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Old 01-27-2021, 12:13 AM   #1
eamonford
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Default Keeps blowing fuses — alternator problem?

My Model A is running a 12V system, with a 60A alternator using the one-wire setup. I recently replaced the coil, and added a ballast resistor. The next time I drove, the car ran fine, but when I hit about 50mph, the headlights blew out and a few miles later, the starter fuse blew. I disconnected the horn/lights wire from the alternator and replaced the fuse, and drove again — after a few miles and when I had been going 50mph for about 30 seconds, starter fuse blew again. Thought it might be a short in the ammeter, so I removed the ammeter and temporarily spliced the wires together (also disconnected the instrument light). But the fuse blew again after a few miles and 30 seconds of driving 50mph. Later after replacing the fuse I tested the voltage at the alternator BAT post while idling, and it was a steady 14.5V. I should note that before removing the ammeter, it would read 30A when I drive 50mph, and these are 20A fuses. So that seems like a problem... but why is the alternator putting out that much current?

But here’s something important: each time the starter fuse blew, the engine cut out. Shouldn’t the alternator be supplying power to the ignition system? As an experiment, I removed the starter fuse and tried hand-cranking the engine, and it did not start. Does this mean something’s wrong with my wiring? I have the starter fuse connected in between the terminal box and the post on the starter.

Thanks very much.

Last edited by eamonford; 01-27-2021 at 12:37 AM.
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Old 01-27-2021, 02:14 AM   #2
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Default Re: Keeps blowing fuses — alternator problem?

Had the same thing happen to me when I made the jump to 12volts and a larger alternator. I moved the alternator charge wire to the non-fused side of the fuse holder (where the positive battery cable attaches) and it stopped blowing fuses. The 50amp fuse was smaller than the alternator capacity, which sounds like the same problem you might be having.
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Old 01-27-2021, 02:16 AM   #3
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Default Re: Keeps blowing fuses — alternator problem?

I suspect you are losing the ground to the battery. When that happens the voltage from the alternator is not controlled and it raises and burns out the lights.

The usual place it is lost is where the ground cable is attached to the frame. Remove it and clean the raised boss on the frame where the hole is with a sanding disk until it is bright metal. Wire wheel the nut, bolt, and the end of the ground cable. Clean both battery terminals while you are at it.

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Old 01-27-2021, 09:03 AM   #4
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Default Re: Keeps blowing fuses — alternator problem?

...

Last edited by Benson; 01-27-2021 at 10:58 AM.
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Old 01-27-2021, 11:03 AM   #5
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Default Re: Keeps blowing fuses — alternator problem?

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Originally Posted by eamonford View Post
Later after replacing the fuse I tested the voltage at the alternator BAT post while idling, and it was a steady 14.5V.
Did you check the voltage w/the engine revved up?
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Old 01-27-2021, 11:05 AM   #6
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Default Re: Keeps blowing fuses — alternator problem?

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Engine won't start without battery voltage at the coil. You can't crank hard enough to provide it from the alternator.

Tom Endy cites the usual cause of what you describe. Clean your grounded battery connection. I would also connect a #12 copper wire directly between alternator housing and battery ground post. Then you won't have to rely on rusty steel connections for a good ground path.
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Old 01-27-2021, 04:02 PM   #7
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Default Re: Keeps blowing fuses — alternator problem?

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Engine won't start without battery voltage at the coil. You can't crank hard enough to provide it from the alternator.

Tom Endy cites the usual cause of what you describe. Clean your grounded battery connection. I would also connect a #12 copper wire directly between alternator housing and battery ground post. Then you won't have to rely on rusty steel connections for a good ground path.
Or getting a ground through paint powder coat!
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Old 01-27-2021, 05:06 PM   #8
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Default Re: Keeps blowing fuses — alternator problem?

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Originally Posted by PotvinV8 View Post
The 50 amp fuse was smaller than the alternator capacity, which sounds like the same problem you might be having.
Whoa! Are you really running a 50 amp fuse or is that a typo? I think I would rethink using that big of a fuse.
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Old 01-27-2021, 05:52 PM   #9
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Default Re: Keeps blowing fuses — alternator problem?

