Hi Fellas, i am getting close to finishing my restoration project of this 1974 DT 175 model A . the rear light works with the light switch on and ignition on but no engine running but no head light.with the engine running the head light does not work either , all bulbs are good ? should i get 6 volt at the yellow wire in the head light bucket from the alternator with engine running ? any help would be great. thanks. Jez...

There's a yellow/white out of the stator that goes to the left handlebar on-off switch for the headlight and gauge illumination lights. 

(Sometimes, especially when there are plastic multi-connector plugs that prevent incorrect connections, Yamaha omits the little white stripe on wires in the harness.)

When you turn headlight switch on, the yellow/white from stator powers left handlebar hi-lo beam switch and a darker blue to the little gauge illumination lights.   On low beam, the Hi-lo switch powers green to headlight and on high it powers the yellow to headlight. 


(Side note, above describes the engine powered headlight circuit.  The taillight also runs through a separate set of contacts in the headlight on-off switch that connect the brown (battery power) to taillight.)

Hi and thanks for your fast reply.i thought that's how it should work.i have no voltage at yellow/white in the multi connector from magneto so I'll have to trace back to behind the flywheel .cheers

Hope you can trace it down without too much trouble!  Good luck!

So the 3 coils in the magneto must supply power for ignition and lighting etc .do we know which one does what ? and is there any way to test them using a meter ?

The coil with finer windings is the one supplying power for spark.
Here's the nearest relevant manual i could find but not easy to follow for me at 1am in the morning haha.
Scroll down to page 125 & not sure if its my computer but had to crank it up to 510% to read it.
Resistance are there if you can follow it. If you see DT125 which i think it covered, then dodge that as without looking i'd guess it's the 12 volt electric start motor.
Best tell us what problem you have & see if we can help. Then again, i'm right confused looking at all that.
Not easy when they combine manuals, but looks like lighting coils are very low resistance like less than 1ohm 0.??of an ohm.
I couldn't see a resistance for the ignition source but might be around 1 to 1.2ohms wildly guessing seeing its got finer windings.
Hope someone has a better manual.
yamaha-enduros.com/index.php/service-man...a-b-c-service-manual

The charts in the front of the manual have some specs too.

I'm not sure they show the headlight lighting coil (yellow).  I will check this evening and see if I can find it.   

 

Thanks for your help guys.i have continuity from the yellow wire at the light switch down to the lighting coil so wiring loom seems ok .the coils look intact ,should I be looking for anything specific.how would you test a coil ? Thanks again ...

Just unhook the mag loom but leave black connected for spark then give it a rev with a cheapy bulb straight into the yellow from the mag loom & see if it blows it straight away. Or AC voltage test.

Ok sounds like you've been running it sorry--but doing as above will have the other charging wires disconnected incase something is dragging it down on the charging side & robbing voltage from the yellow.
I'm likely talking rubbish & not reading the story, fully but hope it helps.

I could not find the specs in the US manual..  RT325 posting nudged me and I dug out my Australian Cycleserv manual which shows the DT175a stator specs as follows:

Headlight - yellow wire 0.48 ohms +/- 10% @ 20 deg C (68 deg F)

Battery charge:
Green/red wire - 0.78 ohms +/- 10% @ 20 deg C (68 deg F)
Green wire 0.48 ohms +/- 10% @ 20 deg C (68 deg F)

Ignition Source coil:
2.5 ohms +/- 10% @ 20 deg C (68 deg F)