Foglight electrical Question.

[CRSmith]

Hello all,

Tried to hook up some fog lights so they would come on at the same time as the stock fogs on my truck. I spliced into the wire that was hot when the stock fog lights were on to get the switched power for the relay that switches the aftermarket fogs. Well, come to find out, both wires are +12V when the stock fog light switch is OFF! Then when you turn the stock fog light switch on, one of the wires becomes a ground, the voltage differential is created, and the stock fog lights come on. The point is, without a switchable hot wire to hook to the relay, I can't run the aftermarket fogs from the stock foglight switch. Has anyone ever run into this? If so, did you find a wire to splice into that served the purpose? Lastly, what on earth are they thinking having both of these hot; why not just power the fogs the conventional way with a constant ground and switchable hot?

Thanks.

 

[Thomas]

You can use a relay with the switched ground. You need to provide 12 volts to one side of the relay trigger coil and use the switched ground to complete the trigger coil circuit.

 

[mikel]

The relay won't care what you switch. Use the switched ground to activate the relay if that's what is already there.



Most automotive relays will have the low-current "activate" pins that need grounding on one pin and +12v on the other pin. Just wire one pin to constant (fused) +12v and the other pin to your switched ground.



Once you do this you can use the other pins on the relay ( the "load") to control the lights. Provide +12v to one side and connect the other to the lights (you can use the same wire that powers the low-current side if you want, but you will have to use a bigger fuse). Ground the lights and when you throw the fog light switch the relay will close turning the lights on.



As to why the lights are wired the way they are, I'm sorry but I just don't know.



Hope this helps,

Mike

 

[CRSmith]

Thanks,

For some reason I thought the relay ground carried high current and I didn't want to draw that much from the stock fog light circuit. But the lights have their own ground I guess. So, I'll take your advice and put constant hot to the relay and switchable ground.

Good idea.