Privacy guaranteed - Your email is not shared with anyone.

12v headlight on a 6v system

Discussion in 'Mechanical & Technical' started by Lrn, Feb 20, 2008.

  1. I have a 1975 Yamaha DT400b and in November my headlight blew out, so a few weeks ago after pulling the bike out of the living room, i want down to a local shop and replaced the lamp with what i thought was the same lamp....
    kicked the bike over plugged the lamp in, it light up, went for a 5 min ride and when i got back the lamp element (filament?) was blown... there were no burn marks anywhere on the plugs or anything.
    my question is whether a 12v lamp would blow out in a 6 volt light system.. my rear break lights work as well as the signals. From what little i could find off google, the hight voltage lamp could blow out on a lower system. I just want to know that this is the problem before a drop another 20 bucks on a 6v lamp that will blow out.

    Thanks for your help in advance


    the markings on the blow out lamp are 4020 6v 35w
    the makings on the new (also blown lamp) are 4020 12v 35w
  2. Yes that would not be a good thing. The bulb is designed to run at 12v. When you slap it into a 6v system, it's drawing considerably more current which will burn out the filament.

  3. tophyr

    tophyr Forum Cripple

    I would double-check elsewhere first. Gixer_racer has it backwards - the 6v lamp is drawing twice the current of the 12v one, because they're both rated at the same wattage. Take a voltmeter and make sure your lighting system is supplying only 6v to the light socket. I can't think of a non-far-fetched reason it wouldn't, but it doesn't hurt to check.

    That said, gixer_racer is right in that lamps are designed to work in the conditions specified. It probably was the fact that you supplied 6v to a 12v lamp that blew it out, despite the current draw being lower.
  4. Woops Tophyr's right. I had my V=IR backwards :) been looking at too many structural drawings the past two days and my brain is fried! (hmmm I better go back and check all my reply letters! lol)
  5. tophyr

    tophyr Forum Cripple

    no worries lol, i double-checked like three times to make sure i wasn't gonna come off as an r-tard calling you out :mfclap:
  6. well i use a voltmeter this weekend but one thing i did change was i swichted to snthytic 2 stroke oil, that cant be it can it?

    thanks for your help

  7. tophyr

    tophyr Forum Cripple

    LOL.. *bites tongue* :mfclap:

    No, that won't be it. However, on that note - it looks like the bike in your avatar is NOT a two-stroke.. why would you switch to two-stroke oil?

    EDIT: Nevermind lol, because a DT400 is a two-stroke. Duh.

    Double edit: Something to be aware of when you install yet another bulb - if you handle the actual bulb itself (not the housing) then when you install it, wear a pair of fresh, clean latex or nitrile gloves. It doesn't sound like this happened to you, but sometimes if you get contaminants (such as oil from your fingers) onto the surface of the actual bulb, then the extreme heat of the bulb can cause the oils to burn and break the thin glass.
  8. that sv is in california.. not the bike in question

    the light is in closed so im not actually touching the lamp, but thanks for the heads up
  9. you BETTER be looking at drawings! I'm getting one of your boats this summer......Cape disappointment is getting theirs for 3 months to break it. then I'm getting it.

    sorry to threadjack:nana
  10. hehe First boat goes in the water in two weeks!!! :)

    I'll have to hide a note in the second one for you! hehehe:secret:
  11. A 60-watt 12V bulb supplied with 6V will only draw 15 watts of power. Halving the voltage will also halve the current, so the total load will be 1/4. The bulb will work just fine, but it sure won't glow very brightly. Find the proper specs for your application... that's the best thing you can do. 8)
  12. james1300

    james1300 Track School Dazed

    Ok, if the batterys toast, the bike will still run the lights. Thats how the old enduro's
    worked. However, the battery is used to help with controling the charging system.
    I had a DT250, and DT 175. BOTH did the same thing. You will first pop the H/L then tail light bulb then insterment bulbs. Sorry about the spelling. New battery, and use CORRECT bulb's.
  13. great, thats the answer is was looking for.... battery and a new light it is then
  14. whats the best way to test to see if the battery is dead, when the bike not running and the ignition on, the signals work perfectly and as bright as they should be.

    But alluding to james1300's theory, my rear light was burnt out as well (the rear indicator not the brake light). and the first headlight did burn out before i stored the bike for the winter.

    Would i just use a voltmeter on the battery terminals? Again i should make clear that i dont have too much knowledge about electrical systems in general.
  15. tophyr

    tophyr Forum Cripple

    Hook your meter up to the terminals and check voltage - it should be at or near 6V. If it's below that, replace it. However, there're still ways it could be bad even if it shows 6V - I'm not sure if your standard AutoZone or w/e can test a 6V battery, but that's what I'd do.
  16. A fully charged, fresh 6V battery should read ~6.40v after resting several hours. Test the battery for voltage stability by placing it (off-bike) on a charger until "fully charged." Disconnect the battery from the charger, and leave it on the shelf for more than four hours. Note the voltage across the terminals. Test again 24 hours later; the battery should not have lost any voltage.

    If the battery lost any significant voltage over the 24 hour period (more than say .02v), keep monitoring; if it continues to discharge on its own, replace it. The approximate "age" of your battery will be its final resting voltage. Anything over 6v may still be used, although I'd start considering a new battery at anything less than 6.20v retained.
  17. ok that makes sense. jsut took it battery to autozone and it was at 6.1, the guy there is trickle charging it to 6.4. il gonna pick it up after may classes for the day
  18. pull your stator/ flywheel cover and check for water. I had a dt 250 and a old 400 both would get voltage problems when there was water in the stator cover.

    the dt250 was just from washing it, it was my street bike at the time . where the wires come out of the stator water would run down the wires. same thing with the 400 but it was from mud puddles then washing.
    not much water needed to screw things up.
  19. A few notes on your bike.

    The 1975 DT400 (I have 3) does NOT come with a voltage regulator. The Battery must be in good shape, as it acts as a voltage sink for the system. The battery, taillight, brake light and blinkers run off of the battery which is on a DC system, while the headlight and indicator bulbs in the instruments run on a separate AC circuit which is unregulated.

    Yamaha published a dealer notice for a "fix" by implementing a voltage regulator to all of the early enduros, and simply added a voltage regulator from the factory to all of the 400's starting in 1976.

    If your battery is not good, and you don't have a regulator, you will blow either a 6 volt or a 12 volt bulb.

    You can adapt an aftermarket one to the bike as the originals are becoming unobtainium.

    You can get a lot of information on the Yamaha Enduros at I am a mod there, and there are a number of good restoration threads there.
Similar Threads Forum Date
2009-2011 Yamaha R1 Headlight Assemble & Front Fairing Gear, Parts, & Accessories Sep 22, 2016
2009 Yamaha R1 Headlights, Front Fairing & Zero Gravity Windscreen Gear, Parts, & Accessories Sep 17, 2016
Headlight Cross compatibility Mechanical & Technical Jun 8, 2015
99-02 R6 Headlights and Speedo went out Mechanical & Technical Apr 16, 2015
Question on "angel eye" Headlight Mods. Law Enforcement Mar 13, 2015