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Motorcycle Repair/1975 yamaha dt400 question- minor electrical


Hi, just purchased this nicely kept Yamaha. The blinkers all work fine, but the headlight-tail light-and brake light do not work.  The headlight bulb filaments look ok.  Is there a common point wire junction or fuse that you know of that might be the cause of this, and where can it be found?  I really don't know where they put these things on motorcycles... thanks.


Hi Martin,

The DT400 uses an AC voltage on the headlight system.
This power comes from the AC magneto on a yellow wire
and goes to the headlight switch. From there it goes to
the dimmer switch.

The ohms on the lighting coil in the magneto is .020 tested between ground and
the yellow wire (disconnected)

The yellow wire should have 6-7 volts reading at 2500 rpm.

There is also a small regulator wired in I believe that helps
to balance the voltage. It would have a black and a yellow/white wire
on it.

I would just check for voltage and then follow the path of the power
with a multimeter.

If the yellow has no voltage then check for ohms as the lighting coil
could be faulty.

Sometimes the switches just get dirty or a wire breaks somewhere.

Let me know if you need more help after checking the above things.

Wayne S

Motorcycle Repair

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Wayne S.


Licensed Motorcycle Mechanic, Knowledge of motorcycles from 1960 up, Japanese, British and most other brand motorcycle repairs.


Worked for Yamaha, Honda, Suzuki, Kawasaki British: Triumph, Norton, BSA Other: Most Scooters and Mo-Peds

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