Fog light, older vehicle, does it get its power from dipped beam?

It must do, right?

Ok, so , if it gets its power from the head lamp dipped beam on the light switch, how does it also need ignition on to operate? Dipped beam doesnt need ignition on. It has to take power from dipped beam though because when full beam is switched on the fog lamp goes out.

Got a wiring diagram?

J

Not all things are switched by the live supply, some are switched by their earth.

marjon said:

Got a wiring diagram?

J

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Nope.

lynall said:

Not all things are switched by the live supply, some are switched by their earth.

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I dont think this is, fog is earthed by the light unit.

It’s magic…
It’s all done on the live side. If high beam is on fogs are turned off…
I thought it was relays but never found them. I re wired mine so fogs can be on at any time(without lights on)

Its a strange set up for sure, fuse to switch, then splits to rear fog lamp - earth and to dash lamp - earth. it draws power from the light switch, from the dash lamp earth to start relay (ignition) which mine doesnt have so it has to earth on the ignition. I still cant figure out why the circuit doesnt earth through the rear lamp thus not earthing through the ignition but there we go.

Turns out it had multiple issues, a faulty wire on the fuse was sending me the wrong signals as sometimes it was live and other times it wasnt so I had to go through the lot testing the circuit everywhere.

nobber said:

It must do, right?

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Mine is a 96 plate and the ignition has to be on and dipped beam before fog light comes on. I've got a wiring diagram at home. I can post later when I'm back if you're interested.

Yes and no! as usual. Red and yellow from the fog lamp to the fog lamp switch. Blue and purple from the switch to the fuse box (it should have it's own fuse). The power to that fuse does come from the lighting switch. It looks like it's spurred off at the back of the fuse box - fed by a blue and red wire. So when you have the dipped beam on, that blue and red feeds the dipped headlights and the fog switch, via it's own fuse.

zeaphod said:

Yes and no! as usual. Red and yellow from the fog lamp to the fog lamp switch. Blue and purple from the switch to the fuse box (it should have it's own fuse). The power to that fuse does come from the lighting switch. It looks like it's spurred off at the back of the fuse box - fed by a blue and red wire. So when you have the dipped beam on, that blue and red feeds the dipped headlights and the fog switch, via it's own fuse.

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Dipped has two fuses for left and right, so it must draw from dipped before the fuses, then into the fog fuse. Its a barmy set up, way too much going on on the wrong side of the fuse.

nobber said:

Dipped has two fuses for left and right, so it must draw from dipped before the fuses, then into the fog fuse. Its a barmy set up, way too much going on on the wrong side of the fuse.

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That's exactly what's on the wiring diagram - as the wiring was "developed" from the SIII wiring, and that in turn from the older Series models, it could be the designer was trying to reduce the amount of wiring used due to post war shortages!