'77 Motobecane 40T 6V lighting issue

For the life of me, I can't figure out what might cause this problem-

With the headlight turned off, my brake light will shine bright.

With the headlight is on, both headlight and tail light shine bright. Once I engage either brake, all lights go out.

* When I remove the tail light bulb with the headlight turned on, I can hit the brakes and the headlight stays bright.

My father thinks it may be some kind of short in the headlight switch, but I don't really know how to verify that. All wiring and connections look really decent; No bare wires that I can see. I believe there is one bulb burnt out within the speedo, but I don't know if that would lead to such problems.

Any ideas would be appreciated!

-Jimmy Lee

Re: '77 Motobecane 40T 6V lighting issue

For starters, check the tail lamp bulb. It should be a type #1154 - 6V lamp.

Re: '77 Motobecane 40T 6V lighting issue

I apologize, I should have mentioned in the first email that I had bought a new bulb and the same occurred. Interestingly, I accidentally bought an 1157 bulb and thought "What the hell?" and tried it in place of the original 1154; This was a few weeks ago, but from what I recall, the tail lamp lit up and the brake light came on but was extremely dim. Afterwards, I thought it must have been the original bulb, but was disappointed to find out a new 1154 behaved the same way as before. Also, another little oddity I noticed while troubleshooting was that both the tail light and head light would shine brighter while turning the kill switch to stop the engine. I wasn't sure if that was normal. - Jimmy

Re: '77 Motobecane 40T 6V lighting issue

That's quite the set of odd symptoms.. but with the design of the 6V Motobecane system, a lot of strange things can happen.

I don't think it's a short.. more like an open or high-resistance connection somewhere, possibly inside the headlight switch. The switch is easy to clean - remove it from the top of the light, and spray some contact cleaner into the switch (holes in the bottom allow this) while working the switch back and forth.

I'd do a general inspection of all the wires and connectors. Pull each connector apart (careful!) give it a shot of cleaner, and put it back. If any of the connectors look bad (evidence of heating or major corrosion) replace them as a set.

There is a wiring diagram in the Motobecane manual on mopedriders.org, but I'm not sure how useful you will find it.. it leaves much to be desired.

Re: '77 Motobecane 40T 6V lighting issue

Oh, BTW - I have a nearly complete 40T in parts. Let me know if you need anything, and I'll see what I can do.

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