Thanks so much for all the responses! I will try what Tom Endy suggested (wire the alternator housing directly to the battery ground, and clean the battery ground cable and terminals) and will report back with results. In the meantime, I also tested the voltage with the engine revved up (albeit in idle) and it showed 20V at the alternator! Is this a problem? I thought it shouldn't go higher than about 16V.

PotvinV8, I did consider moving the alternator charge cable to the other side of the fuse like you mentioned, but wouldn't that effectively be like removing the fuse from the circuit and leaving the wires and ammeter etc unprotected?

Thanks again!
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Old 01-28-2021, 01:10 AM   #10
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Default Re: Keeps blowing fuses — alternator problem?

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Whoa! Are you really running a 50 amp fuse or is that a typo? I think I would rethink using that big of a fuse.
That was a typo, I'm running a 30 amp fuse. Sorry.

Quote:
PotvinV8, I did consider moving the alternator charge cable to the other side of the fuse like you mentioned, but wouldn't that effectively be like removing the fuse from the circuit and leaving the wires and ammeter etc unprotected?
The alternator is the only thing that wouldn't be "protected". Everything else gets power from the "fused" side of things. You could put a Mega Fuse sized larger than your alternator's output if you wanted to be safe. A ground cable attached to the alternator body is a great idea as well.

Here's a shot of my fuse on the starter and how it's wired...

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Old 01-28-2021, 03:04 AM   #11
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Default Re: Keeps blowing fuses — alternator problem?

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Originally Posted by PotvinV8 View Post
That was a typo, I'm running a 30 amp fuse. Sorry.



The alternator is the only thing that wouldn't be "protected". Everything else gets power from the "fused" side of things. You could put a Mega Fuse sized larger than your alternator's output if you wanted to be safe. A ground cable attached to the alternator body is a great idea as well.

Here's a shot of my fuse on the starter and how it's wired...

Ah okay, I understand now. So I see why that would prevent the alternator from blowing the fuse... would that also mean the ammeter would only show power being drawn from the battery, and not power going to the battery, since the alt would be feeding straight into the battery instead of going through the instrument panel circuit?
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Old 01-28-2021, 03:12 AM   #12
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Default Re: Keeps blowing fuses — alternator problem?

Good question. I've always been told "ammeters bad; voltmeters good" but in my Model A, I left the ammeter in place, just didn't pay much mind to the readout. I'm sure someone more versed in the subject will chime in shortly...
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Old 01-28-2021, 10:40 AM   #13
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Default Re: Keeps blowing fuses — alternator problem?

The ammeter is between the generator and battery and measures battery charge or discharge current (either direction). Voltage and power are irrelevant to its reading.

Resistance at the ground connection causes a voltage drop; the alternater senses a lower voltage from the battery and increases the output voltage, causing overcurrent through lamps and fuses.
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Old 01-28-2021, 12:03 PM   #14
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Default Re: Keeps blowing fuses — alternator problem?

I am of the opinion that an ammeter is an essential device that should be on all cars. A glance at it tells a lot about the condition of the electrical circuit.

When you first start the car up the the ammeter should show a heavy charge going to the battery to replace the power pulled out of it by the starter. This tells you that the generator or alternator is working properly. A few miles down the road the needle should return to zero indicating the battery is charged back up an is no longer being charged. This of course if there is a regulator in the circuit.

If you turn the lights on the generator or alternator will pick up the load and the needle should return to zero. If it does not it is telling you the generator or alternator is not working.

On a Model A with an original undamped ammeter it provides a diagnostic tool. The early cars up to November 1929 did not have the ignition circuit in the ammeter circuit. A service bulletin of November 1929, page 390 changed that by moving one wire on the firewall terminal block from one terminal to the other.

The change allowed you see what was going on when you started the car. As you cranked the engine and before it started you will see the ammeter needle swinging back and forth past the zero. This is due to the points opening and closing. This is telling you that the ignition primary circuit is intact and working properly. If the car does not start the problem is elsewhere.

