Since leasing our '15 EX-L, I've wanted to modify the fog lamps so that they function independently of the headlamps, thereby offering better performance in dense fog or inclement weather. I finally got around to attempting this in ernest last week, and with the help of this thread, succeeded in modifying them so that they still utilise the factory switch on the turn signal stalk but will now illuminate if either the parking lamps or headlamps are switched on.

** Disclaimer - I've tested and confirmed that this modification works as described. However, I assume no responsibility for any vehicle damage or voided warranty that may result. Perform at your own risk.

Parts needed:

• 18 AWG wire - approx. 8'
• 12V 30A SPST relay (I used this one)
• Female "quick disconnects" (like these)
• 22-18 AWG wire tap connectors
• 1N540x or 1N400x diode (optional - I used this one)

This modification is fully reversible, and you can opt to solder or crimp the various connections based on your preferences and skill level. Normally I'd solder and shrink tube everything, but I used wire tap connectors on some of the connections since our car is leased.

1. Disconnect the negative terminal of your battery.

2. Remove the two-piece plastic shroud around the steering wheel. The top half snaps off, then the lower half is held on with 3 screws (two in front, one underneath).

3. Locate the wiring connector at the back of the turn signal stalk. There should be a purple wire exiting the connector. Use a wire tap to connect a 2' length of 18 AWG wire to the purple wire from the turn signal stalk. Solder or crimp a female "quick disconnect" to the other end of the wire. This will be wire "A." (Optional - cut this wire and solder in the 1N540x or 1N400x diode. The silver band on the diode must be facing/closest to the turn signal stalk end of the wire.)

4. Find the factory fog lamp relay under the dash. It is located high up, with three smaller wire connectors next to it. The relay is black plastic with blue, purple, green, red, black and grey wires exiting it. The relay will make an audible "click" when the fog lights are switched on or off. Detach this relay from the plastic clip so that you can pull it down towards the floor for better access to the wires. Use a wire tap connector or solder to connect a 2' length of 18 AWG wire to the thin grey wire from the relay. Solder or crimp a female "quick disconnect" to the other end of the wire. This will be wire "B." Re-attach the relay to the plastic clip under the dash.

5. Connect a 2' length of 18 AWG to a chassis ground. I used a bolt on the metal frame under the dash as my grounding point. Solder or crimp a female "quick disconnect" to the other end of the wire. This will be wire "C."

6. Identify a wire that provides 12V (+) when parking lamps are switched on. For convenience, I used the grey illumination wire from the Vehicle Stability Assist button. If you choose to use this wire, you will first need to pop the switch out of the dashboard. A long, flat-head screwdriver can be used from under the dashboard to release the tabs holding the switch in place. Connect a 2' length of 18 AWG wire to your illumination wire. Solder or crimp a female "quick disconnect" to the other end of the wire. This will be wire "D."

7. Identify the 4 pins on your 12V 30A SPST relay. They should be marked 30, 85, 86, and 87. Connect the female "quick disconnect" ends of your wires to the relay as follows:

A = 30
B = 87
C = 85
D = 86

8. Electrical tape or shrink tube all exposed connections, and secure your relay and wires under the dashboard.

9. Re-install the steering wheel shroud.

10. Re-attach the negative terminal of your battery.


Your fog lamps can now function any time the parking lamps or headlamps are switched on, using the factory fog lamp switch.

Notes:

• If you did not install the optional diode, when the headlamps and fog lamps are both switched on, switching the fog lamp switch back to the "off" position will not turn off the fog lamps until the headlamps are also switched off. If the optional diode is installed, all the switches will work as expected.
  
• The factory fog lamp dashboard indicator will only illuminate with the headlamps switched on.

 

Thanks for this.

 

Question. What would happen if a 12v constant was used in place of '86', where it is currently receiving current when the lights are switched on?


Would the fogs turn on when the parking and driving lights are off?