Well, after many months of receiving helpful information from this forum I think I have something to contribute.

Okay, let's get this out of the way. THIS POST CONTAINS INFORMATION ABOUT HOW I MODIFIED THE WIRING HARNESS OF MY LAND ROVER. MODIFYING YOUR WIRING HARNESS IMPROPERLY COULD CAUSE YOUR VEHICLE TO MALFUNCTION. WHILE THIS PROCEDURE WORKED FOR ME, I MAKE NO CLAIMS THAT IT IS CORRECT OR SAFE. PROCEED AT YOUR OWN RISK.

I've always loved and hated the way my Disco handles the fog light operation. I love that it's a momentary type switch, not an on-off one. I love the fact that after your restart the truck they don't turn back on unless you tell them do. That way I'm not driving around shining my HID fogs in people's eyes when I don't need to. I love that it lets you turn them on when only your parking lights are on. This is great in a Minnesota blizzard. What I hate is that they turn off when you kick your brights on.

I've been wanting for a while to mod the wiring a bit so they stay on when your bights are on, but the rest of the functionality stays the same. It's not quite as easy as your might think because of the involvement of the BCU (Body Control Unit). Here's how it works:

When you press the little button, it send a signal to the BCU. The BCU will then turn the lights on or off. However, the BCU will not turn the lights on unless the IDM says that at least your parking lights are on, and the ignition switch is on. The IDM (Intelligent Driver Module) is a bunch of circuitry built into the fuse box by your knees. Why both the BCU and the IDM? The BCU controls tons of stuff in your truck, from the dome lights, to the heated seats, to the windows. It does not, however, control some of the high power stuff such as the headlights. The wiring to the headlights is pretty old-school, power feeds the switch, power comes out of the switch and goes to the lights. The trick is that after the switch it runs through the passenger compartment fuse box, which contains the IDM. The IDM then cleverly detects that you've turned on the lights and tells the BCU. When you turn on your high-beams, the IDM tells the BCU, which temporarily turn off the fog lamps.

Okay, so how do we fix it?

Short of rewiring the fog lamps to a normal switch, which would involved puling wires and would kill the desirable functionally of the system, the only way to do this is to trick the BCU (unless you know how to reprogram one). The easiest solution would be to disconnect the wire that goes from the IDM to the BCU telling the BCU that your brights are on. The only problem is that instead of a bunch of wires going from the IDM to the BCU to communicate data, there is one digital wire. So, now we need to trick the IDM. When need the IDM to not know that your brights are on.

The solution? Go around the IDM. If we connect the high-beam output of the lighting switch straight to the headlights, the IDM will be none the wiser when we turn our brights on, and neither will the BCU. The only problem is that little blue light that tells you your brights are on is driven by the IDM, so we'll need to hook that straight up to the switch as well.

Going around the IDM means going around the fuses. There is still a 40a fuse upstream in the engine bay, but we'll bypass the two 10a fuses dedicated to each high-beam light. I opted to not replaced them with in-line fuses because (1) I run relayed HIDs meaning I am only drawing a fraction of an amp and (2) It was 1:00AM and my truck was all taken apart. I would recommend installing in-line fuses if you're running normal lights.

Now that I've bored you with the details. Here's how to do it:

(1) Open up the fuse compartment door underneath the steering wheel.

(2) Remove that annoying piece of styrofoam. It's glued in, but it comes out easily enough.

(3) Remove the 10MM nut above the fuses

(4) Gently turn the fuse box onto it's side, so you can get to the wire on the back.

(5) Cut these wires free from the back of the fuse box. Your' going to need all the length you can get, but I'd leave enough on the back of the fuse box that it could be hooked back up.
A: The Blue-White wire that's on a connector with two others (see photo). This is the wire that comes from the lighting switch.
B: The Blue-Orange wire that's on the yellow connector (see photo). This is the wire that goes to your front right high-beam.
C: The Blue-Gray wire that's on the green connector (see photo). This is the wire that goes to your front left hight-beam.



(6) Connect these wires together. You can see that I used blue crimp connectors, because it was not the best soldering conditions. Here is where you should do as I say and not as a do, and put in some fuses. A set of 10a in-line fuse holders with 6" leads would work great. That way you pull the fuse holder itself around front of the fuse box, incase you need to change your fuses. The wiring would be like this:



(7) Connect the thin blue wire that goes to the high-beam indicator. This can be found on a connector that's on the front of the fuse box. Cut if like you did the others, and splice it into one of the wires.

(8) Test. Turning the high-beams on should no longer effect your fog lamp operation.

(9) Put it all back together.

Note: In case you are wondering, the green wire in my photo goes up to my brush-guard switch. This is a custom add-on.

Note: After this mod, you will no longer be able to turn on your headlamps from the key fob.

 

The question is why do this?
Fog lights are intended for driving in fog, you do not drive with high beams in fog... so what's the point?

 

It might be good for offroad at night. The more light the better, right?

I think this can be enabled in the BCM without rewiring anything.

Front fog lamps
None ........ The vehicle is not fitted with front fog lamps.
Main ......... The front fog lamps will operate if the headlamps are on main beam.
No main ..... The front fog lamps will not operate if the headlamps are on main beam.

I think (though not sure just yet) that land rover is referring to the high beams as the "main" beam and what I would call the main beams they call the "dip" beam.

I haven't tested this yet, because I don't have fog lights yet. I keep trying to order them for my new bumper from salvage yards, and they keep having broken pieces, so I cancel the order.

 

IMO, this is a good argument if you use the factory wiring and after market fog lights mounted somewhere up on top of your bumper.

The honest truth is that if you do any real off-roading at night, the fog lights at the bottom of the stock bumper won't last long and won't provide much useful light due to their low position. (I learned this the hard way!)

 

This is probably why they moved them up higher in 03, so they wouldn't get busted as often. They probably still aren't high enough.....

 

There is no point, and in fact will make driving at night more hazardous.
Reading "Light mods" threads more often than not illustrates that there are a lot of people who don't understand good automotive lighting, or how the human eye works.

 

Sounds a bit complicated for me to take the time doing, but good job regardless. I've been in many situations where the more light the better...especially lately on the pothole laden roads by me. Also, last night driving through many flooded areas it would have been handy. I had the highs/aux lights on to see well ahead of me (for trees, moving water, etc) but could have used a bit more right in front to dodge the huge craters in the road!

 

Thanks Snafu.

For me it's lighting up the ditches to watch for deer. When it's dark and conditions are bad, the previous setup would only solve one of my two problems at a time. I need to be able to see far down the road, but I also wanted to ability to glance at the ditches from time to time and actually be able to see.

With researching the wiring design, it only took a few hours for me.

~Max

 

I did the same in my Audi... was very simple though and just moving one plug to another and doing the computer. It has the bi-xenon lights and are very powerful in a straight line, but always like to be able to see directly in front when needed. The flat and wide beam of the fogs is helpful.

 

i think its great, why shouldn't you be able to control your own lights in the configuration you want?

I've been at farm gates with combo locks where i had to go back to the car and dim my lights and put on the fogs to get light on the lock. there is a level of practicality in at least saying the driver knows well enough what lights to use and having the ability to do it.