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Old 01-28-2021, 08:03 PM   #15
eamonford
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Default Re: Keeps blowing fuses — alternator problem?

So I cleaned both my battery posts and the battery ground cable, sanded the part of the frame where the ground cable attaches to expose the bare metal under the paint, and ran a #12 wire directly from the alternator housing to the battery ground terminal, but still blew a fuse after driving 50mph for a few seconds.

Thanks Tom Endy for the helpful explanation on ammeter usage. If I'm running a voltage regulated alternator and the ammeter shows a positive charge while driving, but never returns to 0, could that indicate a bad battery? I'm wondering if maybe my battery is just never charging up to full and so the alternator is happy to keep putting out 30A until the fuse burns out.
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Old 01-29-2021, 07:59 AM   #16
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Default Re: Keeps blowing fuses — alternator problem?

I'm going to assume you have an internal regulator. I have a feeling the regulator is bad [ full fielded]. Have you tried full fielding it [ D slot ] to see if the readings change ?
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Old 01-29-2021, 09:54 AM   #17
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Default Re: Keeps blowing fuses — alternator problem?

I'm not sure I understand why some of the components listed are being fused. Fuses are only used to protect wires against shorting to ground or cross shorting to other wires in a bundle. Starters and alternators are not normally fused in the automotive world due to the large changes in current draw or output when they are in operation. The charging wire between the alternator and the rest of the system has to be very well insulated for protection against any possible short that could happen so that a fuse will not be necessary. A fuse will only take the rated amount of current that goes through it. If it's rated at 30-amps and the alternator is capable of 40-amps then the fuse wouldn't last a long time. Starters are only intermittent in operation so they don't need a fuse. They draw a lot of current and even more when they are worn out.

One wire alternators have a built in control system for voltage and the limitation of current output so no fuse is needed between the alternator and the load or buss. An ammeter is usually used in the charging circuit between the alternator and the load. Some ammeters are internally shunted and some use an external bypass shunt that only takes a sample off the line between the alternator and the load. A voltmeter is also a good tool to see if the alternator is maintaining the voltage properly. On a 12-volt system, it should indicate very near 14-volts while operational. It also give the operator a good indication of the condition of the battery before the car is started but it won't tell the operator how much current output is going on.
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Old 01-29-2021, 01:37 PM   #18
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Default Re: Keeps blowing fuses — alternator problem?

In reading the others responses, you have cleaned up the grounds so that can be ruled out as being the problem. A 30 Amp fuse should handle every situation on a Model A with some margin. If you had all the lights on( Not high current Halogen headlights) and blowing the horn and electrical wipers on still should not be using 30 Amps of Current. You have an intermittent short somewhere. Do you have a new wiring harness? Do you have one of the later terminal boxes? Are you using the original style head light conduits which plug in?
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Old 01-29-2021, 06:03 PM   #19
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Default Re: Keeps blowing fuses — alternator problem?

A possible hidden short is behind the terminal block on the fire wall. The originals had the two threaded studs embedded in the composite material. The ones that have been coming from Hong Kong for many years are constructed by drilling a hole clear through the composite and running a bolt through from the back. The bolt can easily back off and touch the tank and short out.

Best thing to do is remove it and put some lock tight on the first nut that secures the bolt and tighten, then fill the back tunnel with JB weld to prevent it from backing out.

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Old 01-29-2021, 09:25 PM   #20
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Default Re: Keeps blowing fuses — alternator problem?

As others have stated, 30 amp fuse should be more than enough. If your alternator is putting out more than 30 amps there is something wrong. Could be a bad ground, short, bad alternator regulator, or bad battery. Keep the ammeter but also install a volt meter so that you can keep tabs on the battery voltage. Should be about 12.3 or so volts after sitting overnight and 14.5 when the engine is running at anything more than an idle, even 50 mph. After driving for a half hour or so, even with the lights on, your ammeter should show close to 0 amps but the volt meter should show 14.5 volts.
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