Tech Article Title Author Date
Control Fog Lights Independently from Headlights and Rear Foglight Peter Harren 2000

I originally performed this modification on a '00 US Spec S4, but it should apply to any '99.5 and later A4 or '00 and later S4. The key thing is that you have the "second generation" fog light buttons. If you modify your fog lights per this FAQ, your fog lights will operate as follows:

  • The front and rear fog lights can be turned on anytime the ignition is on.
  • Neither the headlights nor the running lights need to be on for the fogs to be on.
  • The fog lights won't cut out if the high beam headlights are on.
  • The rear fog lights do NOT come on independently of the front fog lights (stock operation).

This modification requires NO cutting or modification of the wiring harness. It does require the modification of the unit that controls the fog light buttons. But, this part can be easily unplugged and replaced if you have any problems. The unit costs around $80-90 if you need to replace it.

To do this modification, you will need to get into glove compartment cavity. Make sure it is empty before you start! Open the glove compartment and locate the stop mechanism that holds the glove compartment door in its normally open position. The mechanism is located at the top rear corners of the glove compartment door. The mechanism varies from car to car, making it difficult to describe in a FAQ. The ones I have encountered aren't too difficult to release though. The idea here is allow the glove compartment door to open all of the way. Next remove the end cap on the right end of the dash. It comes off exactly like the fuse box cover on the driver's side.

Once you have gained access to the glove compartment cavity, look inside, up above the upper right hand corner of the glove compartment opening. If you feel around you will find a rectangular relay-like module. It's about 2"x2"x1". The unit is also partially visible from the right end of the dash. Once you find it, you should be able to remove it by essentially pulling it out towards the back of the car. The base the module is plugged into should come with it. Separate the module from the base by pulling it out like a relay.

The next step is to cut off pins 12, 15, and 17 at their bases on the module. Then you need to jump pins 15 and 17 to pin 14. There are two ways to do this:

1) Solder a wire between pins 14, 15, and 17.
2) You can open the module like a normal relay and inside you'll find a small PC board. Notice where all of the external connector pins attach to the PC board. You can solder the connectors that correspond to pins 14, 15, and 17 together. They are all right in a row and easily traced. Pin 17 is the second position in from the edge of the PC board. Position 3 is unused, 4 is pin 15, and 5 is pin 14.

I've done both and I highly recommend you try method #1. The cover is VERY difficult to remove in one piece and there is plenty of room for a wire between the module and its base.

All that's left now is to reassemble everything. Reattach the module to the base. It will probably take some patience to get the module and base seated back in their bracket. Notice that there are two cleats on one side of the base. These need to point downward when reinstalling the base and module. The goal is to get the cleats lined up with the cleat cut outs in the bracket. Once the cleats are lined up, you need to push the module / base assembly into the bracket by pushing it towards the front of the car. Once the module and base are reinstalled, close everything up and you are done.

Some notes:

If you want to be able to turn on the rear fog independent of the front fogs, you'll have to modify the control module. Personally I want to maintain this feature. Originally I wanted to restrict the fog lights from coming on without the running lights. But, there appears to be no way to do this without rewiring the wiring harness. This is out of the question for me.

Remember that this modification may allow you to operate you lights in a way that is technically illegal. This will vary by state. For example, I've checked Wisconsin and Illinois vehicle lighting codes. In these states it appears to be legal to operate fog lights with or without running and / or headlights. It appears to be illegal to operate fogs with high and low beam headlights in Wisconsin and Illinois because it brings the total of "driving lights" beyond the legal limit of 4. Whatever the law, please remember to be courteous to fellow drives.

For those of you who want to know more, here is what all of the pins on the module are for:

  • Pin 1 - +12V pulse toggles front fogs
  • Pin 2 - +12V line to rear fog
  • Pin 3 - +12V pulse toggles read fog
  • Pin 5 - +12V line to front fogs
  • Pin 6 - Appears to be ground
  • Pin 7 - Appears to be ground
  • Pin 8 - +12V switched with the ignition
  • Pin 9 - +12V switched with the ignition
  • Pin 10 - Panel indicator light for front fogs
  • Pin 11 - Panel indicator light for rear fog
  • Pin 12 - Holding at +12V PREVENTS the front fogs from turning on (used to cut out front fogs when high beams are on)
  • Pin 14 - +12V switched with the ignition
  • Pin 15 - Holding at +12V ALLOWS the rear fog to turn on (used to cut out rear fog when high beams are on)
  • Pin 17 - Holding at +12V ALLOWS front / rear fogs to turn on (used to prevent fogs from turning out with out the low beams