Just bought an 84 z28 and noticed when I hit the high beams the low beam on the passenger side goes out. Thought I'd try to save some time and see if anyone knows where to check first,headlight switch,dimmer switch,or harness. Thanks in advance.

 

If the driver's side low beam headlight functions normally , the answer would be none of the above , you need to replace the passenger's side low beam headlight .

The low beam headlight has two filaments in it , a low beam filament and a high beam filament . When the low beams are on , only the outside two headlights light up using their low beam filaments . But when you turn on the high beams the low beam filaments in the outside two lights go off , the high beam filaments in the outside two lights come on , and also the inner two headlights (which are high beam only) come on . Your high beam filament in that headlight has burned out ...

 

I kind of thought Maybe. Been 30 years since I've owned one. Looked up the bulb but no mention of it in any of the specs.

 

The bulb is part number H4656 , around $10 at Rock Auto

 

You can take a test light an verify that the plug is getting 12V which the low beam on. Mine low beam was a yellow wire and the high beam is green. If you have power on both high and low it's the bulb.