Hello! I'm trying to get my DT125FC resurrected from the dead (it's life as a farm bike) but I can't get any power to the accessories! So far it runs and drives, which is AWESOME! But no headlight, horn, flashers, tail lights or dash/indicator lights!

I've got a Clymer manual and I've been tracing wires around the bike, repairing anything I see that's broken (the bike had a small fire and melted a few wires). From what I can see, it has a secondary coil that powers the lights and charges the battery.

1. How do I test that coil to make sure it's actually generating power? The battery seems to be charging, at least, because when I start it, voltage jumps up and rises with RPMs.

2. There's a bunch of wires that come out of the magneto. They all are going to things except 1.
- Blue wire - goes to the neutral light on the dash and to ground
- Green/red wire - goes to the rectifier
- Yellow wire - goes to the headlight switch on the bars
- Black wire - goes to the ignition coil and the ignition switch (closed circuit with the flasher relay when key is OFF)
- Green wire - not connected What?!

I'm concerned why this wire doesn't go anywhere. I goes to a 6-pin plug at the back of the bike, but no wire joins it on the other side of the plug?

I'm very inexperienced with electrical, so any assistance is appreciated.

There are two circuits... some stuff runs off the battery (DC) and other stuff runs off the magneto and only works with engine running (AC)



Much of what isn't working runs off the battery. There should be a red wire off the battery, a fuse, and the red goes to the key switch. With key on, battery power (red) is connected to brown coming out of the switch. Brown runs the horn, turn signals, neutral light, and maybe the taillight.

Headlight and dash illumination lights... maybe taillight... run right off the magneto.

With regards to your wires out of the magneto:

- Blue wire - this should be a very light blue wire. Goes to neutral light in gauge, other side of light should be brown (power). When in neutral, switch grounds blue wire and bulb lights.
- Green/red wire - should go to headlight switch and then to rectifier if light switch is on. (Nighttime battery charging)
- Yellow wire - goes to the headlight switch on the bars. Correct. Comes out of headlight switch and runs gauge illumination lights, headlight, and maybe taillight.
- Black wire - goes to the ignition coil and the ignition switch (closed circuit with the flasher relay when key is OFF) I'm not sure what you mean by this??? Black wire simply grounds with key off... has nothing to do with flasher.
- Green wire - not connected What?! Should go to headlight switch and then to rectifier when headlight switch is off. (daytime charging)

So investigate the battery circuit... do you have 6VDC into key at red wire and then 6VDC at brown wire with key on?

On the Headlight, AC circuit it sounds like maybe someone has at least partially bypassed the headlight switch? Or maybe replaced it with something off another bike? Pictures would help... and is your green/red connected directly to the rectifier or could it be wired through the headlight switch? Again, pictures would help.

Two more notes... a lot of literature is for the USA models and all the twinshock US models had the 12v starter-generator (probably would have been called a DT125EC in your country?) Your bike will be wired more like the USA DT175. Second note is that I keep saying "maybe taillight" because in the USA in about 1970 the law required the taillight to stay on with engine off... earlier models and models in other countries powered the taillight off the magneto with the headlight.

Wow, Mark! Thanks for taking the time to post such a detailed reply!

There's no fuse in line with the battery. It goes like this:

Battery -> Red -> Connector -> Red -> Ignition -> Light Blue -> Connector -> Brown -> Front + Rear Brake Switch, Tachometer, Horn

Figure 1


True! My bike goes like this:

Magneto -> Yellow -> Plug -> Yellow/White -> High/Low Headlight Switch (On the bars) Though there's also a secondary Yellow/white wire that isn't connected to anything.

Figure 2


Here you can see how the yellow magneto wire turns into two Yellow/white wires at that plug, one of which does go to the Hi/Low Headlight Switch, while the other is disconnected.



I was confused a bit because the flasher is grounded there too, but it's more like they're both grounded when the ignition is OFF. Why would the bike be grounded only when the ignition is OFF? Does it need to be grounded when it's ON?

Correct! I've tested this and the 6V power is present at the red wire and at the brown wire. I tested further into the harness for power and found 6V power on the brown wire before the horn switch, the brown wire between the switch and the horn, and on the pink wire after the horn. Seems like the horn is broken? Wouldn't it just be constantly BEEPing as soon as you start the bike up?

It changes to a white wire after the plug and goes to the rectifier. (see Figure 2)

So I am concerned why this green wire from the magneto goes to nothing in that plug?

What should that wire do? As you can see there's a missing pin at the magneto plug so the green isn't carried any further.



Also: regarding the concept of daytime vs nighttime circuits, I wasn't sure what this referred to, so I looked up and found a text book that clarified the concept.

(See here if anyone in the future cares: TEXTBOOK

How would the bike switch between these two circuits? I don't have anything I can see other than the high beam light switch that would up the power to run more lighting in the night.

Ok, so I've got some issues solved.

1. I found a ground that wasn't grounding properly in the headlight harness.
2. I found that every bulb on the bike was blown.
3. The horn switch doesn't seem to be working. (There's 6V power before it, after it, in the horn and after the horn, but no horn sound.)

I've also ordered a voltage regulator to protect the electrical system as I switch over to LEDs.

Re the horn, there is possibly a small adjusting screw/bolt on it (maybe with a locking not) and, if there is, screw this either in or out to see if it makes any difference.

Failing that, if the two parts of the horn are held together by bolts or screws, then you can take it apart and clean a) any electrical connections and b) the two surfaces of the points (this item is spring loaded - push them apart and clean the mating surfaces with emery paper or suchlike). The adjusting screw might need to be used after re-assembly.

If you are going to take it apart, remember to take photos or make notes of how you disassembled it, so that the parts go back in the right order/place!

Horn button on most Yamaha's connect one terminal (pink wire) of the horn to earth when button is pressed... other terminal (brown wire) has power with the key on.

So I find it strange that you have power in to the horn button and out of the horn button?

If so, the other terminal on the horn would need to be connected to a good ground.