[jlwdvm]

Before replacing all of my headlights, the outboard driver's side lamp will work when on low, but when I switch to high beam only the inboard light comes on on that side. Both lights come on on the passenger side when highs are on. It is the same after switching to new Sylvania lights.

["QUICK-SILVER"]

High beams work so powers out there. One headlight plug-in should have two wires on the high beam terminal, ones power and the other jumps to the bulb beside it.
(1) Bad bulb
(2) Plug-in problem; lose/slipped terminal or bad connection (poor contact)
(3) Broke wire

That's about all there is.

Some older plastic headlight plug-in's don't fit newer bulbs right because of changes on the back of the bulbs. May not apply to your car but you can tell by looking. It's the headlight plug-ins with a rounded almost clover leaf shape. If originals not a problem, trimming the plastic back will fix it.

Let us know what you find.

[jlwdvm]

I did have to trim the plastic from the cup on the end of the wire harness that connects to the head light. All connections fit tight. All lights work, it is just when the high beams are on that the low beam light doesn't work on the driver's side. If high beams are turned off, low beam light comes on. All lights are new.

["QUICK-SILVER"]

Broke/cut wire OR MAYBE wires have gotten switched around in the plug-in/plastic connector.

Outside bulb has 3 terminals; ground, low beam, and high beam. So check for power on the high beam wire.

For reference,
Looking at the bulb and plug-in from the back: left side is ground, top middle is low beam, and right side is high beam.

Let us know what you find

[66GTO Jim]

The low beam wiring is in series from the wiring harness. It goes to the drivers side low beam connector and headlight low beam spade lug, to the passenger side low beam connector and low beam headlight spade lug. High beams are the same but use a separate wire. They are not hooked together and are separate circuits.

Since you are gettng power to the passenger side low beam, the low beam wiring itslef should be okay since that is the far end of that circuit. Problem should be the connection between the driver side connector and the head light spade lug. That is assuming Murphy's Law didn't enter and you got a new Sylvania headlamp that was defective the same way as the original. That is a REAL longshot but swapping the new Hi/Lo bulbs side to side would disprove that quickly. I have also seen where folks plugged a two lug Hi Beam only headlamp into the outside Hi/Lo connector and get Hi beam only. I'm sure you are not doing that, just mentioning it in case someone searches this thread in the future.

See if you can slide a meter probe or trouble light probe onto the spade lug with the connector partially in place to see if you have power to the center top lug with headlights on. If you have the original 40 year old wiring harness, it is probably the connector that is not making good contact to the center top headlamp spade lug

BTW, on most Pontiacs of this vintage, the Low Beam wire is Black with a single Tan stripe. It goes from the wiring harness to the Hi/Lo Beam (outside) lamp center top spade lugs only. The High Beam wire is Black with a single light green stripe and it goes to all four headlamp connectors and their respective spade lugs. Jim