Brand new DT175...headlamp dims profusely when brake light and/or indicators are used.

[ahilis]

I just bought a DT175 in Botswana. The bike comes from Yamaha South Africa. The headlamp dims so much when I apply the brakes, regaurdless of speed/rpm, that I can hardly see where I'm going. If I use my indicators, it does the same thing and when I use both, the light almost goes out. I managed to measure, with no other lights on, a 50 mV voltage loss on the negative side of the bulb and 450mV on the positive, making me suspect that the earth is good and they should have simply used heavier guage wiring from the factory.

I would like to change the 165mm x 100mm headlamp with a standard, e-code headlamp housing anyway, which would require a different bulb base like the p43t of an H4. This means that unless I can find an H4 in 6V, if they even exist, I might have to attempt a 12v conversion. I'd get less voltage drop, but unless there is a more robust stator that will fit this bike I might not get much of a charge on the battery below about 3000rpm. Otherwise, if I can find a 6V H4, I will just work out heavier guage wiring where it's needed and use 6V H4 globes. That would raise another question: anyone know how much current this magneto is able to produce?

Thanks for any insight on these matters! Keep well!

[drewpy]

I just bought a DT175 in Botswana. The bike comes from Yamaha South Africa. The headlamp dims so much when I apply the brakes, regaurdless of speed/rpm, that I can hardly see where I'm going. If I use my indicators, it does the same thing and when I use both, the light almost goes out. I managed to measure, with no other lights on, a 50 mV voltage loss on the negative side of the bulb and 450mV on the positive, making me suspect that the earth is good and they should have simply used heavier guage wiring from the factory.

I would like to change the 165mm x 100mm headlamp with a standard, e-code headlamp housing anyway, which would require a different bulb base like the p43t of an H4. This means that unless I can find an H4 in 6V, if they even exist, I might have to attempt a 12v conversion. I'd get less voltage drop, but unless there is a more robust stator that will fit this bike I might not get much of a charge on the battery below about 3000rpm. Otherwise, if I can find a 6V H4, I will just work out heavier guage wiring where it's needed and use 6V H4 globes. That would raise another question: anyone know how much current this magneto is able to produce?

Thanks for any insight on these matters! Keep well!

Hi,

Have a look here as I'm sure there will be something for you!1

[ahilis]

Found the problem. It wasn't wire gauge alone. The original designer of the headlamp circuit for the DT series used a combination headlamp switch which has three switches in one. The headlamp runs on AC straight from the magneto, through one of the three throws, unregulated or rectified, and it is supposed to be the only thing running on that AC output from the magneto. (The 6V rectifier/regulator runs on a different output from the magneto altogether.) The taillight runs through one of the other two of the three switches, since it's power comes from the 6VDC through the battery like the whole rest of the bike.

Well, someone sometime somewhere at Yamaha decided they would omit some of the wiring in the wiring harnesses of this bike. They decided to use only one of the three throws of the headlamp combination switch and combine the output to the headlight and the output to the taillight. Now the taillight, headlight, and instrument lights are all on the 6VAC output from the magneto, not just the headlamp. The other two throws on the combination switch are unused and the wires are literally missing from the loom.

So what happens when a person puts on the brake light? The taillight and brake light are the same bulb with the same earth. Now one filament in being powered by +/- 6VAC from the magneto whilst the other filament is being powered by +/- 6VDC. Not only do the voltages not match at any given time, but one is AC and the other is DC AND now the headlamp's current is travelling back and forth across the bike and through all the circuits instead of having it's own path to earth. The headlamp goes so dim that you can hardly see where you're going. The same thing happens whilst using your indicators; so now, if you're riding at night, not only does your headlamp nearly go out every time you use your brakes, but you must now choose between using your indicators to signal your turn to other drivers or being able to see where you're going.

The fix was very simple. I added only one wire, which was from the ignition switch positive output to the throw on the combination switch which was intended to power the taillight circuit, as per the wiring diagram, and moved the pins in the wire harness connectors around so that the taillight circuit comes from the output of that switch...as per the wiring diagram. Painfully simple, yet now my headlamp burns brighter than ever before and neither the brake light nor indicators affect it's brightness. The taillight and instrument lights burn much brighter and are more consistent since it has the capacitance of the battery, increasing visibility to other drivers, especially at lower engine speeds. I have no way to verify this, but it seems like there is a little extra power from the engine as well...either from more current being available to the igniter or from less current being drawn from the magneto or both. As a matter of fact, the engine RPM used to drop slightly when you switched the headlamp on at idle speed, where as now it seems to have no effect.

Anyway, just thought I'd let you all know before you take the time to reply to my original post. I will be sending a formal complaint to Yamaha because this lighting problem was so extreme that it was quite unsafe. Keep well!

[yoda]

The battery acts as a buffer for changes of load. Therefore the engine load remains much the same. When drawing load directly from the electrical generator on the engine, any change in load will affect the running of the engine.

It should not make any difference whether the earth (ground) has ac or dc or a mix of the two if the connections between the engine/gearbox, battery and frame are good.