I started this thread as this issue was under my original thread of a 1973 DT3. So, I have the Yamaha service bulletin regarding how to hook up an aftermarket voltage regulator via the "blue wire" from the instruments. I tried this, no luck. I also tried hooking to yellow wire inside of headlight, no luck because that was only the high/bright light circuit. It blew the low circuits, but high seemed to work ok for a bit but eventually blew.

I have now just tried tapping into the yellow wire RIGHT OFF THE STATOR. The headlight is burnt out both high and low filaments and I am not installing another just to blow it up $$$. I am using the taillight bulb as my tester. After hooking up the original style STANLEY regulator at yellow wire off the stator, the low circuit either immediatly blew or never came on. The high circuit worked but as soon as I revved engine, it blew the taillight bulb. I do have an AGM battery hooked up to "maybe" provide some load.

I am kinda stumped here because I have done this modification to many other pre-74 Yamahas with great results. Hooking directly to yellow wire off stator. So, either my regulator is bad (how do I test it?) or I am hooking to the wrong things. There is a discussion about AC vs DC wiring on these bikes. On this particular bike, I have not ever hooked up the regulator to DC....as far as I know. But without a proper translated wiring diagram...I am not sure.

I am willing to try the instrument light wires again, but maybe it's not working since this is JAPAN SPEC? Any help appreciated. I am going to try a finned newer regulator today at yellow wire off stator and see if that does the trick. If only three wires off stator....what does the green one do? Looking at USA wiring diagram, it's hard to tell. Yellow looks like it is main power supply to light switch.

Sorry to ask, although will be one more thing to discount, but is the regulator earthed ok?

Yes, regulator case must be solidly grounded!

We are at a huge disadvantage because you can see what you're working on, we can't... and you're working on an unusual model variant from what it sounds like.

DEET posted a video he did showing regulator testing.

Yellow off stator is headlight (and probably taillight) circuit. Easy way to tell is on US model wiring after 1970 or so the taillight comes on when the lights are turned on... it runs off the battery. Earlier models and non-US model taillights often only come on with engine running as they are on the same circuit as headlight which sounds like what you have.

Green goes to a rectifier (diode) to convert output to DC and charges battery. If you only have one green, do you have the option of turning off the headlight with engine running? Most models where headlight can be turned off have a "daytime" battery charge circuit and a separate "nighttime" battery charge circuit... second green not 100% needed but that's normally how they are wired.

Here's a link to DEET's test. 

yamaha-enduros.com/index.php/forums/how-...lator-shootout#43886

UPDATE!!!!! First of all, the JAPAN SPEC version DOES have a separate on/off light switch, the lights do not automatically come on via the ign switch.

So, I finally discerned which wires that go to instrument lighting by following by hand/eye. I have a JAPAN SPEC wiring diagram but it is useless until I can get it translated. Hooked up everything today and so far things are fine, headlight high and low and running brake light all working fine with no visible spiking from revving.

Now my "high beam" brake light does not come on anymore....good God. Neither the hand lever nor the foot lever will activate the high beam in the rear brake light. I am not sure why this happened. I guess I will hotwire the footswitch and see if that shows any results.

I still need to replace all the indicator and running lights in the gauges from when they got blown out previously. I hope to report back soon. I do report that the regulator was running a little hot when hooked directly to the stator, it seems to be running much cooler now. The great thing about the new finned regulator is that it has an indicator light so you know if it's functioning or not. This is pretty cool.

UPDATE PART TWO:

So now the headlight and running brake light dim when I apply handbrake or footbrake. The stoplight circuit was working fine with regulator hooked straight to yellow wire off stator. 

Any clue what might be going on? I tied into blue wire for instrument lights in headlight bucket. I cant be 100% certain, but pretty sure Im tied into the actual wire that comes from the headlight switch which on the JAPAN SPEC bikes is separate from the ignition switch. Kinda losing my mind here. Thanks for any help.

Is this happening with the engine running at, say, above 2k rpm?

I think you posted about this bike a few years ago?  I think I mentioned then that you can translate the diagram easily by using Google translate app with a phone camera?

Anyway, someone posted a scan of the Japanese DT250 wiring diagram but the file is too small to read...  if I enlarge it, it gets all fuzzy and unreadable.

I'll PM you my email address and if you can email me a good quality scan of diagram I can try to decipher it.  (best copy of file I have is 100kb which is likely what the forum software reduces uploads to...  probably need something in the few MB range to be able to read it.) 

email sent

Yes it happens at all rpm.