Fog Light DRL Mod

Thanks to DanMan's posting of the fog wiring diagram, I was able to work up a DRL mod for the fogs.

With this mod, the fogs will operate only when the taillights are off. Essentially, that means they work in the daytime and cut off at night when you fire up parking or headlights. They also turn off normally when you open the door.

The mod requires the addition of a single normally-closed relay with a 12v coil, which can be added to the relay box in the engine compartment. I don't yet know whether there are any OEM relays available in a normally-closed version, because I haven't seen the other wiring diagrams. In any event, the part is readily available. (e.g. http://www.allelectronics.com/cgi-bin/item...DT_RELAY_.html). Also a Radio Shack 275-241 will work. The new relay doesn't need to carry much current, only enough to energize the existing Fog relay coil---almost any relay will do.

The Original Wiring
The fog relay coil takes its 12v power from the taillight feed, and it grounds through the Body ECU (to cut off at door opening), thence through the fog switch, thence through the low-beam switch. If you turn off the fog switch, the relay-coil ground opens and the fogs turn off. Similarly, if you flip to high beams, the ground is opened and the fogs shut off.

The Modded Version
The new relay is wired to allow daylight use of the fogs, except when the taillights are lit. Thus the fog relay has power available immediately upon power-up, and its coil power drops when you turn on night lighting.

When you turn on the fog switch, the fogs illuminate (subject to the driver's door being closed). The orange wire from the headlight stalk connector gets a permanent ground. This prevents the headlight switch from ever entering the equation.

When you turn on night illumination, the taillights light and the new relay energizes, pulling power from the fog relay coil. The fog relay drops out, killing the fogs.

The fog switch works as you expect: when it's off, no fogs. When it's on, you get daytime fogs.

Since you never need to turn on parking or headlights in the daytime to enable this DRL mod, you needn't worry about panel and MFD dimming.

Easy Wiring
(This is based on a quick examination of the relay box, not from actually doing any wiring yet.)

Steps 1-4 can be done entirely within the engine-compartment relay box:
1. Disconnect the brown wire from terminal 1 of the Fog relay. Re-route it to the coil of the new relay.

2. Connect the new relay's other coil terminal to pin 2 of the Fog relay.

3. Wire terminal 1 of the Fog relay to the common contact point of the new relay.

4. Wire the NC contact point of the new relay to terminal 3 of the Fog relay.

Step 5 must be done somewhere under/within the steering column area:
5. Cut the orange wire going to the headlight combo switch. Ground it. (This may be more elegantly done by disconnecting the wire at the connector.

On the attached drawing, the red wires are the changes. The words "disconnect" and "move" indicate where existing wires get disconnected.

Click for the DRL Wiring diagram.

 

The next neat thing would be to find a wire that's energized when the car is moving (or one that tells us we're stopped.) A diode or relay fed from that wire could be used to kill the DRL-fogs when we're stopped, and light 'em up as soon as we start moving!

DanMan, I'll bet you can find such a wire, and perhaps in a convenient location?

 

<div class='quotetop'>QUOTE(FireEngineer @ Apr 26 2006, 06:02 AM) [snapback]245435[/snapback]</div>

I haven't had time to do the mod; I was out of town for about a week, just got home.

As for a simple NC relay replacement, the logic wouldn't be right.

- The Fogs would light as soon as you power up the car. This is good.

- But they would only go off if [/i]all[/i] these conditions are met:
1. Door is open;
2. Fog switch is on;
3. Lo Beam switch is on.

Obviously, that's screwy because of the need for the lobeam and fog switch settings.

Danny, it's prudent for me to wait to move it to the Articles section till I've actually made the mod. Hopefully in the next couple of weeks I'll get time to do it.