[Gear Head]

So reading this thread over at Scooby mods:
http://www.scoobymods.com/showthread.php/ultimate-aftermarket-fog-light-oem-5784.html

I’m installing some Hella 500’s this weekend and I want some confirmation as to if I can use a NEGATIVE trigger from the foglight switch to run the coil on my relay.

86 - +12v fused
85 – (-) trigger from Fog switch and Alarm Aux 1 channel
30 – +12V fused 30A
87 – to Hella 500’s

I'm looking at Pin 1 as a possibilty, i dont want to run another wire through the firewall and i like dealing with neg. triggers as if they rub on anything metal it doesnt matter...

thanks!

 

[ryanlarue]

Absolutely! You can use a ground to switch a relay.

 

[vicali]

Assuming your 500s are fogs and not driving lights that would work. :biggrin:
If you are not putting in the hella switch and just using the stock one you don't need to go through the firewall at all, just grab the 12v and ground from the fog lights themselves to switch your relay..

 

[bbottomley]

Tapping onto a switch that is already controlling something else can lead to unexpected sneak paths which produce unexpected results. I'd endorse vicali's suggestion above.

 

[Gear Head]

Absolutely! You can use a ground to switch a relay.

ha yes i can! i like them more than positives when running a distance.. question is, which pin on the switch will be the one i want or the one where a ground becomes active when the switch is in the ON position.

Assuming your 500s are fogs and not driving lights that would work. :biggrin:
If you are not putting in the hella switch and just using the stock one you don't need to go through the firewall at all, just grab the 12v and ground from the fog lights themselves to switch your relay..

Well grabbing the +12V isn't what i want to do... i want a negative trigger wire from either the switch, the relay or something that connects to ground when the fogs are on

thanks!

 

[Gear Head]

Tapping onto a switch that is already controlling something else can lead to unexpected sneak paths which produce unexpected results. I'd endorse vicali's suggestion above.

I would be diode isolating each individual wire as well to keep that from happening and back-feeding anything

 

[vicali]

As always, Peaty knows.

 

[bbottomley]

I would be diode isolating each individual wire as well to keep that from happening and back-feeding anything

That's how I'd go too, but first I'd look for a solution that didn't require this, such as the one that vicali suggested. Everything is right up front in his approach. No firewall involved. Nice short leads.

 

[Gear Head]

thanks guys

 

[ryanlarue]

Assuming your 500s are fogs and not driving lights that would work. :biggrin:
If you are not putting in the hella switch and just using the stock one you don't need to go through the firewall at all, just grab the 12v and ground from the fog lights themselves to switch your relay..

I did this when I converted my headlights to HIDs. Cut the old harness plug, wired in a relay harness, +12v from battery, boom done, still use the stock headlight relays, fuses and switches.

So you need to use a negative trigger because you want to switch the lights on with your alarm auxiliary, right? So I am guessing that its only a 200ma output. Definitely wire in a diode, I'd hate to see your alarm system get damaged from a backfeed!