

Originally Posted by Bob Bidonde

<snip>

So the fuse in the generator or alternator wire to the terminal box needs to be 2.5 times the current rating.

What does "2.5 times the current rating" mean in real numbers?

2.5 times is pretty high. For a 30 amp alternator, that would be 75 amp fuse. That is so high, it is almost like no fuse at all.

[nkaminar]

Fuses are usually rated at some percentage over the normal maximum current, say 20%. So a circuit with maximum of 30 amps would use a 36 amp fuse. If the fuse size is not available then the next available size is used, which in this case would probably be 40 amps.

Circuit breakers are available to fit in the fuse holders. These have the advantage of re setting after the short or overload is fixed.

[Bob Bidonde]

Thanks all for your feedback. The fuse rating should be 1.5 times the current rating. My mistake.

[Jim/GA]

Even 1.5 is high. I use ~1.2 or so, rounded to an even 5 or 10 amps, depending on the actual value.

The biggest short risk is the lighting system, fed by one wire into the headlight switch. Adding a dedicated fuse to that feed wire into the headlight switch can protect your wiring and not kill the ignition, so the engine does not die at an inopportune moment and leave you vulnerable. Depending on the amperage of your headlight bulbs, you can install up to a 30 amp fuse there.

My brake light switch shorted out once when I stepped hard on the brakes and it blew the single fuse I had on the starter. I was getting ready to turn left and suddenly found myself with a dead engine. Luckily I had not yet pulled out into oncoming traffic. It could have been very ugly. I have since added a fuse directly to the headlight switch feed wire.

Another short risk is the horn, because it has a hot wire running into it all the time. A second dedicated fuse there can prevent a problem, yet not kill the engine if it blows. I find a 10 A fuse is adequate there, YMMV.

After taking care of those two items, then you can protect the entire system with a 40 A fuse down at the starter switch. It should never blow, unless you have a bad short in the wires running between starter post, terminal box, ammeter and generator (alternator). If that happens, you want everything to be cut off by that fuse, and (unfortunately) the engine will probably quit running. But no smoke.

[Rob Doe]

Jim, Bob, all others, thanks for this thread.

What are the wire gauge sizes of the wires in the Model A looms? 14 and 16?

Shouldn't the length, diameter and material type be considered in making the fuse size decisions??? In other words, the fuse is sized to protect the wires???

I have a generator look-alike alternator that supposedly has a 60 amp rating. I question whether or not the standard Model A wires will tolerate a 30 amp fuse, let alone a 40 amp or even larger one.

[katy]

Originally Posted by Rob Doe

I have a generator look-alike alternator that supposedly has a 60 amp rating. I question whether or not the standard Model A wires will tolerate a 30 amp fuse, let alone a 40 amp or even larger one.

Put the larger fuse or CB on the alternator output.

[rotorwrench]

A fuse only protects the wire to keep it from burning. Each wire size has a current rating limit so the fuse is sized by the wire size or gauge. Model A cars generally are only fused at the wire from the terminal box to the battery terminal at the starter. If the fuse blows, the question is, will the generator alone carry the load of the ignition coil? The next question is, what will the ammeter read if the fuse blows to the battery since the connection is open. The battery controls the ouput of a 3-brush generator so what will the generator do if the battery is taken completely out of the circuit.

On a car with an alternator, what will the alternator do if the wire to the battery is open? If you can answer these questions then you know what will take place and what to watch out for. Disconnect the battery while the engine is running and you will find out what happens.

[Jim/GA]

Originally Posted by Rob Doe

Jim, Bob, all others, thanks for this thread.

What are the wire gauge sizes of the wires in the Model A looms? 14 and 16?

Shouldn't the length, diameter and material type be considered in making the fuse size decisions??? In other words, the fuse is sized to protect the wires???

I have a generator look-alike alternator that supposedly has a 60 amp rating. I question whether or not the standard Model A wires will tolerate a 30 amp fuse, let alone a 40 amp or even larger one.

Yes, wire is mostly 16 ga, some 14.

You are over thinking it. You size the fuse to a little above the expected load and the wire can handle the expected load. If you have a short, the fuse blows before the wire has had much time with the high current, so no damage to the wire.

I like to try to protect the various systems with smaller fuses and hope that the larger fuse never blows, because then your ignition might go out, or your generator has no battery to regulate the voltage.

So if the short is in the headlights, you lose your headlights but the engine keeps running. If the short is in the horn, you lose the horn but keep everything else. Better than losing everything.