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15 November 2005

Rear Fog Lights

Making the rear fog lights functional on the A3 is a matter of getting the right switch, and making the right settings in the Central Electronics computer (using VAG-COM). There's no excuse for Audi not doing this from the factory.

From the Wikipedia article:
Rear foglamps

(ECE Regulation 38, SAE J1319) In Europe, vehicles must be equipped with one or two bright red "rear fog lamps" (or "fog taillamps"), which are switched on manually by the driver in conditions of poor visibility to enhance vehicle conspicuity from the rear. The allowable range of intensity for a rear fog lamp is 150 to 300 candelas, which is within the range of a U.S. brake lamp. For this reason, many European vehicles imported to the United States have their rear fog lamps wired as brake lamps, since their European-specification brake lamps may not be sufficiently intense to comply with U.S. regulations, and rear fog lamps are not required equipment in the U.S.

Most jurisdictions permit rear fog lamps to be installed either singly or in pairs. Proponents of twin rear fog lamps say two lamps provide vehicle distance information not available from a single lamp. Proponents of the single rear fog lamp say dual rear fog lamps closely mimic the appearance of illuminated brake lamps (which are mandatorily installed in pairs), reducing the conspicuity of the brake lamps' message when the rear fogs are activated. To provide some safeguard against rear fog lamps' masking of brake lamps, ECE Regulations require a separation of at least 10 cm between adjacent illuminated edges of brake lamps and rear fog lamps.

That last sentence is why the US and non-US A3's have differently-configured taillamps. Fortunately, this is all done through coding, with no re-wiring necessary. In Europe on our cars, on each side the outermost tail light bulbs are tail & brake, the next bulbs (still in the lens on the car, not the hatch) are tail only, and the innermost bulbs (the ones in the red lenses on the hatch) are rear-fog only. The US-market configuration for the A3 is to have all three of those bulbs on at 24% of max brightness as taillights, and all three of them at 100% brightness as brake lights. Activating the rear fog function will use the innermost red lights on each side (the ones on the hatch).

Note: Also see my article on European taillights with amber turn signals.

Finding the right switch to get is an adventure. Since this feature uses existing bulbs in the taillamp assemblies and a software setting, the only physical difference between cars 'with' the feature and 'without' is the headlamp switch on the dash. Instead of making a few different switches for the most necessary combinations, Audi made 67 different switches (parts list PDF). Think of all the overhead expense to make smaller quantities (less economy of scale), documentation, shipping, stocking.... lots of unnecessary expense that could have been simplified. (Update, November 2006: Switches with aluminium trim used in the new-generation Audi TT will work in the A3. I don't have a complete parts listing available for those.)

So, out of those 67 different switches, you have to pick the one you need. Some cars have the 'AUTO', and 'Coming/Leaving Home' features, mine does not. (Those cannot be added by getting the 'wrong' switch, so don't try. It can't be done.) What I do have is a DRL switch on my headlamp switch assembly, which turns on/off the dedicated Daytime Running Lamps in the bi-xenon headlamps. Again, this is dependent on those headlamps being factory-installed on my car, and cannot be used on cars not so equipped.

The part number for the switch that I needed was: 8P1 941 531 AH 5PR Audi describes this as "multiple switch for side lights, headlights, front and rear fog lights, for models with day driving headlights". In other words:
  • WITH: Parking lights, headlights; front & rear foglights; panel dimmer; DRL switch; black; left-hand drive

  • WITHOUT: 'AUTO'; coming/leaving home

I ordered it from Martyn at VAGParts, the international shipping was no problem for them at all. You will need to pick the appropriate switch for how your car is equipped.

Installation of the switch is trivial: Simultaneously push-in-and-twist the knob clockwise, which releases the clips, and while maintaining that pressure wiggle the switch and pull to get it out of the dash. Swap the corresponding wiring plugs between the two, and put the new switch in the same way.

The next step is to change the Central Electronics coding so that they are usable. A few people have reported that their cars were already coded this way (i.e. just plugging in the new switch was all they had to do), but that is not the norm. Decide whether you want one (driver's side) or two (both sides) rear fogs activated, and make the coding change using the steps described in my VAG-COM Coding Reference.

New switch installed. Note the extra icon on the left of the main switch indicating 'pull two clicks out for rear fog'.

Taillights on, rear fogs off. The US configuration uses all three bulbs for taillights, and all three brighten for brakelights.

Taillights on, rear fogs on. You can see that the innermost bulb (the one on the hatch) is as bright as brake lights would be, to make you more visible from the rear in heavy fog or rain. DO NOT drive around like this in normal weather! It's dangerous and obnoxious.

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