Headlight circuit issue

With your meter, follow the blue wire coming out of the dimmer switch.
With the ignition on and black probe on battery neg, use the red probe along the blue wire. Whenever you find 12v along that wire you will have found the issue. Headlight must be plugged in.

So I'll investigate the blue wire that comes to the left switch
 

Actually the blue is going to the LH switch. It’s the 12V power. It is feed from the brown wire coming out of the ignition when in the “ON” position. White is 12V to the ignition. 

Yep Mikaw,
The blue wire supplies 12v to the hi/lo headlight switch.  By following the blue back toward the battery he can find where the break is.
His RH controls are missing so no telling where the blue wire is spliced.

Thanks guys,
The plan is to find where it failed-yesterday it was so much rain I couldn't do anything, and it was summer hot 90 degrees too
I'm getting to it right now.  

Best
JB

Well, I traced the blue wire from the bullet connector under the tank. it goes to the fuse box and the fuse is OK.  I don't like those glass tube fuses and replaced the main fuse with the newer type.  But since the fuse checked out, the blue wire then becomes blue with white stripe and goes back to the area under the tank where it was joined by a brown and a brown/white wire.  One of the brown ones has no juice, the other had about 11V... I'll have to charge the batt again overnight since the turning it on and off to test probably drained it and the coils do start to get hot.

I'll have to investigate more tomorrow, got late.  There are some bullet connectors joining quite a few things there, they're mostly brown wires.  A bit of a mess and as I've read on these forums, sometimes wire colors don't match the diagrams and people did crazy stupid things over the years.

My other '74 is an H2 and practically all wiring is original with no issues.  But an H2 runs a lot cooler than a 900 and the wiring is a bit simpler.  I've owned that bike for 26 years and I know it well 
This bike I've owned about 6 years and didn't even want to look at this spaghetti under the tank until this happened 
 

Sounds like you have quite the mess under that tank.
So--is the blue/white wire spliced to the brown AND brown/white?
 

Yes, that's what it is, but I also have to see what else is under there-there are these bullet wire connectors that are double female, I'll probably have to take a picture to analyze it tomorrow

Thanks again
John

That is weird---You have 11v on a wire that is spliced to 2 other wires that are dead.
All 3 wires should have 11v.

Hello, how many fuses do you have?, Z1 has only one, Z1 does not have blue/white, and brown/white wires, later models have more fusing for the lighting , unless these wires were added by PO, have a later model harness, I have a 74 Z1 and a 81 650,  650 has 3 fuses, power/ignition(20a) and 2 for lighting,  in the 81 the blue/w and br/w are butted to the brown in the rh control harness, and they are the line side of the lighting circuit fuses, which turn into blue to dimmer control, and  red/blue (running lights),  Z1 has none of that, one switch turns it all on,  like your H2 very simple, do you have a Z1 diagram? is in this forum, or get a new Z1 harness (Z1ent, Z1 parts, Niche, or ebay, about $100)  and a new RH control  about $40 from same sources, plug and play and you'll have switched lighting, good luck

 

Thanks for all the replies!
After days and hours cleaning connections this thing came back to life 
I may never know where it was, but meter finally showed 12V on the blue wire, after I reconnected everything the light went back on. High and low beam switch working too, and it’s also suspicious.
I had sprayed Deoxit D5 pretty much on every connector, and inside the sliding low/high switch I scraped the brass or bronze contacts where the switch moves.  So that’s that, it certainly fried me out chasing the cause and getting it to work.
This bike is a ‘74 but then the harness is something else cause there is a fuse box and I went through it too.  There are 3 fuses in there including the old main one which I bypassed-

The main fuse I bypassed a while ago and installed a modern one because in the past that glass tube one failed at night-no lights or ignition which was insane at 60mph+ but fixed on the side of the road-

So this failure could have been the high/ low switch or any other connector that I cleaned.
My H2 is a much cleaner bike and I've owned it for 26 years.  I've had fuse issues with my '68 Karmann Guia and other vintage VWs years ago, even my newer cars 'cause Miami is basically a swamp!    Bad contacts happen even if they look ok
And only a few months ago I did the H4 headlight conversion 'cause basically you can't find sealed beams anymore, not US made at least.

Best,
JB

Pic of my H2 which I've owned for 26 years and rebuilt countless times