VCCA Home Forums TECHNICAL - GENERAL Electrical - 6 volt Fuse burns out with one headlight on

The car is a 31 sport coupe completely restored about 8 years ago. A problem came up recently with the fuse burning out when I turned on the headlights. I attempted some troubleshooting and tried disconnecting a headlight. I disconnected the left headlight and the fuse did not burn out. Reconnected it and disconnected the right headlight. Same result, no blown fuse. So the fuse burns out only when both headlights go on at the same time. There is only one fuse and it is 15A. The horn works when the fuse is good, and the cowl lights do not cause the fuse to burn out (both cowl lights turn on without a problem). I have been checking for bad wires, but I have seen no worn, frayed, or otherwise suspect wires anywhere. I noticed that the left headlight low beam was not working, but that has happened before and no burned fuse. The tail light (only one) works fine. I replaced it with LED a few years ago with no problems arising from the switch. Stepping on the brake pedal (causing the brake light to illuminate) does not cause a problem. The system is still 6 volt and works well otherwise. Never had a problem with a dead battery. Car starts easily.
Anyone have any pointers?

Possibly a bad dimmer switch???

I would start by fixing the low beam, and then see what you have.

At 15 amps, you have about 90 watts available. You can't load a fuse to 100% though and expect it to be reliable, 75% is more of a typical design load, so about 68 watts.

Are you using extra bright headlight bulbs? What else is on the fuse? I gather the horn (thats weird), the cowl lights, the LED tail light, the brake light too?

Do the cowl lights run at the same time as the headlights?

Does this car have a floor button for the dimmer switch? If so, those have a nasty habit of getting conductive and bleeding current to ground (when exposed to moisture a lot).

It should be possible (but not exactly easy, since they rate these old bulbs in candlepower instead of watts) to add up the loads, and it should come to less than 15 amps (or 90 watts) for everything that can be on at the same time. If the dimmer switch is leaking current to ground it should get hot whenever the headlights are on.

Yes, I would try a 20 amp fuse.

I agree, a 15 amp is pretty small for headlights etc.

I guess my biggest concern is why this is happening after seven or nine years. Nothing is changed. The headlight bulbs are the type and size that came with the car in 1931. The headlights do not light up at the same time as the cowl lights. Cowl lights turn on at the first click of the switch and go off when the headlights come on. Thank you for the ideas. I must do some more sleuthing this weekend.

For 31, there was only one fuse on the entire car, and it is on the headlight switch behind the dash. Car is in a heated garage.

I will also try the 20A fuse. I am unsure what was in the car from the factory. I thought it was 15, but likely wrong.

I thought most were 20 amp but I don't know that. It seems that after several years of running a 15 amp and it starts blowing makes me think that there must be a short or high resistance some where in your electrical system. It must be a fairly small draw if you can run 1 headlight at a time. I guess I would try a 20 amp fuse and then look for warm or hot spots at all connections and at the fuse holder. Hopefully there will be someone on here that can give you more useful advice than I can.

Given all of that, I'll bet its the dimmer switch. See if it gets hot.

I suspect a couple things since something has changed or degraded over the years.

1) A problem with your bulbs themselves:

You may have been fine with a 15A fuse for years but as incandescent bulbs age, the tungsten burns off the filament or the seal can leak allowing air to enter so the bulb can draw more current than it did when it was new. The current draw can vary wildly between bulbs even when new. Bulbs are also mechanical and we have all seen the coil filaments break and then arc and reattach themselves and actually shorten the filament, drawing more current.
A 20A fuse will likely finish off the suspect bulb at some point so easiest solution as suggested above.

2) If you can, measure your voltage at the battery with lights both off and on while running and not running:

Electricity has a direct mathematical relationship and if your generator normally produces 6 volts and drops by 1 volt due to another problem, your current on all loads will go up 17%. The 1 volt is just an example to show the significance of a small voltage drop increasing the current flow through the fuse on a 6v car. Voltage drop can be caused by several things like poor ground or other connection (examine light sockets), bad battery, regulator, generator problem or a large load added to the system, like a shorted dimmer lol (add anything recently?).

I will work with these suggestions and report what I am able to find. Thank you for the help.