Model A fuse.

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frank d kirkstad

Member
I drive a 36 ford now,but I have had a number of Model A's in my past.I was at a car show recently and a gentleman with a newly acquired 30 A, could not get it to start.He thought his battery was bad because nether the lights or horn worked,but I saw that the starter was turning engine over fine.I noticed he had a fuse mounted next to his starter switch that was bad and when I replaced it, all was fine. My question is,should the cars ignition be hooked into this fuse also?,it doesn't seem right to me.I never added a fuse to any of the A's I've owned although I probably should have.If it is wrong I will rewire it for him, especially if it's a safety issue.Thank's
Frank
 
frank d kirkstad said:
My question is,should the cars ignition be hooked into this fuse also?
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I could be wrong, but I think the logic of locating the fuse immediately after the starter in the wiring sequence is that if you located it later, you'd have to have multiple fuses. Current travels from the battery to the starter, then to the ammeter, and then to a node in the junction box that connects to multiple independent circuits including the ignition. So if you wanted to exclude the ignition you'd have to put fuses on each of those separate circuits instead.

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I added a fuse block to help prevent burning wires from a short at the headlights and horn. Makes it easy to isolate and eliminate a fire source.
 

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The fuse located on the starter is an aftermarket mod that is widely accepted as a worthwhile safety device. It is spliced in between the battery cable downstream of the starter stud. The fuse carries the entire electrical load with the exception of the starter current. A blown fuse will allow the starter to run, however, no current goes to any circuit including the coil. I had inadvertantly bumped a torque wrench against one of the junction box wing nuts when checking a head bolt. Did not think I did any damage dispite the flying sparks. Went to start and it cranked fine but would not start. Checked the fuse which was indeed blown. Took a fresh fuse from the driver's door pocket, installed it and the engine started immediately. I would say the subject car at the car show is wired correctly and does not warrant any additional modification.
 
I consider the fuse modification a must have on any Model A. It is also a good idea to remove the fuse from the holder and stand it on its head at either end of the holder when finished driving for the day. This will remove power from all the circuits in the car except power to the starter, which is not normally a problem area.

Should there be a partial short circuit somewhere where the current draw is less than the value of the fuse, say it is drawing 20 amps and the fuse is a 30 amp. It will sit there and smolder long enough to light a fire. Removing the fuse will prevent that.

Tom Endy
 
Removing the fuse is also the fastest and easiest way to cut power when you need to mess with the electrics, e.g., working on the horn.
 
One mark against having one fuse to protect all of the circuits is that is you have a short somewhere, everything dies.



Normally this isn't very dangerous but one night I was driving home and forgot about the intermittent short in my dash light and when I went to turn it on I blew the fuse and lost everything... no headlights, no taillight, no brake light... and no ignition. Luckily I was heading into town when this happened and no other cars were nearby so I got it into a parking lot and changed the fuse. Had that happened any earlier I would have been on the shoulder of an unlit 50mph road with no lights in a dark car.. no good.


This post is getting long but in the end I still think having a fuse is the safer alternative. You may, however, want to consider putting individual fuses on short prone circuits.
 
Thank's very much guys for all the helpful replies!I just wanted to make sure his system was wired correctly.I will relay all your replies to him and make sure to tell him to carry extra fuses. Anyway,I did make a new friend that evening.
Frank
 
ryanheacox said:
Normally this isn't very dangerous but one night I was driving home and forgot about the intermittent short in my dash light and when I went to turn it on I blew the fuse and lost everything.
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Not disputing your overall point at all, but in this particular case I think it would have been unlikely that someone (who didn't already know there was a short there) would have fused that individual circuit. The dash light comes off the ammeter, which is right after the starter and before the junction of the lights and ignition. In order to isolate that one component you'd have to put a fuse inside the instrument panel or run wires from the ammeter back to a fuse box and then back up to the dash light. I mean someone certainly could do that, just seems unlikely they'd bother.
 
alexiskai said:
Not disputing your overall point at all, but in this particular case I think it would have been unlikely that someone (who didn't already know there was a short there) would have fused that individual circuit. The dash light comes off the ammeter, which is right after the starter and before the junction of the lights and ignition. In order to isolate that one component you'd have to put a fuse inside the instrument panel or run wires from the ammeter back to a fuse box and then back up to the dash light. I mean someone certainly could do that, just seems unlikely they'd bother.
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Yup, I was thinking the same thing after I posted that. No easy/convenient way to isolate the dash light.
 
Always try the horn. If it won’t work, your fuse is blown.
On using the fuse as a disconnect switch: it was not designed for the many cycles of a switch. If you need a switch, put one in. That many ins and outs will loosen those little rivets and leave you stranded. When that happens it is very hard to find.
 
frank d kirkstad said:
Thank's very much guys for all the helpful replies!I just wanted to make sure his system was wired correctly.I will relay all your replies to him and make sure to tell him to carry extra fuses. Anyway,I did make a new friend that evening.
Frank
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Frank,
You touched on something that all Model A enthusiasts appreciate. Model A guys are like marines in that there are no strangers in the community. Even in Jersey (I am originally from Exit 13) where they drive with two hands (one on the horn and one out the window), two Aers unknown to each other will immediately talk about their rides like old buds. Would the world be a more peaceful place if everyone owned a Model A ? I think so.
 
I believe I read here or somewhere else there is a possibility of a problem with the Starter Mounted Fuse. If the Generator can keep the motor running with the fuse blown, then teh generastor will peg voltage and can damage things - True, or not true?
 
In theory I think this is true but every time I've blown a fuse with the engine running it has died immediately. Maybe someone else can add more info?
 
depending on how its wired. i custom wired mine. before the rewire I had a loose battery conntection the lights burned out. if your fuse is in between your ignition and generator the car will shut off. i have two 30 amps near my starter that feed a 8 spot fuse box. i wanted to have my ignition and lights/horn separate. So if I had a horn short or tail light short. my car would hopefully still run.
 
mike657894 said:
depending on how its wired. i custom wired mine. before the rewire I had a loose battery conntection the lights burned out. if your fuse is in between your ignition and generator the car will shut off. i have two 30 amps near my starter that feed a 8 spot fuse box. i wanted to have my ignition and lights/horn separate. So if I had a horn short or tail light short. my car would hopefully still run.
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Seems you have a customized fuse set up. I was referencing the standard starter mounted fuse set up where only the starter is not fused by the single starter mounted fuse.
 
Generator output voltage varies with applied load. Higher load, lower voltage. That's why you have to readjust between day and night driving.

If you remove the fuse with the engine running, voltage will go up due to battery charge load being removed. But it will continue to run, perhaps damaging the coil from the higher voltage.

If a 30-amp fuse blows because of a short, then all generator output goes into the short. 0 volts at the coil - engine dies. A good thing, until you remove the short.
 
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