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I have 1965 Crusader Sports, converted to 12v. I would like to wire headlight so that it can only work with ignition on. I am not great with the logic of wiring diagrams :?
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, so hoping someone with better electrics nous can help.

The bike is late model Sports, so has later Continental wiring, as per the attached diagram, though due to 12v conversion the third wire from alternator no longer goes to the headlight switch.
Ok. Someone else check this through first.

Two ways I can see. Number 1 would use a relay because the only switched live I can see supplys the coil and I would worry that drawing the lights directly from here might interfere with the spark whereas the relay would only have a very minimal current draw. Number 2 doesn't worry about that and assumes the switch terminals and wires are man enough to cope with the additional current (which if it's on 12v would be running at half their design spec anyway and the switches are pretty beefy as i remember).

If it were me, I'd probably try number 2 first and if it gave any ignition issues, revert to number 1. In fairness, both of these would avoid the battery power going through the lighting switch before it gets to the ignition anyway, removing a potential failure point from the original design.

The horn and stop lamp would still operate with the ignition off but the lights would only operate with the ignition on.

Both of these assume that the small black line that appears to join terminals 12a and 13 in the diagram is a printing error, which I think it is as other bridged terminals are more clearly marked.

Number 1

If you disconnect the wires from terminals 2 and 10 of the lighting switch and connect them directly together, that will supply power to the ignition switch at terminal 12A.

You could then use the output of terminal 13 which looks like a switched live to trigger a normally closed relay that connects battery feed to terminal 2 (or 10) on the lighting switch.

So if I was doing it, I'd get a standard 4-pin, normally open automotive relay (SPNO), non diode type to avoid positive earth issues.
I would disconnect the wires from 2 and 10 on the headlamp switch and connect them together.
I would attach terminal 13 on the ignition switch to pin 85 of the relay.
I'd attach pin 86 of the relay to earth.
I'd attach pin 30 of the relay to a live feed (say terminal 12a on the ignition switch).
I'd attach terminal 87 of the relay to terminal 2 (or 10) of the headlamp switch.

Number 2

If you disconnect the wires from terminals 2 and 10 of the lighting switch and connect them directly together, that will supply power to the ignition switch at terminal 12A.

Run a new wire from terminal 13 on the ignition switch to terminal 2 (or 10) on the lighting switch.
For what it's worth all the later 1960s British bike exports to the USA had to be wired for the lights only to come on with the ignition on. A wiring diagram which might help you would be for the 1969 BSA Starfire, this shows the alternative wiring you would need to achieve this, allowing for differences in switch gear, of course. You might prefer to follow the relay option as above.

Jamesw wrote:
Fri May 13, 2022 5:30 pm
Yes, thanks for the comments. I like the relay option, so will go for that.
Another refinement you could do is feed terminal 30 of the relay directly from the battery with an inline fuse, or use a fused relay. Then you have a seperate lighting fuse, so if the lighting pops the fuse, the rest of the bike will keep running.
Now trying to figure out the wiring on my bike. I did a 12v conversion a few years ago and since then have also fitted a new wiring loom. The 12v conversion meant combining the GY lead from the alternator with the GB one that goes to the recifier (now a Boyer Powerbox) and to terminal 7 on the light switch.

According to my logic (not a strong point) that means the GY lead to terminal 17 on the ignition switch is redundant (I have checked voltage on this lead and it is nil whether IGN is on or off, and whether engine is running or not.

Therefore, with the ignition OFF or ON, am I right to think that the black lead from terminal 18 on the IGN to terminal 5 on the light switch is also redundant?

Could this black lead be sued to connect terminal 87 of the new relay to the light switch, thru the harness instead of running a new lead?

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