Topic: Headlight Problems - 76 CB750F

[MacM2010]

Hi folks.  First, the obligatory apologia regarding searches and whatnot.  I searched, and I have a Honda manual and a number of the in-color wiring diagrams sitting next to the bike.  If this is covered ground, I apologize.  Here's the situation:

The PO, upon changing to drag bars back in the late 70's, did some surgery on the wiring for my bike.  Fun stuff, like making the right turn signal illuminate when the left switch is engaged, and vice versa.  This was fixed easily enough, so my signals all point in the right direction, but the headlight is still giving me guff.

It won't light up.  The bulb is fine, I checked that.  I replaced the fuse, so that's not it.  And when I test the wires coming out of the bucket (the green, blue, and white ones), the blue one has power, the green one (ground) doesn't, and the white one doesn't.  Is this correct?  The white one is the high-beam wire, yes?  So should it have power when the high beam is disengaged?  Otherwise, I can't figure out why the bulb isn't illuminating. 

I've chased the wires all over, and the PO soldered them fairly well, but I can't find the loss of power.  From the headlight, I chase the power back, but tend to get lost - can anyone point me down the right road to find my lack of power?  I'm getting tired of peeling back tape, only to find everything looks good. 

If anyone has any ideas, or at least approaches to figuring it out, I'm all ears.  I'd really rather not pull the whole harness, especially since that seems to be fine (at least everything else is working). 

Thanks for reading.
Best,
Mac

[HondaMan]

It's a long way from Las Cruces to the Honda shop! My wife hails from Ruidoso...

That GREEN wire is Ground. It should be connected in the wiring harness to ground, but might have had some other misadventure, it sounds like. Try to (temporarily) jumper from the Green wire on the headlight (and the Green in the headlight bucket, for that matter) to the engine and see if it lights up.

The BLUE is HI beam, the WHITE is LO beam. On your bike, the power for the headlight goes like this:
From Keyswitch on BLACK, the power goes to the headlight bucket, then up to the START button. When this button is released (i.e., not pressed), the normally-closed side of this switch sends power to the [Black/White, Black/Red, or Black/Yellow, or Brown/Red, depending on which harness you have] wire, back to the key, out of the key on BROWN-WHITE, to the fuse, back to the headlight bucket, over to the left HI/LO switch, then to the headlight (whew!). There's a lot of [aged] connections in between, any one of which can cause some issues. It is possible to simply unplug the Brown that goes to the left Hi/Lo switch (in the bucket) and connect it to the Black in the bucket, to see if the Hi/Lo switch is at least working, too. Don't leavie it this way if you have a halogen headlight, though, as over time it will overheat the splices in the Black wire in the body of the harness. It's OK for testing, though. 

[MacM2010]

Ah-ha - I have my blue and white switched.  Okay, that helps a lot.  I'll try jumping from the brown to the black like you said and see if I have any luck there. 

Thanks very much for the help.

Mac