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Old 07-16-2016, 09:09 AM   #1
Dick Steinkamp
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Default Fuse

I'll admit that electricity is basically magic to me and this is going to sound stupid to those that have a good grasp of it.

The common place to fuse a Model A is on the wire between the starter and the terminal box. It seems to me that this would work fine if a short developed anyplace in the system when the engine was not running (except of course IN the starter or starter switch).

With the engine running, however, if the fuse blew, wouldn't the generator back feed everything including the ignition, horn, and lights? Wouldn't the engine continue to run and continue to feed the short?
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Old 07-16-2016, 09:31 AM   #2
Kurt in NJ
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Default Re: Fuse

Yes, but the generator can soon overvoltage unless the load was adjusted to the output-----when the generater is putting out 40 volts it soon burns up, then the car stops, usually the car runs bad, and bulbs that are on burn out, then the engine stalls the first time it comes back to idle and the generator goes off charge
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Old 07-16-2016, 09:41 AM   #3
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Default Re: Fuse

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Originally Posted by Kurt in NJ View Post
Yes, but the generator can soon over voltage unless the load was adjusted to the output-----when the generator is putting out 40 volts it soon burns up, then the car stops, usually the car runs bad, and bulbs that are on burn out, then the engine stalls the first time it comes back to idle and the generator goes off charge
Then it is probably worse than I thought. If a short occurs while the car is running, generator goes over voltage and continues providing the short with power...even MORE power until the generator self destructs along with other electrical devices. Probably a pretty crispy car too by that point.
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Old 07-16-2016, 10:22 AM   #4
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Default Re: Fuse

The model A electrical system is essentially a single circuit with different branches... If a short happened anywhere's in the circuit the car would lose spark and shut down...
If a fuse was installed that may also blow and cause a no start... If the short is intermittent the car may have intermittent spark causing a drivability issue and may not blow the fuse right away...
An EVR in the generator will protect it from burning up or if you had high resistance in the battery connections it will help preserve the generator that way also...
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Old 07-16-2016, 11:05 AM   #5
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Default Re: Fuse

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Originally Posted by Mitch//pa View Post
The model A electrical system is essentially a single circuit with different branches... If a short happened anywhere's in the circuit the car would lose spark and shut down.
I should probably explain why I'm looking into a fused system.

Yesterday on a drive, the cockpit started filling with smoke. Engine still running fine. I pulled to the side, shut off the engine (which was still running), got out and opened the driver's side of the hood. The wires at the left front corner of the radiator were on fire. I was able to put the fire out, and determined that the wire from the generator to the horn/light switch had shorted out and started the fire. Direct short, but the car did not loose spark and shut down.

I'm just wondering if the normal fuse between the starter and the terminal box would have prevented the fire. It seems to me like it wouldn't since even if if it had a fuse and the fuse blew, that the generator would continue to output.

(BTW, what's an EVR? Do Model A's have one or would this be an add on?)
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Old 07-16-2016, 11:27 AM   #6
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Default Re: Fuse

"I'm just wondering if the normal fuse between the starter and the terminal box would have prevented the fire. It seems to me like it wouldn't since even if it had a fuse and the fuse blew, that the generator would continue to output."

On the original stock generator, the battery is the only control for the output, so if you have an open in the charging circuit the generator has nothing to control the output and the generator keeps raising the voltage until it meets the limits of the design, which is about 40 volts. 40 volts will cook the generator.

If you have a SHORT, then the short becomes the reference for the generator output, and the generator will be safe at 0 volts because the output is directly shorted to ground. But since the short is also being fed by the battery, something will burn up, either the fuse or the wires.

The EVR (electronic voltage regulator) will help give the battery the correct charge under varying conditions, as needs dictate, and it will also limit the output voltage to a safe level if you should have an OPEN in the charging circuit.

It sounds like you had a short in the lights/horn circuit. Did you have the lights on when the trouble happened?
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Old 07-16-2016, 11:30 AM   #7
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Default Re: Fuse

There are different severitys of a short.. Some can just be a slow burn where others can be an all out fuse / circuit breaker blower..this can also effect whether it causes a drivability issue or not . Depending if your headlights were on or not may effect the situation also.. It's also possible maybe the offending wire was vibrating intermittently against the ground. Not all shorts are created equal..
I'd rather have a fuse then not have one, along with a batt cut off

An EVR electronic voltage regulator goes on or in the generator and controls output according to load.. Basically is an automatic 3rd brush... I use Tom W 's units in my generators...
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Old 07-16-2016, 11:55 AM   #8
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Default Re: Fuse

FWIW, with a fuse between the starter and the terminal box, I also put separate fuses in both the horn and lighting wires coming off the generator terminal.
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Old 07-16-2016, 02:21 PM   #9
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Default Re: Fuse

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FWIW, with a fuse between the starter and the terminal box, I also put separate fuses in both the horn and lighting wires coming off the generator terminal.
Me too...
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Old 07-16-2016, 07:12 PM   #10
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Default Re: Fuse

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Originally Posted by Tom Wesenberg View Post

It sounds like you had a short in the lights/horn circuit. Did you have the lights on when the trouble happened?
Lights were not on.
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Old 07-16-2016, 07:28 PM   #11
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Default Re: Fuse

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FWIW, with a fuse between the starter and the terminal box, I also put separate fuses in both the horn and lighting wires coming off the generator terminal.
Although a lot of good info here, I think only Kurt (post #2) addressed my question...

