Tech Article Title Author Date
Control Fog Lights Independently from Headlights and Rear Foglight James Wu 1999

Disclaimer: North American law requires that any accessory lighting (i.e. fog lights) on a vehicle be only operational when the vehicle's low beams are on. The factory fog light wiring for the Audi A4 sold in North America conforms to this law. By performing the following modification, you are bypassing the factory (lawful) wiring which prevents the use of the fog lights except when only the low beams are on. Please note that it is now up to you to obey this lighting law, and only use any non-standard lighting patterns now available when off of public roads. Also note that the electrical system on any modern automobile is very complicated. Please be sure you know what you're doing before performing any modifications to the electrical system. This rewiring has proven to work on the author's own vehicle. However, please note that the author will not be responsible for any tickets, fines, damages, or personal injuries which occur before, during or after this modification.

Make sure you have read the entire instruction and understand every step before performing this procedure. And yes, we will need to cut some wires. Are we ok with that?

The results of this modification will be:

  • The ability to turn on your fog lights anytime you have the iginition on
  • Front fogs will not automatically come on when you turn on the rear fog lights
  • All fog lights will automatically shut off when you turn off the ignition, even if you leave the switches in the on position.

Tools and materials you will need:
  • 16 gauge automotive electrical wire
  • 1 In-line fuse holder (capable of 15A)
  • 1 15A fuse
  • 1 ring connector (to connect to the power post)
  • 2 butt connectors (to connect the wires)
  • Wire cutter
  • Wire striper
  • Wire crimper (these three can often be found on the same tool)
  • Electrician's tape
  • A rachet with a 8mm and 10 mm socket
  • A flat head screw driver
  • 2 thin files (for removing the radio)
  • Your radio's anti-theft code (as we will be disconnecting the battery)

How to perform this procedure:

Disconnect the car's battery!! (using the 10mm socket)

Follow the FAQ on A4.org written by Dan Masi on how to remove the driver side knee bolster, and then follow a seperate FAQ also by Dan Masi on how to remove the radio (these two procedures is where we use the 8mm socket, the flat head screw driver, and the files). Slide the radio out, but there's no need to unplug it. We just want to be able to reach in where the radio sat and push up and out on the fog light switches to free them from the center console. To remove the fog light switches from the center console, stick your hand in where the radio was, find the back of the switch, and push up. This should free the top of the switch from the console. Now, hold on to the bottom of the switch in the back, and pull it straight out. It should pop out with a little force.

Take your inline fuse holder, and connect the ring connector to one end. Now, look under the dash where the knee bolster used to be. You should see four power posts in a row below the relays. There are three red posts, and a yellow post on the very left. remove the bolt on the yellow power post, and connect your ring connector to it, and replace the bolt.

Next, run a wire from the other end of the in-line fuse holder to the front fog light switch. What I did was used the two rings underneath the steering column that's holding in another thick wire, and lightly wrapped the new wire around it to get it to the back of the center console. From there, just get the wire up to the fog light switch.

On the back of the front fog light switch, there should be two thin wires (for the switch lighting), and four thick wires going into two slots of the harness one on top of another. So basically there are two wires going into the bottom slot, and two wires going into the slot just above it. The reason there are these two extra wires going to the front fog switch as compared to the rear fog switch is because these wires power the front fogs when the rear fogs are on, that's why the front fogs go on automatically when the rear fogs get turned on. If you like this feature, don't cut the following wires.

For those who want to prevent the front fogs from automatically coming on with the rear fogs, take the wire cutter, and cut the lower wire going to each slot; that means you cut two wires in total. Please note this is what I cut on my car and it worked. It may be that your vehicle is not exactly the same. If it turns out you should have cut the top wire instead, just reconnect the wires and cut the other wire. (you'll know if you've cut the right wire by turning on the rear fog lights, if the fronts still come on automatically, you cut the wrong wires).

Now, connect the wire you fed from the in-line fuse holder to the wire you've cut in the center slot of the front fog switch. Connect it to the end coming from the wiring harness of thw switch, not the other side. When you do this connection, connect an extra wire to this connector, and feed it to the rear fog light switch. Remember to feed the wire behind the center console, as you'll have to push the switches back in place when you're done.

After that, take the electrician tape and tape up all the ends of the wires you've cut. Take the wire you've fed from the front fog switch to the rear fog switch, and stick it in the center slot of the rear fog light switch (the same slot as the one you connected the power to in the front fog switch). If you prefer, you can cut that wire and connect it to the harness that way, but it's not neccessary, just make sure you've got a good connection there. Now, tape the wire in place so it won't slide out from the harness. (This is also what you would do at the front fog switch if you decided not to cut the wires there before).

To finish, place the 15A fuse in the in-line fuse holder, and reconnect the car's battery. Turn the ignition to the on position, and without turning any other lights on, try to turn on the fog lights. The fog lights should come on. Now, without turning off the fog lights, turn off the ignition. The fogs should shut off automatically. (because the yellow power post we connected to is only hot when the ignition's on. The red ones are on all the time. even when ignition's off). If everything checks out, then you can replace the fog light switches, the knee bolster, and the radio (punch in your anti-theft code to get out of the radio's "SAFE" mode). Reset the clock, (how's that for comprehensive instructions = ) and enjoy your total control over the vehicle's lighting.

An alternative to the in-line fuse:

When I did the modification, instead of using an in-line fuse holder, I actually cut into the fuse box and wired the original fog light fuse (slot 36) into the circuit. This way, I have a cleaner result, and I can replace the fog light fuse as if no mods have been done. If you feel comfortable with doing that as well, go ahead, just make sure you know what you're doing, and make sure you're cutting into the right fuse. Test it by taking the fuse out and trying the fog lights first. They should not work without the fuse in place.

If you want to use the original fuse like I did, you must first make sure you've found the right fuse (as described above). Now, you need to remove the fuse box. It's held in place by two 8mm bolts behind the left side cover of the dash. Remove these bolts and work the fuse box out. Once freed, unclip the back of the fuse box to reveal the wires inside. Locate the wires that correspond with the fog light fuse, and cut them. (You've got the car's battery disconnected right?) Connect one end of the fuse to the wire coming from the yellow power post (you'll need one extra butt connector here), and the other end of the fuse to the wire going to the fog light switch. It doesn't matter which end you connect to because it's basically serving as your in-line fuse. If there was any other circuits before, you've bypassed them by cutting at the fuse. Remember to tape up the open end of the wire you've cut, close the fuse box back cover, and bolt the fuse box back into place.

Another way to rewire the fog switches:

In many North European countries, the standard fog light wiring allows the fog lights to be operational with just the parking lights on. The reason for this is because in heavy fog, even the low beams are aimed too high and give off too much glare. By having just the fog lights and parking lights on, the car and the road is lit without as much glare. For a true "European" spec fog light wiring, you can check out Stan Jirman's instructions (with pictures) on his web site. Stan will show you how to wire your A4's fog lights to operate like they do in Northern Europe. His instructions will also show you how you can wire your front fog lights to come on automatically when you turn on the high beams; because when you turn on the high beams, it usually means you want as much light as possible, right?