OK. I can start and run my DT100 without a battery...the magneto supplying power. So, why do I need a battery at all? The lights do dim a bit when I have too many on (for example, if I have the turn signal on and I put on the brake, then the turn signal light dims). Is that all the battery does....back-up power for the lights? Someone please educate me. Thanks, Pink

Ran my 175 MX with no battery but it poped the rear bulb all the time and the lights were piss poor.

When some guys wanted to ride dirt with their cycles (and get rid of the street-necessary parts), they would only hook up the hot lead in their wiring harness. Those not planning to go back on the street even stripped away the rest of the harness. However, if you need your lights, the battery is required for a constant, regulated voltage output. This prevents the bulbs from receiving either too little or too much voltage/wattage. Without the rectifier and/or voltage regulator/battery in the circuit, the output from the magneto varies, and this unregulated current output will burn out your bulbs. With the early "badge" cycles, it was found that the system was not protected adequately enough, and a Yamaha modification directive was issued. A auxiliary voltage regulator was to be installed in the headlight bucket/circuit, to help prevent the bulb from burning out in the event of a over-rev of the motor when your headlight was on. (what you get when you miss a shift). Most guys never knew of this, and they are seldom found today, so if you see one in a scrap headlight bucket, snag it!

Yes.

I believe that Forum Moderator DEET is able to supply new 6v voltage regulators and the Yamaha service bulletin on this point can be found in the Tech Library on this forum.

Yamaha started fitting the regulator as standard around 1975 or 1976, but I don't know exactly when.

If you run lights without the battery, it's extremely likely the (expensive) headlight will blow.

Lots of posts explaining how the system is balanced to regulate voltage.

You didn't mention the year... a later model may have a regulator and might be okay.

DEET is 'da Man! Made his acquaintance over 10 years ago on "the other Forum". His work-arounds, creativity in making ersatz specialty tools, and general knowledge is outstanding. If memory serves, I think he may have supplied the copy of the Yamaha directive about the voltage regulator. I remember making a hard copy, but was not able to locate it earlier today, or would have posted it w/the .jpg of the VR...

'ere we go!

yamaha-enduros.com/index.php/workshop/se...e-voltage-regulators

OK. I think I am starting to understand this. My DT100 is a 1982.

In looking a the wiring diagram in the Clymer manual there is a rectifier that takes the AC from the magneto and converts to DC that both maintains the charge on the battery and is also wired to the ignition switch.

I also found this description of a rectifier: "A motorbike rectifier is an electronic unit which converts 12 volts DC out of the 11 to 14 volts AC the generator/alternator in the engine delivers. " So it is a transducer (in my business), converting one signal to another. The input is a range of 11-14 VAC and the output a constant 12VDC. I think it also effectively takes the place of a voltage regulator, but someone correct me if I am wrong. There is no voltage regulator in the wiring diagram

If there is no battery, then the same DC is just going to the ignition switch. This is why the bike runs and everything (the lights) work even without a battery.

So, I don't think the battery is needed, except maybe to provide constant 12 DC voltage when the revs are low and the output from the magneto may be at the lower end of the AC required. That is, if there is less than 11VDC input, then you would end up with less than the 12VDC output required to have the lights at full brightness.

Finally, The only "reason" I can think of for a battery, and this is just speculation, is that maybe the magneto will come under "electrical stress" (heat up) if it is working too hard to keep lights going bright a low revs. An electrical engineer/technician may say this is a bunch of hokey, but I am neither.

Thanks Gr8uncleal...I saved that document.

Sarge is that you!