With the engine running, however, if the fuse blew, wouldn't the generator back feed everything including the ignition, horn, and lights? Wouldn't the engine continue to run and continue to feed the short?

From Kurt's reply (post #2) and the fact that two posters here also fuse the horn and lighting wires it appears that the one fuse between the starter and the terminal box does not protect a running engine.It also appears that even if all three fuses blew that the engine would continue to run and back feed a short in the terminal box or behind the dash.
I think (and again, I'm not electrical expert) that to actually protect a running engine, the fuse would have to be downstream of BOTH the battery AND the generator and all electrical devices would need to be downstream of that fuse. The way the A electrical system is designed the fuse cuts the battery as a source of power in the event of a short, but does not cut out the generator as a source of power.Even fusing the wire coming off the cut out to the horn and lights still does not protect those electrical devices between the generator and the single fuse at the starter.
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Old 07-16-2016, 08:53 PM   #12
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Default Re: Fuse

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Originally Posted by Dick Steinkamp View Post
Lights were not on.
The headlamp power would stop at the light switch, so it must have been the horn power feed wire that shorted to ground.
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Old 07-16-2016, 09:06 PM   #13
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Default Re: Fuse

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Originally Posted by Tom Wesenberg View Post
The headlamp power would stop at the light switch, so it must have been the horn power feed wire that shorted to ground.

...or the always hot wire that FEEDS the light switch (which I believe was the culprit).
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Old 07-16-2016, 10:26 PM   #14
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Default Re: Fuse

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Originally Posted by Dick Steinkamp View Post
...or the always hot wire that FEEDS the light switch (which I believe was the culprit).
Did you find the spot in the wire that shorted to ground? Pictures of the burned wire might also help.
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Old 07-16-2016, 10:33 PM   #15
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Default Re: Fuse

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Did you find the spot in the wire that shorted to ground? Pictures of the burned wire might also help.
By the time I got the fire out, the bundle at the left corner of the rad had the insulation pretty much burned off all wires. That hot wire that feeds the light switch looked the worse for wear, but it certainly also could have been the hot wire to the horn.
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Old 07-16-2016, 10:44 PM   #16
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Default Re: Fuse

I don't know much about electricity either, but I have a fuse mounted on the starter between the starter and terminal box. I had a shorting problem with my lights a while ago. The short in the lights blew the fuse and stopped the engine and all other components from running. It would not let me start the car (without blowing another fuse) until the short was corrected. As Mitch said, it is a single circuit so the fuse likely will blow with a short somewhere in the single circuit.
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Old 07-17-2016, 12:54 PM   #17
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Default Re: Fuse

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I don't know much about electricity either, but I have a fuse mounted on the starter between the starter and terminal box. I had a shorting problem with my lights a while ago. The short in the lights blew the fuse and stopped the engine and all other components from running. It would not let me start the car (without blowing another fuse) until the short was corrected. As Mitch said, it is a single circuit so the fuse likely will blow with a short somewhere in the single circuit.
Ken's post gave me an idea. I could duplicate a "blown fuse" in my car by disconnecting the starter post to terminal box lead. Starting the car with a jumper from the starter post to that lead, then disconnecting the jumper to see if the car still runs.

I found the opposite that Ken experienced. My car continued to idle with the jumper disconnected...

https://www.youtube.com/watch?v=YpXun-kX-5w

Again, my knowledge of electricity is severely limited, but it seems (at least in my test) that the generator will continue to power the ignition with the battery out of the circuit. Of course it won't start with this wire disconnected (or the single fuse blown), but the car will run the same with or without once running.

This tells me that you really need 3 fuses to protect the wiring. One at the wire that feeds the terminal box from the starter post, one on the wire coming off the generator that feeds the lights and horn, and one on the wire from the generator to the terminal box. Any less and I don't believe you are actually protecting the wiring from a short while the car is running.
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Old 07-17-2016, 01:20 PM   #18
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Default Re: Fuse

There is a difference of merely disconnecting the wire as you did VS shorting out the circuit.

I don't recommend experimenting with either

Last edited by Mitch//pa; 07-17-2016 at 02:11 PM.
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Old 07-17-2016, 02:31 PM   #19
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Default Re: Fuse

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Originally Posted by Mitch//pa View Post
There is a difference of merely disconnecting the wire as you did VS shorting out the circuit.

I don't recommend experimenting with either
There very likely could be a difference, but I won't be carrying out the second half of the experiment.
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Old 07-17-2016, 02:48 PM   #20
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Default Re: Fuse

If you want to protect your generator from burning up due to an open in the charging circuit you could install an EVR, or you could do as some generators used to do years ago. Some makes of generators put a fuse between the adjustable brush and field winding, so if the voltage shot up the amps would also increase through the field winding and blow the fuse.

Some people use a 30 amp fuse at the starter switch, then use a 20 amp fuse at the terminal feeding power to the lights and horn. This way if the lights or horn short out and blow the 200 amp fuse, the car still works fine with the good 30 amp fuse. You simply won't have lights and horn until the short is fixed.
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