I have an 87 300zx GLL model and I can't get my left highbeam working. The symptoms are a bit odd: Both lowbeams work fine, but when I switch to highbeams I the driver's side beam doesn't work. The weird part is that the light works while the housings are actively moving up. So if I toggle my high beams on with the headlights switch off, and then I turn the switch to on, the driver's side highbeam will be lit until the housing is fully up. My first guess was that a wire was getting pinched but I've checked for that and it's not the case. I measured the voltage from various pins on the connectors and found a normal 12 volts on everything except the power lead for the light in question, which sits between 5 and 7 volts. This is my first time doing electrical work on this car and it's not off to a great start. Any suggestions would be appreciated.

Obviously you have tried a new bulb, correct?

The wire does not have to be externally broken or pinched to be flawed internally from the repeat directional changes. Broken all the way across to simply barely touch could easily account for the only partial volts there. May also touch to work or not depending on how the headlight assembly moves. The copper wire inside will break before the plastic insulation if it is still pliable.

And yes, if the bulb broke a filament on one end it could work then not work depending on how the headlight shell movement positions it. You can do a bulb swap to check that quick enough.

Another vote for the filament. if it's not he bulb I'd suspect the combination switch, this is common part to fail, usually on the low beam side of the switch.

I should have specified, the first thing I did was replace the bulb. I've checked the first 12 inches of the suspect wire and it has the low voltage problem through all of it, so if it's damaged somewhere it's gotta be pretty far down. There isn't much left between the tested areas and where it disappears into the wiring harness.

Totally dumb stuff, but my car has separate fuses for each bulb. Have you checked the fuse with a voltmeter in addition to a visual inspection? Sometimes the fault is super tiny and hard to see inside the fuse. I think the combo switch would knock out both lights, not just one side. If not a fuse, perhaps the ground or power don't make it all the way anymore. Then you'll need the voltmeter and FSM.

'I think the combo switch would knock out both lights, not just one side.

Me too, never heard of a switch that split the sides up. Any split there is usually between high and low.

Temporarily wire a junk piece of spare wire as a jumper past the light assembly to further back and see if the problem stays in place. Or two pieces if you can't tell if the load or ground is the issue.

I've tested the output of the power for the highbeam with it grounded directly to the body and it's still only half of what it should be, so it's not the ground. Neither fuse is giving the resistance it should so I'm going to replace in the morning and we'll see where we're at.

Both fuses have been replaced with no difference in final voltage, however it is notable that there voltage AT the fusebox is a full 12 volts. So the problem is somewhere between the fusebox and the bulb.

Bypass or swap relays to see if they are the culprit. Replace as necessary.

Swap parts left to right and see if the problem moves, headlights, relays etc.

I had the exact same problem on my 88T. The highbeam only working while the lights were in motion was baffling but, with some research and a multi-meter I found that my combination switch (aka "headlight switch" or turn signal stalk) had corrosion on the contacts. I still can't figure out why it works while the light is in motion but replacing the switch was the cure.
You can search for "z31 headlight switch" and find instructions on how to clean the contacts. I tried this but ended up bending the contacts too much and got all sorts of headlight malfunctions.
You'll also find instructions on how to swap the internals of the switch with one from a 1991 240sx but I'm not sure if that works for 84-87's. You can find brand new 240sx switches from a bunch of suppliers for about $50 (Nissan Part #25540-44F00). You're essentially swapping a clear plastic box that houses the electrical contacts and plug. That is what I ended up doing. Now hopefully the problem won't return for another 30 years.

Edit: I originally posted that the 87-89 OEM switches were not available but that is incorrect. You CAN find new OEM replacements for 84-87(AIRTEX/WELLS #1S1714). I'm still not sure if swapping the internals with a 240sx switch would work with 84-87's. If it does, the new 240sx switch is about half the price as the new z31 switch.

Okay, so I've tested the switch on the steering wheel and it appears to connect everything up right with nearly zero resistance. The power to and from it seems fine as well. In addition, I completely removed the head lamp wires from the housing so there's no way that putting the lights up and down could jostle them. I did my previous tests again and found that with the highbeams on there is still only 6-8 volts going through the wire, but while the lights are moving up there's about 22-24 volts. So I'm definitely starting to think it's a short somewhere. Maybe there power line is touching the circuit for the headlight motors so it receives high voltage when they're running and is losing power when they aren't? I really don't know. I think the next test needs to be at the relays.

I don't know why I didn't do this the first time. I just took the wheel apart again and bypassed the switch with some wire, completely fixed the problem. So at the end of the day it's definitely the switch that's bad.

You may be able to clean up all the contacts inside the switch and get it working again. It's worth a try and worked for my 240sx. Otherwise, good luck with shopping. I'm glad you figured it out. I'm surprised a switch knocked out one bulb and not the other. Interesting.

I really can't figure out how the light gets power only while it moves up. Maybe something to do with the headlight timer and how the lights will automatically raise when turned on. It really the me off when trying to diagnose the problem. Anyways, the swap with the 240sx switch does not work with 84-87, the connectors are different. Luckily for you replacements for those years are easy to obtain.