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1948 Desoto possible electrical issue

1948 Desoto still 6 volt. Driving at night a few days ago and all the lights suddenly went out on the car. The car did not lose power. It kept driving quite nicely. 30 seconds later the lights came back on. 30 seconds later the lights went back off. This continued for the next 10 minutes until I got home. The next day I started the car and let it run for 20 minutes or so with the lights on. Nothing happened. However, I put my hand on the light switch under the panel and noticed that it was quite hot. Is this perhaps a sign that my light switch is going out? Maybe a bigger problem. Also, if my light switch is going out, are light switches volt specific? Thanks.

@kfunnymn hot wires are a sign of poor contact and/or electrical resistance. Degraded wiring could be a factor, in addition to loose/corroded connections

The electrical system is original? It’s 65 years old. You may really want to consider re-wiring it before rotten insulation causes a fire.

But to your questions, a hot switch needs to be rebuilt or replaced to avoid a fire, and switches are rated for wattage. A 12v switch would work just as well.

This problem does sound like a faulty switch and those switches will get a bit hot anyway due to the 6 volt system. Any switch should not be volt specific but could possibly (as in very slim) affect any dash light dimmer function. To be honest, I don’t know or remember if there was a rheostat for the dash lights on these cars or not.

While I could not even begin to relate anything about one of these cars with any degree of certainty, it may be possible to remove the switch and repair it as a lot of that old time stuff can be disassembled and put back together after repairs. It could be there’s an issue with a wire end at the switch also but no matter; any fix should be easy.

I’ve got 2 antique Harleys (40s and 50s) and there’s not a switch on them that can’t be fixed as needed. Maybe that kind of serviceability will carry over to your DeSoto.

The two things to check for the headlights turning off and on are the light switch or the dimmer switch on the floor. Your lighting system passes the voltage thru the switch, thru the dimmer switch, and to headlights. So the switch can get hot from the current passing thru it.

The switch should be sized for the current that passes thru it. Not the voltage.

Tester

I believe Chrysler products of this era had a circuit breaker built into the light switch which would open if the current draw is too heavy. I had a 1948 Dodge and this was the case. The light switch/circuit breaker is performing as it should when there is a heavy current draw. The breaker opens and closes automatically to prevent a fire, but not leave you in darkness should an overload occur. I remember reading this in the owner’s manual.
Check the wiring from the headlights to the tail lights. Look for worn places where the conductor may be touching the ground. Installing a new headlight switch/circuit breaker will probably do the same thing.

I’d be voting for the switch on this one but wouldn’t hurt to check the wiring and insure good connections.

Is this headlight on/off switch the kind that you pull out to turn on the lights and rotate to dim the dashboard lighting? If yes, this switch will get pretty hot as part of normal operation due to the rheostat incorporated into the switch. The floor high/low beam switch could also be a source for this problem.

There’s a great deal of good information already posted but I will add that if one or both headlights short circuit the high and low beam circuits so that both bright and dim elements are lit regardless which is turned on the circuit breaker in the light switch will function just as the OP has stated. If both headlights each have both high and low elements lit the problem is most likely the dimmer switch.

The headlight switch did not have a rheostat for dimming the panel lights on the 1948 DeSoto as I remember. There was a separate control for the panel lights. At the bottom center of the dashboard were a set of knobs. From left to right you had the following (as best I can remember from my dad’s 1947 DeSoto):
ignition switch headlights panel temperature defroster heaterfan

The headlight switch was a pull switch–first position parking lights, second position hedlights. The panel light switch rotated for off, dim, bright. The temperature control was operable if the car was equipped with the more expensive heater. the defroster pulled out to direct air to the windshield. The heater fan had 3 positions as you pulled it out–low, medium and high. Above the ignition switch and slightly to the left was the pushbutton for the starter and on the opposite end was the cigarette lighter.

As I recall, most of the circuits in the car were protected by an automatic reset circuit breaker on the headlight switch. If the car had a radio, it was protected by a separate inline fuse and I believe the clock had an inline fuse.

Try this: Start the car but leave the headlights off. After running the car for 10 or 15 minutes, see if the headlight switch gets hot. If so, something is pulling excess current through the switch. If it stays cool, turn on just the headlights. If the switch becomes hot, the problem is in the headlight circuit. If it does not, switch on other devices in the car–the turning signals, the heater fan, etc to see if one of these circuits causes the switch to get hot.

A wiring diagram for your DeSoto would help. I’m going from memory for a car my dad owned in the mid 1950s and this is all I remember after 58 years.

I think the dimmer switch, the connector to it, or the wires to it at the floor are the likeleist culprits. Your headlight switch is doing what it should. Also check your headlight and taillight wiring anyplace it goes through metal.

It used to be that a bad or poorly grounded (rusty floorboard) dimmer switch would cause the lights to go off. Some of the old switches would get loose and you would have to tighten it by turning the chrome ring. And some had to be replaced.

@Trideq: I completely agree. I’d also add that as these breakers age, they can get ‘weak’ and open up even when there is no overload present. I believe a bi-metal strip that flexes is used to break the circuit. When the spring tension gets weak, the breaker can open from the normal amount of heating that is present during normal operation. I actually had this happen with a car as late as a 1980. 60+ years is a pretty good run for a mechanical/electrical component. I would check to see how much current the lighting system is drawing, but I’d suspect a bad switch.