electrical fuse problem

[NeillB]

I keep burning my main fuse I am using a 25amp fuse on my 1929 model A, I loose all my lights and horn I have checked all the grounds and power can not find anything the problem is intermittent it may last several trips or burn the first few minutes of driving,I have checked the the generator at an idle seems to be charging wondering if the charging system is not working correctly and over loading it a higher RPM. wonding if any has had this issue and could point me in the right direction to troubleshoot.
thanks

[Mitch//pa]

depending on how its wired usually the main fuse also causes the car to not start as it also feeds the igniton system..

[Patrick L.]

What exactly happens when the fuse blows ? How is the fuse wired into the system ?
Does the engine run till you blow the horn or turn the lights on ? Are you running an alternator or generator or aftermarket ignition and what does the ammeter show ? Have you looked at a wiring diagram ?

[1931 flamingo]

I'd start looking at the headlight wiring from the switch thru the buckets.
Paul in CT

[NeillB]

I am running a electronic ignition I have that wired seperate to the battery direct the engine does contiune to run I just loose all power to the lights,horn usally this happens when the headlights are not even on. I do still have a genetator 6vd the amp meeter shows between 5-7 amps. I have looke at the wiring diagram just have not gotten to check it all with a meter. as far as I can tell everthing else is wired stock the fuse on the side of the starter .

[Mitch//pa]

Sponsored Links (Register now to hide all advertisements)

Disconnect the headlight conduits and see what happens as paul said

[harleynut]

I had a similar problem. Every time I stepped on the brake, I blew the fuse. Turns out my new (junky) brake light switch would short out internally. Put in a new (junky) switch......problem solved.

[Tom Wesenberg]

Originally Posted by harleynut

I had a similar problem. Every time I stepped on the brake, I blew the fuse. Turns out my new (junky) brake light switch would short out internally. Put in a new (junky) switch......problem solved.

A couple guys in our club had the same problem several years ago. The one terminal would short to ground due to missing or poorly placed insulators, or the contact strap hit the side of the case.

The fuse can blow even with the generator supplying the needs because a dead short will overload the system and attempt to draw a heavy current from the battery. Since it happens with the lights off, you can skip the headlamps for now, but do look inside the headlamp switch for stray wires touching the cover. Also check the ammeter for poor insulation inside the case. If you have a repro terminal block, check that the two terminal screw heads aren't touching the firewall.

A poor connection at the fuse can cause excess heat and melt the fuse near the end cap.

[Mike V. Florida]

Originally Posted by 1931 flamingo

I'd start looking at the headlight wiring from the switch thru the buckets.
Paul in CT

Originally Posted by Mitch//pa

Disconnect the headlight conduits and see what happens as paul said

Ad me to this list. With the ign out of the pictute you only have lights and horn wiring to worry about.

I also suggest you get a resetable circuit breaker to replace your fuse.

[Y-Blockhead]

Is a 25 amp fuse large enough? Most of the Vendors provide a 30 amp fuse with the holder. I use a 30 amp ciruit breaker with no problem.

From Snyder's;
"A-11450-SF Safety Fuse Installation Instructions

Disconnect Battery! Bolt your safety fuse onto the engine side of the starter switch using the same screws that attach the switch. (Note: if the screws are too long and penetrate the starter body by more than 1/4” a short will result)

Remove the wire coming from the terminal box at the starter switch and attach it to the empty screw on the safety fuse.

Attach the yellow wire from the safety fuse to the bolt on the starter switch along with the battery cable. Replacement 30 amp fuses can be purchased at any auto supply store."

[RonC]

Originally Posted by Y-Blockhead

Is a 25 amp fuse large enough? Most of the Vendors provide a 30 amp fuse with the holder. I use a 30 amp ciruit breaker with no problem.

From Snyder's;
"A-11450-SF Safety Fuse Installation Instructions

Disconnect Battery! Bolt your safety fuse onto the engine side of the starter switch using the same screws that attach the switch. (Note: if the screws are too long and penetrate the starter body by more than 1/4” a short will result)

Remove the wire coming from the terminal box at the starter switch and attach it to the empty screw on the safety fuse.

