Yes, stock switch. Everything seems fine for now. I'll report back when I plug in the new headlight.

A thought on the regulator heating up if running in the "daytime" ignition position..... Could one just install an on/off switch between regulator and tapped AC wire from stator?

DT3 uses B+ for brake ( brown) among other loads, and taillight ( blue from L terminal) you want regulator on the LW (also blue) side for headlight inst lites, power for headlights (and regulator) will go nowhere in pos 1 if wired to LW side of switch

I don't follow your terminal designations. I don't have access to those "that I know of."

I definitely am not educated on where the power goes from wire to wire...but figured if I'm tapped into the yellow wire, right off the lighting coil, I should be completely "regulated."

As to the position 1 thing. I guess you are saying that i should tie into the light blue (LW) wire after the ignition switch? I tried doing "that wire" on my dt250 and still blew low beam circuit.

If I stay tapped into the yellow wire off of stator and always run position 2...I should be fine right?

UPDATE: I looked back at my notes. I had ORIGINALLY tapped into the rear brake circuit (blue wire) with the regulator. I had switched over to the YELLOW wire off the stator on my second attempt. The reason I didn't realize this was because after my first failure...I "mocked up" the YELLOW wire connection before I put the bike up. So when I pulled it out for the SECOND attempt...I thought the YELLOW wire was tapped into the first time. So, I guess I have to decide which wire to tap into now. Stay tapped into the YELLOW wire or the LW BLUE wire after the switch?

Thanks for all the help on this. I have multiple Yamahas I am trying to get this issue resolved on. I ride them on the street mostly. Sure is more fun than a street bike.

The rear light wires after an early year (1970?) in the USA are on the DC "battery" side of the electrical system. The laws here said the brake and taillights had to work with the engine off.

If you tapped into a wire after the ignition switch on your DT250a like you just said, that's why it didn't work to protect the headlight. The headlight power does not go through the ignition switch on a DT250. (74 on)

Tapping into a tail or brake light won't work either. The yellow to the brake light is NOT the same yellow that comes off the stator.

And again, none of what you've tried is what Yamaha recommends doing in the Service Bulletin. It's not as clear as it could be, but they say to connect to the speedo/tach gauge illumination wires. Those lights are only on with engine running and headlight on either high or low beam.

Hopefully the AC regulator wasn't damaged connecting it to DC?

If you don't want to follow the Service Bulletin, unfortunately you can't just tap into any blue or yellow wire anywhere you want to without at least checking two things:

1. Is the wire you've chosen connected to the AC light side of the system? Only has AC power with engine running?
2. Does the wire you've chosen have AC power only with headlight switch on and with beam selector on either high or low beam?

If the answer to ALL the above questions is "yes", then that's a good wire for the regulator to connect to. If the answer to any of the questions is "no", then you may defeat the regulator completely, have the regulator work on only high or low beam (not both), or have the potential to overheat the regulator if riding around with lights off.

Since you seem dead-set on not following the instructions in the bulletin, connecting to the Yellow right where it exits the stator is your best bet. Others have done this and as long as you ride with lights on most of the time it should be okay... Yamaha OEM regulator is good quality and they tend to hold up well.

Yes, I am not an electrical wizard...it's hard for me but I'm learning. My biggest challenge is "finding" that wire that the service bulletin tells you to use. I thought I had found it...but as per your comment...I did not.

I'm not "dead-set" on anything except finding the right solution.....I definitely want to do what the service bulletin says...it's just hard for me...even following the wires by hand/eyes....trying to find that magical wire that is referred to in the bulletin. The only reason I went with the yellow wire right off the stator was for simplicity sake. AC/DC are terms I know, but don't really know how to apply professionally or in a working situation. Your post helps though. I just thought that there was no way tapping into the wire that controls the instrument lights could be the same for the headlight. I will revisit this on each bike and see what I come up with.

Sounds like yellow wire right off stator with lights on is safe for now at least.

