Debugging a headlight issue on 1990 VF750F

Riding down the highway and the low beam winks out. High beam still works. A little research and the bike has original OEM bulbs, both identical, appear to be single filament Halogens (I assume the beam is determined by the shape of the reflector). On low beam only one side should light up, on high beam both sides light up. So swapping bulbs has no affect (still no low beam, full high beam). Headlight fuse is good (but I assume that would take out both low & high beams). Next I swapped the relays which had no affect (still no low beam, full high beam) which I am assuming means the relays are not the problem. So the only things I have left are the switch and the harness. Is that all logical or have I made a leap of faith somewhere along the line? I guess I'll look up the test procedure for the switch and then start checking various wire continuities (ugh!). Worst case could I just install the bulb on the low beam side and leave the high beam switched on? (And just don't ride fast enough to require the high beam.)
Chris

 

Are you positive both low beam filaments are good? (Or at least one)

 

So the bulbs are dual filament? If so the low beam filament burned out on both? Even though one is not lit on low beam? Or are they both supposed to be lit? I know some bikes only light one side.

Before I pulled the front end apart I ordered some cheap LED standard H4 bulbs. I could try one of those and see if it lights on low beam.

 

Im just starting with the most simple explanation. Both my low beams are lit on my U.S. bike, my previous 1990 and current 1992.

Also, I know my U.S. 1992 (same as 1990) does not take a standard H4. The tabs are different and it's a 45/45W. Confirmed that 2 days ago.

Sounds like you should do a little bulb testing and research.

 

D0 you ride with the high beams on alot? So, getting to those relays on the gen3 bike is beyond a pain in the nut sack right? I had head light troubles on my high mileage gen3 bike years ago, I was working for Honda at the time and ended up buying a new wiring harness for the bike (last one in the country supposedly) That cured the problem. When I was looking at the old wiring harness (un-cut) I saw that below the fuse panel for the fuse associated with the head lights, was suspect, fuse would not sit in there tight. I think, if you cut the wires below the fuse block for that circuit and replace it with an inline fuse that could solve your problem.

 

The wiring diagram shows that the healight bulbs are wired in parallel and have dual filament bulbs. Both low beams, or both high beams, should operate together. Given you only had one low beam previously, I'd suggest you need new bulbs as a first step.

 

Thanks for the help. Yes both low beam filaments were burnt out on the original OEM bulbs. Installing 2 new low cost LED H4 bulbs (by bending some tabs) seems to have fixed my issues. (Along with a rectifier swap to address a possible over voltage cause of stuff burning out). Anybody want 2 OEM Honda H4 bulbs with just high beam filaments, LOL.