Headlights not working together

[JR F100]

Searched the forums but couldn't find anything on this. 1969 f100.

Out of nowhere, one of my headlights stopped working. So, I replaced it. Nothing. I then found if I hit the dimmer switch on the floor, it started working but the other stopped. Checked the grounds and all wiring looks decent for their age. Replaced the dimmer switch, no change. I know both lights work but can't get them to stay on together. High beams never worked.

Going to replace the headlight switch next I guess, but I'm an idiot. Figured someone might have had a similar issue. Any advice would be appreciated.

[basketcase0302]

"JR F100"]Searched the forums but couldn't find anything on this. 1969 f100.
Out of nowhere, one of my headlights stopped working. So, I replaced it. Nothing. I then found if I hit the dimmer switch on the floor, it started working but the other stopped. Checked the grounds and all wiring looks decent for their age. Replaced the dimmer switch, no change. I know both lights work but can't get them to stay on together. High beams never worked.
Going to replace the headlight switch next I guess, but I'm an idiot. Figured someone might have had a similar issue. Any advice would be appreciated.

They both get their power via the headlight and dimmer switch, (any issue with the dimmer will usually effect both lights). I believe you on the right path to first check the grounds very well which is usually a problematic area and also the switch, (which has the over current built into it). Another thing to consider looking at is the type of bulb you are running. Is it a halogen bulb that is taxing on the old wiring, (which is also a problem area) and if so maybe consider the headlight upgraded wiring most her favor to do. A one time cost of less than $150 to add relays and new wiring in the truck that will increase your visibility by 4X.

[fordman]

Probably a bad dimmer switch. Or the wring to the switch. Ie losse wires in the plug maybe.

[71Fe2O3]

Ground problems or bad dimmer switch seem likely candidates. The light and dimmer switches are available from NAPA.

I encourage you to rewire your headlight circuit with relays, an inexpensive project if you buy the supplies and build your own.
Modern cars are set up this way; it allows full battery voltage to go to the headlights rather than dissipating away as it goes through old, thin wires.
You will need two relays (one for high beams, one for low), two fuses, some wire (heavier gauge, within reason, is better for less loss) and a bunch of connectors. I also replaced the headlight sockets.

If you haven't done this before (I hadn't) you will need to spend some time studying the wiring diagram and learning about relays. The installation took me a few hours, and my headlights are much brighter now.

If you are interested, send me a pm and I can send you a set of relays with fuses and associated wires for a few bucks. I bought a 5-pack of relays, so built 4 wiring rigs just for fun and now have 2 laying around in case I decide to do my plow truck, but that won't happen real soon. Either way, good luck. Wiring problems can keep you busy.