I get it. Electricity is confusing to me too.

It might help you knowing that these bikes were designed to have a simple, reliable electrical system back in the times before computer chips and electronic semiconductors were cheap and readily available.

With a fixed magnet alternator generating the power and no voltage regulator they had to "balance" the output of the stator against the load of the bulbs to prevent the voltage from getting too high and blowing the bulbs. This system was simple and worked great to run the simple AC electrical systems before batteries became needed.

(My best guess for why the systems are 6 volt is because a change in resistance (load) on a 6 volt system makes less of a difference in voltage than the same change in load on a 12 volt system... so 6 volt system voltage would tend to be more stable)

Then to complicate things safety laws were made that said certain lights had to work with the engine off... and turn signals became required. You can get a horn that works on AC (early scooter) and there are even AC turn signal flashers... But you must have a battery to operate any of those things with the engine off. And a battery requires DC power. (It's also hard to "balance" a simple AC system with turn signals and brake lights because they are not on all the time... some stators (not Yamaha) solved this somewhat by having separate stator windings for each circuit... like a wire off the stator to just run the brake light.

So to get a battery to work a separate stator coil was connected to a plate rectifier (en.wikipedia.org/wiki/Selenium_rectifier) ...very "high tech" and expensive in the day but today just a simple diode... that only let the + through from the stator. This is not very efficient but allowed basic DC battery charging.

Because of cost and inefficiency of this primitive conversion to DC, the manufacturers kept the headlights on AC and for safety and legal reasons used the battery circuit to run the horn, brake and taillight, and turn signals.

One of the goofy things is that even though the battery is charged off a separate winding(s) in the stator, they generate power using the same spinning magnets so if you remove the battery charge load it will increase the available voltage to the headlight circuit.

So while it might be a simple and reliable system in the respect of not having a lot of parts... it took genius engineering to get a combined AC and DC system designed and balanced.

As long as connections and everything is well maintained, they work great.

Adding a regulator to the AC circuit just provides that extra insurance to protect against voltage spikes and blown bulbs when everything isn't perfect.

To find the Service Bulletin wires, it's easiest to start at the gauges themselves. Wire color should be a a darker blue and runs the illumination for the gauge... usually there's one illumination bulb in the top of each gauge. You could splice in right there at the back of the gauge or trace back to the headlight bucket and splice in there.




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Revisiting this saga here. Installed a vintage factory stanley regulator at yellow wire off stator on the 1974 JAPAN SPEC DT250a. Blew taillight once I revved it up. Blows low circuit immediately, or low cicuit is just not coming on. Yes I have a battery hooked up, but I am not running a headlight bulb due to crazy $$$ cost. So, I have a feeling the old stanley is shot. I have a newer regulator(finned) I will try. I really don't understand AC/DC. I tried to tap into the "blue wire" as advised in the service bulletin. I simply cannot find it or it didn't work. I have had success running directly off stator yellow wire on all other bikes, CT3, DT3. I'm damn near at a loss here. I will try the finned regulator next and see if that works. Ordered 10 taillight bulbs for testing. Starting to get expensive. I guess I might try one more time to find the MAGICAL blue wire off gauges and tap in there? I just don't understand how tapping into gauge lights are going to stop headlight and tail from blowing? I guess it's the AC/DC thing I simply just don't understand. So, does a stator provide AC or DC?

I'm really bad at electrical but from what I've read here if you don't have all lights working the excess voltage will go to the other light and overload it. I'm sure there's a better explanation to come but that's what I've gathered from similar topics.

Do you have the speedo and tacho fitted? Do they have their illumination lights fitted? If yes, the wires to each bulb holder should be one black and one blue and it is either of these blues that you can splice the single wire from the voltage regulator into.

Check out my reply #52 (towards the end) of the attached thread to see my alternative to splicing.

Headlight keeps blowing out. - Page 6 - Vintage Enduro Discussions (yamaha-enduros.com)