There is a table on my wiring diagram that shows 3 positions on the left axis: Off, I, and II. My ignition switch only has two positions, Off and on. What is position II referring to?

That's one of the (unfortunately many) boo-boos in Yamaha literature. Before 1974, the lights were controlled by the main key switch (position II). Somehow the legend for the earlier switch made it onto the 1974 diagram.

The Service Manual in the Tech Library has the correct legends in the Electrical section itself.. (not on the diagram!)









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Brilliant! Thank you!

A couple more questions have come up. I seem to have a Grey wire in the headlight cup, but I don't see Grey anywhere on the diagram.

Also your table mentions Y/W as a wire color, which is nowhere on the diagram either. Are these more mistakes?

Not "my" table... it's Yamaha's.

Yamaha is not too precise... the "Y/W" is the AC headlight circuit power wire from the stator which is traditionally solid yellow in Yama-wiring... could be solid yellow out of the stator and changes to Yellow/white at the switch harness. (Similar thing happens on many models with points ignition and the black or black/white. )

Not sure about the grey. They do list grey in one of the Service Manual wiring color legends but I don't see it on any of the diagrams. Is it a European model? They had slightly different wiring. Pictures of what you're looking at might help. Can you trace where it goes?

Good idea to trace it, which I find leads from the headlight shell up to the Tachometer indicator lights. It goes into the same receptacle as the Db wire, so according to the wiring diagram it must be Dg. Sure doesn’t look like it to the naked eye, but with the camera flash at the instrument end on I can see some green. In any case, I’ll call it Dg. So that mystery is solved but here is another one:

The wiring diagram says that the wires to these instrument indicator lights are Dg, Db, B, and Y. Y shows as going to three indicator lamps. However, on my bike I find that I only have one Y wire, and two L wires.

• Db and Dg go to the flasher indicator
  
• Y and B go to the high beam indicator
  
• L and B go to the Tachometer meter light
  
• Also, L and B go to the speedometer meter light (not pictured).

So I think the wiring diagram is wrong and where it shows the Y wires to the meter lights they should be L.

High beam indicator?

Yes, traditionally the Yamaha instrument light illumination wires are blue. (They are powered by yellow when light switch is "on")

If your diagram shows yellow wire from switch to bulb, it's wrong.

That would be from page 204 of the service manual in the tech library.

74 DT250-DT360 service manual

There are also several diagrams in the body of the Service Manual...

Page 132 Figure 7-4 is far more accurate (and in my opinion, easier to read). It shows a blue (L) in the main harness connecting to blue/red (L/R) at the instrument illumination bulb harnesses.