Headlight problem

[Peterwh]

Tonight, in the dark, I went to start my 78 CB 750K to ride home from work and found the headlight failed.  All other lights work fine but not the headlight, low beam and high beam.  Got home and swapped the sealed unit for another one I have and that one doesn't work either.  Checked the fuses and they are all fine.  Does anyone have any suggestions?

[TOOLS1]

See if you have power to the switch on the handle bar. If you have it there, check for power at the headlight plug. Also just because a fuse looks good, does not mean it is good, or making a good connection.
TOOLS

[Peterwh]

Thanks Tools.  I will check the switch tonight.  The reason I said the fuses are good is that I tested them with a light tester across the fuse holder terminals, not the fuses themselves.

[Re-run]

Its the old wiring harness issue. You have 4 points of failure: Fuse or block, switch, plug and bulb, or the wires themselves.

Fuse is simple, just swap a new one in to test. Takes a few seconds. After that, it gets harder.
It turns into testing for voltage at different spots. I would open the switch first though and check it out. These switches are simple but still fail. It is a metal ball the rides on copper plates. This ball can flatten out and fall out and once that happens, switch won't work. The plates could also be really dirty. So, clean that thing first and makes sure the ball and the spring behind it are still there. Heck, it needs to be done anyways if you have never done it before.

After that, if still no joy, then it is testing for voltage at various spots such as before and after the fuse block.

It also wouldn't hurt to hook a couple wires directly to the bulb while it is out. Just to make sure. Could be as simple as a bad bulb though they rarely lose both hi and lo at the same time.

And as always, check those grounds. A bad ground can be a bastard to chase down sometimes.

[Peterwh]

I think I found the problem - bad fuse.  I have the original fuse block that Tools hates.  Does anyone know what the three fuses are for?  It was the middle one that had gone.

In the meantime, I dismantled the hi/lo beam switch to clean it and, as was almost inevitable, I lost the little copper contactor.  Does anyone have a spare switch they want to sell?

[shinyribs]

Peterwh wrote

I think I found the problem - bad fuse..... Does anyone know what the three fuses are for?  It was the middle one that had gone.

I betcha a dollar that middle fuse is for your headlight.





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[Peterwh]

I fixed the problem short term - I wired round the switch.  Unfortunatley I now only have a low beam but at least I have a semblance of legality until the new switch arrives.

[Re-run]

Always do those stupid switches over a large white cloth, and now you know why!

[Peterwh]

So, the upshot of all that is that I replaced the whole left side switch assembly and wiring with the one I bought on ebay.  Seemed as though I had to dismantle half the front end to do this but it was quite a simple thing to do.  While I was in the headlamp bucket I noticed there are two wires that are not connected to anything:  an orange one terminating in a male connector and a brown one terminating in a female connector.  Should they be connected to something or maybe to each other?  Any ideas anyone?  Everything seems to work fine however.

[TOOLS1]

I think those are for the turn signal buzzer.
TOOLS

[Peterwh]

Tools, thanks for the thought but the turn signal buzzer works annoyingly well without those two connected to anything.

[TOOLS1]

Well, then that's not what there for, and the bike is operating normally, so I would leave them alone.
TOOLS

[Re-run]

The 78 does have a couple wires that are not used. Kinda like the green wire by the turn signal flasher. Most likely, the harness was either used on a different bike, or the 78 was meant to have some accessory that never made it to production.