Itinerant Air-Cooled

On my '68 I had the passenger side low beam go out. The fuses are all good and clean.

I removed the sealed beam, cleaned the connections including the ground.
I then replaced the light, but still just the high beam on that side.

I then swapped in two different junkyard halogens and got the same result.

Then I switched the known good lamp from the driver's side. Although it works fine on the left, it only works on high beam on the right side. The passenger side lamp works, both high and low beams, on the driver side, thus all four filaments are good. Where to go from here?

Using a test light or meter, check for voltage at the lamp connector and at the fuse. It could be a bad connection. Sometimes the lamp connector goes bad. Good luck!

You can replace head lite plugs with individual spade connectors, just use a test lite to determine which is which and write the color code on the headlite bucket with a permanent Sharpie. Sometimes the wire will corrode up under the insulation an inch or so from the plug. Thirty-some years of wind driven weather gets forced into the wires.

Bill

Flip the dimmer switch back and forth a few times.

When it is just one filament, you can stay clear of upstream guesses. The Law requires that headlamps have different fuses, so check your R-Low Beam fuse, not just the fuse element which may be OK, but the sockets the fuse rests in. The headlamp ground wire is screwed to the headlamp bucket just in front of the kick panel. Windshield leaks can wreck the ground connection even if it is sitting there looking OK.
Colin

Bingo. Loose wire to the fuse. It's always, always the simplest solution.