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We are working on a 1948 Chevy 3100 ½ Ton truck. We ask why when you pull the light switch to first detent or Parking light position front and rear parking lights come on, but pull the switch to second detent position turn the sealed beams on front and rear parking lights turn off? Is that by design?
We’ve used the 1948-1949 wiring diagrams and we’ve traced the wires and all wires from the switch to lights are where they should be.

We have encountered an issues where that after turning on the sealed beams minute or so afterwards the thermal switch built into the light switch activates and we loose all systems that are ran off the light switch activates. The switch was replaced years ago with aftermarket switch. We’re able to turn on parking lights but after pulling the switch to the second detent to turn on sealed beams minutes later the light switch thermal switch disconnects all power running of the switch. We’ve verified that all wires are connected per the attached wiring diagram. Any ideas? I’m inclined to believe that the light switch is the issue.

Do the lights stay off or do they come back on by themselves after awhile? Do you turn off the lights and then turn them on again and the whole process repeats?

6volt?

Sounds like you have a hot wire that goes to the headlight/headlights that is shorted to ground, or you have a bad dimmer switch.

I'm guessing defective switch. There is only one wire that runs from the headlight switch to the tail light. If it works in the parking light position, it should be OK for the headlights on position. Try shooting the switch with some contact cleaner (Remember that stuff from when we had mechanical channel changers on TVs? You can still buy it.) and then operate the switch back and forth a bunch of times between the parking and headlight positions. If that doesn't fix it replace the switch.

Thanks.
It’s a 12 volt.
I’ll turn the switch off and after the thermal switch cools I can turn them on. But on one or two occasions I’ve left the switch on and they come on but shortly after they’ll go off again.

Bill, I've copied pasted your below response because for some unknown reason I can this content in my iphone but not on my desktop computer, so forgive me for restating your below response.
"Turn the headlights to low beam.
Turn off the headlights and go drink a beer.
Turn on the headlights and time how long it takes for the thermal switch to interrupt the first time.

Turn the headlights to high beam.
Turn off the headlights and go drink another beer.
Turn on the headlights and time how long it takes for the thermal switch to interrupt the first time.

The high beam pulls a little more current than the low beam, so the thermal should trip a little quicker. Actually, neither should trip, but I'm just checking to see if one trips and the other doesn't. If they both do I'd replace the switch, which should also fix your taillight problem."

Bill, I did something similar as you suggested but I was not as thorough as you. I plan also to disable the dimmer and jumper power from the light switch and feed 12 volts directly to fuse port for the upper and lower sealed beam. In my mind if the issue is with the dimmer the thermal switch should not open the circuit and thus light stay on. I'll do this on the independently to the upper and lower.

Thanks also to 52Carl.

The dimmer switch is sort of like a distribution block. It has 3 terminals. One is is power that it receives when the dashboard switch is turned on. The other two terminals send the power to either Hi beam or Lo beam when the floor button is pushed.

A home made trouble shooting tool I have consists of an old -30 0 30+ underdash ammeter with a 4 foot set of #12 wires attached. Each wire is terminated with an alligator clip. One wire has a 40 amp inline fuse near the meter. Most modern volt/amp/ohm meters have an internal 10 amp fuse. Anything more breaks the modern meter.

My tool allows you to unhook one end of the wire of the circuit to be tested, attach the tool between the wire end and where it had be connected and activate the circuit to see current draw. It doesn't really make any difference which way you hook up the tool, the meter will read + one way and - the other. If current exceeds 40 amps the fuse blows, but is easy to replace.

I probably use this thing twice a year.

Where are you in Texas?

College Station

Today I’ve sort of concluded that the light switch is the most probable cause. Perform the steps proposed by Bill. Based on the results I wasn’t sure which is the culprit. So I removed the dimmer switch and connected the power in line to the dimmer to each of the dimmer output wires (independently of course.) Applying power to high beam wire or to the low beam wire caused the thermal switch to open and remove power to circuit. So I’ve decided to replace the light switch. The switch never failed in the first detent for the Parking lights. Assessing the old switch operation I feel that the switch’s shaft is hard to move through its full range.
I’ll report back once I order and install the new switch.

Well I installed a new light switch. Now I have High / Low Beam and the High Beam indicator light comes on. Also the Park lights front and rear come on with the switch in the Park or Headlight On position.
However, I've discovered that the dimmer switch wiring had to be wired to the center pole (power in) and one of the terminals at the dimmer along with the instrument panel indicator light for the High Beam.
I consider this case closed. Thanks for for all the ideas.

I would double check your wiring on the dimmer switch. I happened to change mine out today and had it wired incorrectly (power in was on the center terminal, so had no high beams.) But the switch was intermittent high resistance anyway, so I put in a NOS dimmer switch. Both were marked with "BAT." on the upper terminal. (See pic.)
As far as the high beam indicator light, that's shown connected to the fuse box for high beam, so there are just a single wires terminated at the dimmer switch, (battery feed, high and low.)

I also replaced my headlight switch, as it was a bit high resistance also, and discovered that the mounting nut was chrome plated plastic, which promptly stripped it's threads. This replacement switch was the same style as the one in the truck when I got it, but it at least had a pot metal nut (which I reused.) The knob that screws on the shaft was also chromed plastic. Replacement are getting cheaper (read that as crappier) by the day.