DIY: Rear Fog Light Wire Via BCM and Stock Wiring

edconline

Ok, that makes sense, tested it last night and I would get a reading on the multimeter for an instant when it came on then nothing again. I'll try to find a resistor that does the trick. Will using a resistor dim the light at all?

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ChrisSlicks

No it shouldn't dim the light. You will be adding it in "parallel" to the existing circuit and not inline. If you know the current draw of the Evo light and the current draw of the stock fog then you can calculate the extra draw required using V=I*R or in this case R=V/I. The wattage (power handling capacity of the resistor) will need to exceed P=V*I where V=12V and I is the calculated current draw of the resistor.

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jgvill

Sorry but I'm just clueless when it comes to electronics. Can an ON-OFF switch be used so that in the ON position the Fog light turns on w/ the headlights and in OFF it's off even when the headlights are on? Can you show how
Thanks!

spearfish25

An ON-OFF switch can be used, but it will be annoying if you wire it the way I describe (controlled AT the BCM instead of AFTER the BCM). My DIY describes how to tell the BCM to turn the rear fog on/off so that you maintain the factory interaction between the headlights and rear fog. IE, the rear fog can only go ON when the headlights are on, and goes off if you turn the headlights OFF. The way the BCM reads the On/Off signal is momentary. Imagine you have two bare wires that make up the On/Off circuit instead of a switch. The BCM just looks for the two wire ends to briefly touch, and it turns the light ON. Another brief touch then turns it back off. This is what a momentary switch does. An On/OFF switch is like keeping the two wires touching when it's ON and having the two wires separated when it's OFF. However, once the BCM gets its momentary 'touch' input, it doesn't care if the wires are still touching or not. What this means for an On-Off switch is as follows. Assume the headlights are ON. If the fog is off and you switch your ON-OFF switch to ON, the rear fog will go ON. If you then switch the On-Off switch back to OFF, the rear fog will stay ON (the switch doesn't give another 'touch' when going to off...it's just like you touched the two wire ends together and delayed while separating them again). You'd then have to switch the On-OFF switch to ON again to get the rear fog turned OFF. Thus, using an On-Off switch instead of a momentary would mean you have to give it two switch throws every time you want the rear fog to go either On or Off.

Short answer....find a momentary switch and follow my DIY. Option 2 is to wire an On-Off switch from a 12V power source to the orange fog light power wire. You'll lose the headlight dependence, for better or worse.

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ChrisSlicks

It is possible but not with a simple passive circuit. You would have to use a NAND gate IC to sort out all the logic combinations and get feedback from the BCM fog light output. As already explained the fog light circuit uses a momentary toggle signal where as the headlights use a on/off signal.

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jgvill

I guess what I want (fog turns on w/ headhlight until switched off) is too complicated via the BCM. I'll just find a lighted rocker MOM switch and follow your DIY. Thanks guys!

spearfish25

Bump given this has become a topic of discussion once again.

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gurneyeagle

Incredible write-up. Thanks!

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nmjaxx9

Yeah good write up, this makes me think of doing mine now.

LukasC

Ok so i got the european fog light with the harness switch from the zstore. Is there a way that i use this setup to where when i turn the light on it will work with the brakes but i can also turn it off and it wont turn on when the brakes do or it can be solid on?

onzedge

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pedZ

Thank you Spearfish for the great DIY. I installed the switch for my rear fog yesterday. I used the following momentary switch with built-in LED for this project.

Amazon.com: Oracle Lighting MSGQ16FR Red LED Momentary Flush Mount Switch: Automotive

Here are some pics of the install.

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spearfish25

^ Looks great!

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onzedge

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happytheman

Perez, that looks fantastic!

I would love to be able to hook the Evo-r fog to flash on reverse AND flash on brake. Is that do-able?

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