oisact

Thank's for the additional info. In your wiring diagram, how do you know how the switches are wired internally? IE double pole double throw, etc. Did you determine that from an actual schematic?

Everything is stock, however, I really don't believe the voltage regulator is part of the headlight coil / circuit. I get up to 20V AC at my headlight socket - it's not being clipped. You said on your bike the headlight is not part of the battery circuit, so your headlight is not affected by the regulator either, is it? What voltages do you see directly at your headlight socket?

Now what is the "pilot in the headlight?" I just have a low beam and high beam, both 35W full brightness. The only front-facing lights on the bike are the headlight and front turn signals.

Just introducing myself real quick. My name is Dan, and although I just turned over-the-hill this last month, I'm still as active as ever riding dual-sport cycles, rockclimbing, hiking, etc, etc. I got my first cycle for my 6th birthday, and have been riding ever since. I've owned a couple Yamahas over the years - an MX 80 when I was around 9 or 10, and currently a 1978 DT-175. I bought the DT-175 6 years ago. It had 1,500 miles on it, and even the tires were still original. 4,000 miles later (most very hard riding on trails) and it's still going strong. I live in the Appalachians where there are great roads (smooth survice, nice curves, scenic views), and lots of back, back roads and trails for dual sports. So it's pretty much a dream for both streetbikes and dual sports. Last winter was so mild that I road 400 miles during the month of January (all "pleasure" - I don't commute). My neighbor just bought a 2013 Honda CRF250, and his friend bought a 2013 Kawasaki KLX250 (both dual sports). Well I've ridden them both, and I'll take my 78 yamaha any day, on the road or off. Way more pep and power, plus my bike feels like it's at home in the dirt and rock, and not something with lots of plastic to damage and areas that are impossible to ever clean the dirt and mud out of. I found these forums because of a thread where NE0 converted his 78 DT-175's electrical system to 12V, which is something I'm interested in to make it easier (and cheaper) to find replacement lights and batteries.

Now that it's finally dark out I swapped in the 12V 35W headlight to see how well it works, and it's a no-go like this. Way too dim and orangish looking. As I mentioned above, I get 20V AC at 6000 RPMs at the headlight socket, and with the 12V bulb it's still too dim. I put the 6V headlight back in and it's nice and bright. I've never had a problem with the 6V headlight not being bright enough, but they are expensive and cannot be found locally. So I'm still not quite sure what the differences are between the UK and USA version of the bikes, but there are obviously wiring differences with the headlights as you can turn yours off and on, and as you said, based on the switch position different windings of the coil are being connected to the voltage regulator and headlight. When you turn on the headlight, are the white and yellow coil wires BOTH flowing to the headlight and battery? If so I bet that's my problem, since the headlight stays on all the time on my model and is not connected to the battery, it's a completely separate circuit running off of a single coil only. If the two coils are combined and so there's a single unified electrical system (not counting ignition) then I bet there is enough amperage at the higher voltage to operate the 12V headlight. That makes a lot of sense, because the one coil just charges the battery and runs the indicator lights, turn signals, tail light and horn. Most of that isn't even drawing power 99% of the time, so there would be a lot of excess current available to help with the headlight.

Great thread!!! It's even greater since I own this exact bike (US version). I've gone through a couple headlights and taillights now (including two 6V LED taillights that essentially cannot burn out). So I'm pretty sure my voltage regulator is bad. Since I already need a voltage regulator, headlight and taillight (battery too, it's shot)..... I might as well switch to 12V. I have some notes, questions and comments. First off, I already changed all my gauge lights to 6V LED lights that I got from superbrightleds dot com. I was able to get each LED bulb in the exact color of the various lenses (amber, blue, red, green and white for gauges) and that made a huge difference. Gauges are much brighter and don't flicker, even without a battery. That is especially good for the neutral light, because the instant you hit neutral it comes on bright and clear. I would strongly recommend updating the gauges to LED bulbs even if keeping the bike 6V. I found the EXACT replacements (exact same base, etc) in LED form, so it was a no-brainer to install them. I think they were around $2 a piece. Now for a question. You have a UK version of the 1978 DT 175, which apparently allows you to control whether the headlight is one or not via the ignition switch (my 1972 Suzuki TC-125 is that way as well). However, the USA version only has a 2-position ignition switch - the headlight is always on when the bike is running (they must have added that requirement in the USA between 1972 and 1978 then). Further, you say that according to the wiring diagram, your headlight is connected to the battery. Does that mean if you turn the switch on and the bike isn't running, your headlight comes on? The USA model doesn't work that way at all, which may mean your conversion process is different for (or might not apply to) the USA version. Another thing I don't understand (and not having a wiring diagram is probably to blame) is there is only a single regulator, yet there are 2 (not counting ignition) lighting / charging coils. If there is only one regulator then only one of those circuits is regulated (unless the two are simply tied together at some point, which may be the case). So I hooked my multimeter up to the headlight socket, and it is clearly AC (0 reading on DC), which means the headlight circuit does not run through the rectifier either. I measured 20VAC at 6k RPMs, which doesn't match either of your charts. Obviously the voltage is going far, far above 6V, which can mean only one of two things. The regulator is supposed to be part of the headlight circuit but it's bad, or the regular is not meant to be part of the circuit in the first place. Thus I was curious, since the headlight power is not clamped to 6V on my bike, how well a 12V 35W headlight would do right off the bat. The local auto parts store had exactly what I was looking for - exact same size headlight as the 6V, but in 12V 35W. I hooked it up and fired up the bike, and it is noticeably orange tinted, and certainly not as bright as the 6V (my 6V bulb is 30W and the high beam still works so I can compare). So if I'm already getting unregulated power to my headlight, then that's as good as it can get - regulating the system to 12V will could only make the problem worse, not better. So that makes me wonder if your 12V battery is simply averaging things out - it charges when you're at higher RPMs, and is draining at lower RPMs, keeping the headlight bright across the spectrum. If that's the case then, again, that won't work on a USA bike because the battery is not part of the headlight circuit. Anyway, I'd really like to do the 12V conversion, as I believe my regulator is bad anyway, but I'd like to know for certain that a 12V 35W headlight will be as bright as the 6V light. Thanks!