fog light

Hello everyone! I often visit here to get help with stereo installation and most of the write ups have been very helpful but now I'm venturing into unknown territory. I'm trying to figure out a way to install a set of fog lights but keeping it legal by having the fog lights shut off when the high beams come on. I have read that by using a relay and a low beam wire to energize the coil, I should be able to control the fog lights as intended. However, my low beams stay on even when I turn on my high beams. I can't use them to control the fog lights because I can't shut off the relay since my low beams are always on. Is there any way I can connect the fog lights and have them shut off when I turn on the high beams? Thanks in advance for your help!

It says I need to post info on my car. Not sure how relevant it is, but it's a 1997 BMW 318i.

You should be able to use the high beams to energize a NC relay that will open up the fog lights when the high beams are on. This relay would be an additional relay to the relay that is used to actually turn the fog lights on.Kevin Pierson

Or ground the fog relay to the high-beam +ve. (If halogen etc, NOT HID.) Hi-beams off means its a short to ground (through the filament/s). When hi is on, it's +12V to no foglight relay coil grounding. That methods suitability and alternatives (including HIDs) may vary with application.

I'm working off of this diagram. Oldspark mentioned grounding the relay to the positive high beam wire. Does that mean connecting pin 86 to the positive high beam wire instead of leading it to a ground? How does that cause the fogs to shut off? (Feel free to get a little technical on me, I'd like to understand the inner workings of relays.) http://www.classictruckshop.com/clubs/earlyburbs/projects/bosch/foglite.jpg

Alternatively, is there anything I can do with pin 87a?

jay.low wrote: Does that mean connecting pin 86 to the positive high beam wire instead of leading it to a ground?

Based on THAT diagram, yes. But based on convention (important if using internal-diode spike suppressing relays), they have interchanged 85 & 86. Conventionally (or normally), 86 is the coil's +ve & 85 is its negative (GND). Not that I ever knew those pin numbers until some years AFTER starting on the12volt.com - I would always wire according to the schematic typically included on Bosch/Hella type relays. For a bit of optional extra explanation... But yes, it is THAT Ground that can instead go to whatever is +ve when the beam or high-beam is on (whichever you want to disable the fog/driving light relay) noting that that connection must be to a low impedance (ie, resistance) load - eg, the beam or hibeam bulb(s) themselves. If connected to the switched side of the beam or hibeam relay COIL, the fogs relay may not turn off (de-energise) and it might even turn on the beam/hibeam that the "grounding" relay controls. If you want to know why the tech behind NOT to use the other beam's trigger (or relay coil) circuit... Voltage Dividers: The reason for NOT using the beam/hibeam's COIL switched +12V is that both coils are high impedance/resistance. That's the tech way of saying that they are probably close to being the same resistance, hence forming a voltage divider. A voltage divider is merely the center point of 2 joined SERIES resistors (see Wiki etc, but ignore Wiki's first bit using "Z" - go to Resistive divider) where Vout = Vin times (R2)/(R1 + R2) where R1 is the "top" resistor (to V input, eg, +12V) and R2 is the bottom resistor to GND. In my suggestion, R1 is your fog light's relay coil. R2 is what you connect the coil's ground to. Example of Voltage Divider in THIS situation: If R2 is small (well under 2 Ohm for a bulb) and the relay's coil R2 is large (say 60 to 300 Ohms etc) and Vin = 12V, then Vout = at least 60/(60 + 2)Ohms x 12V = 60/62 x 12 (= .977 x 12) = 11.6V which is fine for a 12V relay. (They typically pull in at 7V or above, and let go below ~4V etc.) In short, the grounding resistance is negligible compared to the "load" (ie, 2 compared to 60 Ohms = 1/30th) so it has negligible effect on the circuit (ie, the load has close to full voltage across it). And when that "grounding load" gets 12V, then the "ground" of the relay coil raises to 12V, hence 0V across its coil so it de-energises and turns off. If the grounding load (R2) is high relative to the fog's relay coil R1, then Vout aka the -ve side of the fog's relay coil is nowhere near GND. EG, if R1 = R2 (same fog and beam/hibeam coil resistances), then Vout = Vin x R2/(R1+R2) = Vin x R2/2R2 = Vin x 1/2. eg, 12V x 60/(60+60) = 12 x 60/120 = 12 x 1/2 = 6V. That "center point" 6V may not be enough to GND the fog relay (ie, it only has 6V across it) hence it won't turn on, and it may be enough to turn on the beam/hibeam relay - ie, you turn your fogs on, but they stay off and the others beam might come on. There are other issues like 12V relays with only 6V (or 8V etc) may "bounce" off with bumpy or vibration conditions. But you can play with different resistances (coil resistances) and see the impact. My simple method is the ratio of the resistors - eg, 2:60 is negligible (but not 60:2!), and if close - say 1:4, then the voltage divider ratio is 4:5 (80%), or if 2:5 = 5/(2+5) = 5:7 (71%) etc. FYI - I never remember the voltage divider formula though I know it's "R divided by the sum of the R's" (eg, R1 or R2 divided by (R1+R2). But the current through series connected resistors is the same (it has to be, else where does the current go?), and with Ohm's Law V=IR, since currents I are equal, then the Voltage V across a resistor is proportional to its resistance. So the bigger the resistance, the bigger the voltage drop across it. The above may sound complex, but it's a mathematical application of simplification of Ohm's Law with 2 bits of circuit theory (ie, current through any series path is equal everywhere, and that SUM of the voltages across each component/resistor equals the total voltage - ie, the supply voltage. [Is that Kirchoff's Law or... Oh, yes, BOTH are Kirchoff, but DON'T read Wiki's pages - see something easier!!]) But if non mathematical (or unfortunate enough to read complex explanations like Wiki, or mine LOL!), then a few calculations on paper will probably enable you to see or understand the above in your own way.     Incidentally, don't blame me....

jay.low wrote: (Feel free to get a little technical on me, I'd like to understand the inner workings of relays.)

Ok, so I don't know what "a little technical means... And that's more than relay stuff, but it's something that applies to ANY "load". Otherwise relay can "simply" be described as a "digital" (meaning on-off) amplifier -cum- switch. In other words, a small current (the coil's; 86 & 86) can turn on a bigger current (the contacts; 30 to 87, or 87a). How? It's an electro-magnet (coil, aka solenoid) that "pulls in" heavy contacts. Or "other contacts" for more complex relays (DPDT, 9PDT etc). And relays usually provide isolation - eg, a 12V signal or coil to turn on 24V or 110VAC or 240VAC contacts or loads. TIP (albeit too late down here LOL!) - DO NOT try to absorb all the above in one go. Just a bit at a time for what YOU want. If possible, try to get the overall picture or remember some of the gobbledygook crap I mentioned in case it comes up in the future. But re-read, and look up alternative sources (google!). For me, it took years going over that airy-fairy crap. But once something clicked, it made the next stage possible. (Or rather, as some things clicked, the other things suddenly made sense.) Another short topic-101 brought to you masochistically by courtesy of you know who. PS - nothing you can do with the fog relay's NC contact. (Only the others' NC if available might be useable.

Wow! Thanks oldspark! That was very thorough.