Attach the yellow wire from the safety fuse to the bolt on the starter switch along with the battery cable. Replacement 30 amp fuses can be purchased at any auto supply store."

I use a 30amp but I have also used a 20amp with no problem

[Tom Endy]

Originally Posted by RonC

I use a 30amp but I have also used a 20amp with no problem

If you are running an alternator that is capable of putting out 30 amps it should be considered that if the battery is extremely low after start up, the alternator may be sending 30 amps to the battery, which will have to travel through the fuse.

Tom Endy

[Tom Endy]

On a Model A Ford wired per Henry, with the addition of a fuse located on top of the starter, the only current passing through the fuse while the engine is running is the amount going from the generator to charge the battery. When the engine is running and up to speed, the generator (or alternator) has assumed all the electrical loads. A short circuit in any of the loads should not blow the fuse because the load is receiving current direct from the generator (or alternator). If the fuse should fail (or fall out) while the engine is running it will continue to run because the generator (or alternator) is supplying current to the ignition. If you shut down the engine, (or go to idle and the cut-out opens) you wont' be able to start it again because the ignition needs current from the battery to start and the failed (or fallen out) fuse is blocking it.

The danger here is that with the fuse failed and the engine running, the Generator (or alternator) does not see the battery for reference and it will run wild with increased voltage and in the case of the generator burn it out.

Later cars came equipped with a fuse for each load and there was no fuse between the generator (or alternator) and the battery. If a load shorted it took out only its own fuse.

The addition of a fuse on top of the starter on a Model A is a compromise. It is protecting the car from a short circuit and a fire when the car is parked. Rolling down the road it is not doing much for you, and if it should fall out you will likely burn up your generator. However, I think that fuse is very important and when I park my Model A for the day I remove it from the clip and stand it on its head at one end of the clip. This way everything except the starter switch is isolated from the battery.

A couple years ago a well known Model A repair shop here in Southern California burned down due to an electrical short in a customer's Model A. The customer brought the Model A in for repair late in the day. The customer or the shop owner failed to remove the fuse (if there was one). A few hours after closing time the electrical short occurred. If there was a fuse the short circuit did not pull enough current to take the fuse out, but the wiring got hot enough to start a fire. The shop and several cars were destroyed in the fire.

The good news is that the shop was insured and is today back in business.

Tom Endy

[Al 29Tudor]

Tom, thank you.
Great explanation on how the electrical system works. I hadn't thought about running with a blown fuse and what is happening to the generator.
Al Leach

[BILL WILLIAMSON]

A 30 Amp, G.M. Bayonet type Circuit Breaker is a GREAT addition, instead of the TROUBLESOME fuse holders.
Bill W.

[30Ford]

I had a grounded Amp meter once ..Burnt a few wires

[Y-Blockhead]

Originally Posted by 30Ford

I had a grounded Amp meter once ..Burnt a few wires

You may need to get some of this...

[BILL WILLIAMSON]

Originally Posted by Y-Blockhead

You may need to get some of this...

Why do the English drink WARM BEER?--LUCAS builds their refrigerators.
Why doesn't the English build TV sets?--They haven't figured out a way to make them LEAK OIL.
We should invent an aerosol can that SOUNDS & SMELLS like a FART!!
Bill W.

[Patrick L.]

Originally Posted by bill williamson

why do the english drink warm beer?--lucas builds their refrigerators.
why doesn't the english build tv sets?--they haven't figured out a way to make them leak oil.
we should invent an aerosol can that sounds & smells like a fart!!
bill w.

lol !

[RobertB]

Originally Posted by Y-Blockhead

You may need to get some of this...

When I was into Spridgets (MG Midget/Austin Healey Sprite), the model A of sports cars, one of our best restorers always added a "smoke test" guage to his dash. He converted an old guage, added a "test" button, and mounted a hair dryer heater smeared with grease inside the guage.

A push of the button caused smoke to pour out of the guage. All Brit car guys know the wire if full of smoke, and and your smoke leaks out all at once, the car is toast...(pun intended).

Wonder if there would be a market for "original" Ford Model A smoke test guages?