San Francisco is synonymous with fog, and for good reason: fog banks rolling into the Bay Area almost every morning in summer, providing beautiful views for tourists and serious problems for commuters. If one of the fog lights on your Audi TT goes out, it can seriously hamper your ability to see when these low-lying clouds roll in. How do you fix the fog lights on your car, and how can you make sure they're working at their best?
How do Fog Lights Work?Fog is low altitude cloud, and just like a cloud, the water droplets inside obscure your vision. When you have your headlights on, the light bounces off of these droplets, further reducing your vision. Fortunately, while fog may look like a solid wall, it's usually between 12 to 18 inches above the ground. This means light can pass underneath it without reflecting back. Fog lights are mounted low with specially-shaped lenses designed to only shine light at this specific height, giving you a wider view of the road. Keep in mind that while fog lights might help you see, they don't do much to make your car be seen. When driving through fog, you should always have your low beam headlights on. Using your high beam lights increases light reflection, limiting the visible area for you and other drivers in your immediate area.
Why are some Fog Lights Yellow, and My TT's Lights are White?There was a theory that the longer wavelength of yellow light would keep it from bouncing off of water droplets, helping it penetrate fog banks. However, the droplets are dense enough to reflect all light, regardless of color. Worse still, most aftermarket yellow lights are simply a yellow-tinted lens over a bulb. While this works fine for turn signals, it reduces the amount of light significantly, hampering performance. Audi's engineers know this, which is why they design these cars to use white fog lights.
How Do I Change the Fog Light Bulbs and Housings?The procedure depends on whether you have a first generation or second generation car.
1999-2007The fog lights are built into the headlight housings. To access the bulb, open the hood, then look at the back of the headlight for the connector closest to the center of the car. Twist the connector counter-clockwise and slide the bulb out of the housing. Lift the bulb retention clip and slide the bulb out of the connector. Reinstall in reverse order.
2008-2015To access the fog light, the headlight needs to be removed. To perform this repair, you'll need small flathead screwdriver and a Torx T30 screwdriver or a T30 bit with a ratchet and a long extension. Open the hood and look down at the rear of the headlight. The wiring harness for the headlight bulbs ends in a single plug that connects with the rest of the electrical system. Follow the cables from the headlight to this plug and disconnect it by using the screwdriver to push in the tab on the side of the connector. Next, remove the two bolts on the top of the headlight. Pop off the cover the goes over the top of the inner half of the light housing. Unscrew the three remaining bolts holding the headlight onto the vehicle. There are two under the cover that will stay in the headlight once they're loose and one just behind the outside corner of the lamp that is removable. Slide the headlight out of the front of the car and set it aside. If you look straight down through the hole left by the headlight, you should see the back side of the fog light. To access the bulb, twist the connector counterclockwise. To remove the bulb, press in the tab on the connector and slide the connector off of the bulb. If you need to replace the fog light housing, remove the bulb, then remove the screw on the inner side of the housing and gently pull it out of the lower grille. Reassemble in reverse order.