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D7 twin switch wiring, what goes where, and how to modify to give you battery fed headlights
Exeter, Devon.
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February 28, 2019 - 12:38 pm
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The twin switch D7 wiring has always been difficult to understand. More so as it switches from DC to AC depending on switch positions. So, as I've been looking at a D7 twin switch model that just wouldn't run well, I had to work out a) what goes on in different switch positions, and b) what I could do to modify the circuit to simplify it and make it more reliable. The colours are not the wiring colours; I've used them to identify if DC or AC current is, or should be, in the wire.

First I looked at the two switches standard wiring circuit and looked at each situation starting with both switches in the 'off' position:-

1 . both switches OFF


The horn and the brake light should work if the battery is charged up.


2. Ign ON Lights OFF


Now the engine should run as DC from the battery goes to the coil (blue colour), plus the brake light and horn work via the battery, which should be being charged as well (green to rectifier).


3. Emergency ON Lights OFF


Now the ignition is being run on AC volts direct from the generator (yellow) AND the battery is being charged from generator separately (green to rectifier). Brake and horn will only work as the battery charges or from the battery charge circuit.


4. Ign ON Lights LOW


Battery feeds ignition, and also side, speedo, rear lights as well as brake and horn. Battery charging from rectifier as before.


5. Ign ON Lights HIGH


Now all lights (side, head, speedo, rear) are being fed by AC coils (yellow) and NOT the battery. The battery (DC, blue -ve and red +ve) only feeds the horn and brake light and ignition. The battery is still being charged by the rectifier circuit (green).


6. Modified Circuit


This relies on the battery being charged, and uses all the generator coils to provide a charging circuit. The wiring colours reflect what is actually on the guinea-pig machine now, so sorry if it's a bit confusing. It needs the wiring in the generator to be slightly modded (nothing too drastic) and all the lights are now run from the battery/battery charging circuit using a rectifier/regulator unit to convert the AC generator output to DC. It can be 6v or 12v as the output from the generator (if in good condition) is around 18v. If you choose 12v you will need to change the lamps, coil and battery; the horn just gets a bit frenetic. At least the lights stay bright (-ish on 6v) at tickover.

The headlamp mounted ignition switch is unemployed so we used a new switch (with key) in the battery box front and put in a fuse at the same time. If the fuse blows the bike stops, ditto if the battery is flat, so a dedicated charge line (not shown) from the -ve provides a point for the trickle charger to clip to, saving having to take the seat off etc. while the bike is in the garage. I always trickle charge my bikes all the time with Optimate-type chargers.

I thought about modifying the (new) harness on the bike but actually it's easier to start afresh with the wiring to and from switches to battery area; small spade female connectors with heatshrink will fit onto the switch blades so you don't need to muller the switch sockets.

Good luck!

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March 1, 2019 - 11:24 am
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Looking at figure 6 you are only getting output from two coils. The Dark Red cable needs to be joined with the Orange (not White) output from the alternator to get a full flow alternator.

Once this happens there will be no ac current entering the circuit so, although redundant, there is no need to remove any wires, just link the dark red to the brown in the lighting switch, as you did, and isolate (put a bullet connector on the end of) the redundant dark red cable from the ignition switch.

That way the old loom and ignition switch remains in use with minimal modification.

The D7 diagram, though complicated, is so much simler than the later D14 diagram! My only reservation is that I can't work out what the purpose of the maroon cable from the ignition switch was.

Chatham Kent
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March 1, 2019 - 5:13 pm
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Hi Peter.
      I can answer the question about the maroon wire from the ignition switch. It connects to the ignition coil (points terminal). With the ignition switch in the "E" position the lighting coil dark red wire is connected via the ignition switch to this maroon wire, this effectively shorts the coil when the points are closed and allows a large current to be generated in the lighting coil. Then when the points open this creates a high voltage spike due to the collapsing magnetic field in the lighting coil that is transferred to the ignition coil.
In the "E" position the side of the ignition coil that is normally connected to battery -ve via the ignition switch in connected to earth.

Hope that makes sense.


Exeter, Devon.
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March 1, 2019 - 6:03 pm
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Thanks Peter. I have played around with different combinations of coils being attached and found the largest output - the problem being is that I've forgotten and didn't write it down. I'm always uncertain exactly which coils are acting in harmony and which are opposing as the rotor revolves but in the circuit I have shown I got around 18v AC on the meter so I assumed that would be the right way.

The owner had a separate lockable ignition switch so it was easier to simplify the wiring for him so that should he get a problem he could see what went where. I'm not absolutely certain the (newly-purchased-and-fitted-before-it-came-to-me) loom's colours matched the diagrams to add a frisson of difficulty to it all. Such fun....

mike p5xbx
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March 2, 2019 - 12:13 am
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this is the easiest way to test that the third coil is in phase with the other two
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