I'm helping my 85 yo neighbor revive his 1939 Chevy business coupe after it's last drive in 1971. After getting it running and replacing almost everything to get it back on the road we've hit a snag. The car has no lights. He says he can't remember ever driving it at night. The headlight switch has all the wires disconnected and when going through them one by one, anything to do with the headlights seems to be a problem. I can get a right then a left headlight but the draw is tremendous. The jumper gets hot and quick! Since I've have no first had knowledge of an OG 1939 Chevy, I can only speculate that someone has put sealed beams in place of the originals. There is no headlight relay that I can find. I've traced back to the junction block on the inner fender and all three posts have power, with the dimmer clicked either way, high or low. High beam indicator just stays on. There's no color code on anything! Shouldn't that block be one be high another low and one ground? And does anyone know where i can get a good diagram of the switch? I'm guessing that the dimmer switch is bad? Thanks!
there isn't much fuse protection in the original wiring on these cars, and the wire insulation lasts about 20 years...so you're kind of looking for things to catch on fire. I think the 39 is very similar to the 38 wiring shown here. https://chevy.oldcarmanualproject.com/shop/1938/38csm259.html
Back when nothing had relays, switches were built to take the amperage, at a guess the dimmer switch is shorted. Switches up to the 60s can be disassembled and cleaning usually solves most problems, use care bending the tabs and watch for little springs that can fuck off to nowhere at moments notice. I’d look at the dimmer for sure and the main headlight switch as well, some also have the fuses as part of the assembly, then follow the power wherever it needs to go. Also make sure of paths to ground, most older vehicles used the body to supply ground for lights, also dash lights and switches, the headlight switch should have its own ground to the battery
I can offer some troubleshooting guidance, but given how somebody has been at the wiring you'll probably need to check every connection/wire. First thing I'll say is a swap to sealed beams shouldn't increase the load enough to matter unless someone has installed high-wattage 'off road' lamps, very unlikely. Second, 'ground' wires are not common for lighting, with most lamp assemblies being grounded through the metal socket as attached to the body. The one common exception is the headlight socket, but these almost always just have a short 'ground' jumper to the body structure. The other two wires at the socket are the headlight low/high beams. If the issue is in the headlight wiring, your best bet is isolating each individual wire (noting where it goes) and checking for continuity between each end. Also check to make sure each wire doesn't go to ground. If you find that, it's mis-wired OR you have a failed wire with a bare spot somewhere going to ground and you'll need to find either the wrong connection or the bad spot. The fact that your high-beam indicator is always on proves there's something mis-wired somewhere. And the OEMs didn't use headlight relays, that's an aftermarket 'solution' to mask inadequacies in their harnesses. This will be a somewhat slow slog, but being methodical and 'proving' each wire is the path. One anomaly is the posted diagram shows the headlights getting their ground from the headlight switch, something not normally seen. I'd look very carefully at that to make sure it's not connected somewhere it shouldn't be if that is what you have. And don't assume if you find one issue part way through your checking that that's all it is. Check it all, that way there shouldn't be any surprises. Once you get the wiring 'proved', then turn your attention to the switches and various terminal blocks to verify proper operation/connections. Thoroughness will pay dividends....
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