I'm wiring my 1939 truck using what I believe is the original headlight switch. None of the original wiring was left as an example. I'm puzzled by how and what the switch does. It appears to have 5 positions. best guess...1. off 2. possible accessories? 3 taillight parking and dash 4 head light 5 taillight again? very confusing. Also, what is the resister? on the back. Any help about how it works and what goes where appreciated.
Last edited by Donf; 06/11/20252:24 AM. Reason: correction
Here is a wiring diagram from the 1930s. Car and 1/2 ton truck were nearly identical. 15A fuse goes in the holder. There is no resistor for dimming the instrument light(s).
I am replying based on my knowledge of my '37. I don't know if the '39 is the same.
My '37 does not have a voltage regulator, only a voltage cutout. This keeps the battery from turning the generator into an electric motor. On my headlight switch, there are 4 positions; 1) Off, 2) remove field resistor (that resistor on the back of the switch), 3) parking lights, 4) headlights.
One of the problems with having no voltage regulator is that you can overcharge your battery. I have been told that if you go on a long road trip you should put your headlights on to help prevent that. Also, to prevent overcharging, the off position of the headlight switch includes that resistor, called the field resistor. Let's say you have an accessory, such as a heater and you are doing stop and go driving. You put the headlight switch in the second position, which removes the field resistor. You can see the result with the ammeter which shows a bit more charging. For the parking light and headlight positions, the field resistor is also removed (bypassed).
That system was probably perfectly satisfactory for the mid 1930s where people didn't regularly drive long distances. Battery technology wasn't what it is today so I imagine they didn't hold a full charge for as long as they do today so overcharging the battery wasn't a big concern.
I imagine today's batteries probably stay at full charge more readily than the old ones and are more susceptible to overcharging.
What year did they introduce a voltage regulator to the system?
1950 Chevrolet 3100 (Ol' Roy)
1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)
Thanks for the help. It explains a lot. My understanding is that my 39 once had a voltage cutout and not a voltage regulator. The voltage regulator came along in 1940. I'm wiring my 39 for a 12-volt generator and regulator and using a general-purpose fuse panel. No need for a fuse or the resistor. Kent's 4 position descriptions now makes sense but I'm still seeing a 5th position on my switch past the 4th headlight position. seems to duplicate the 3rd parking light position.
Back to the headlight switch, it looks like, from the wiring diagram that the connections are: ammeter, park lights, horn, inst/rear light, dimmer switch.
Headlight switch was probably used as a wiring junction; thus the horn connection.
Thanks again for the help. Headlight switch is now wired in and functioning properly (minus all of the 39 voltage cutout and other wiring junction options). I was wrong about a 5th position on the switch. There was a bit of a 'catch' that I thought was a position before the headlight position. Four positions just as Kent described.
