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Discussion Starter #1
I need to add a headlight to my '55 640--still a 6v positive earth machine.

Looking for general understanding of positive earth wiring and some pointers:

-where to tap off the power so the light will not draw when ignition key is off
-where to mount a switch to turn light on/off

Thanks,

Jeff
 

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1981 Ford 1100 4WD
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6V positive ground is no different from 12 volt negative ground when it comes to wiring lights, with the exception the current is usually twice as much as a 12 volt system for the same wattage.

I don't know if the ignition switch can handle the current of the headlight. I single 55watt headlight will add 10 amps to the switch, that's a lot of extra current.

If you really want the light switch to have no power when the ignition is OFF then you might have to add a 6 volt relay into the circuit. This way, the relay gets its power through the ignition switch but the headlight gets its power through the relay contacts and straight from the negative terminal of the battery (thru a fuse of course).
 

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Here is a 6 volt system wiring diagram for your 640. To add lights run the wire from terminal block to a fuse then to a switch then to the light. The second diagram (although it's 12 volt diagram) will show you how the lights are run from the terminal block on. Do not use this drawing for anything else. You will also need 6 volt lights not 12 volt.

Kirk
 

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Discussion Starter #4
Thank you. I assumed that there probably had been a light on the machine when it was new but maybe not. There is evidence of an aftermarket light but I can't tell how it was wired.
 
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