Does anyone understand how the electrical works for the fog lights?

What I know on my rig:
Front and rear seem to follow a similar electrical pattern
When headlights and key switch are on - working from outside to inside of wires - far left wire reads 1 volt, next wire reads 6.8 volt, next wire (which is black) is at 0 volt, and 4th wire is at 0
When I engage the switch for the rear fogs, the glow light indicator comes on in the switch, the 4th wire is now at 12v and the rear fogs come on -- is this British reverse logic that has me confused? How does this wire go to 12v when wire 1 is still as 1v, wire 2 is still at 6.8v and wire 3 is still at 0.

I'm pursuing this because I can't figure out the front. When I engage the switch for the front fogs, nothing happens. But, if I bridge a hot wire to the fourth wire, the glow light indicator and fog lights do come on which seems to indicate that the light relay is okay. If it is the switch, then I've luckily gotten a second switch that was bad out of the box.

Anyone?

 

When you say "reads" - where is the second meter lead? I would expect it to be on a metal frame ground reference.

 

Yes, by reads I mean the voltmeter reading with the voltmeter negative lead connected to ground.

 

Sorry for the delay , had to drive about an hour and a half.

The connector you are looking at is C0065, per those tiny numbers that the RAVE schematic uses. The electrical guide lists the connectors and pin outs.

C0065-1 - Pink/black - data level sense wire from BCU. The switch connects this point to ground to ask the BCU to operate the fog lamp relay.

C0065-2 - Red/brown - battery volts via the adjustable dimmer to provide the dash light function inside the switch so you can find it in the dark. So it may be at some voltage set by the dimmer, and not there if dash lights not on.

C0065-3 - empty

C0065-4 - Black - chassis ground via header (group ganged together plug) to tie point to chassis (earth header C0017).

C0065-5 - Blue/purple - plus volts when fog lamp relay is engergized, lights the other light inside the switch. If second light (not the dash lights) does not come on suspect fuse F3 in engine compartment fuse box.


Now, Would I guess you are wanting to connect some additional fog lights? The relay that runs the fogs is in the under hood fuse box, and it put + battery volts out to fogs on a blue/purple wire at connector C0574-pin 2.

I have attached the fogs page from the circuits guide. It does not show the dash light in the switch, you find that over on "interior lights".


The system is basically a switch that asks the BCU to engage the front fog relay, the BCU does that and lights a small lamp inside the switch as well. The small switch is not carrying the current for the fogs, or even the coil of the relay that drives the fogs. Just a baby current to the BCU.

 

If the front fog lights are connected similar to the accessory lights on the push bar then that 12V is just there to light the indicator. The momentary push of the fog light button probably just toggles an input on one of the control modules which then actually turns on the lights.
I am working on a replacement circuit for our accessory lights that retains the momentary push button and some relay control logic that toggles the relay with each momentary press of the button, versus replacing the 2nd aux light switch with a cruise control switch and re-wiring the whole circuit.

Ron
'04 Discovery II Trail Edition

 

Pretty much like I said, the knee bone connected to da leg bone..... The BCU is the "control module" and it does command the fog lamp relay to engage. The same 12 volts from that relay lights up the fogs and the itty bitty light in the switch.

 

Thank you both for the responses. I will work through it this evening to trace problem (not being able to use the switch to turn on the front fogs) and let you know. If it is the BCU I'm thinking a bypass switch might be in order....

 

Looking at the schematic, the BCU supplies a ground to turn on the fog relay. So even if all the parts work, an open connection between the switch and BCU means so request signal is received, and an open from BCU pin C0663-4 would prevent activation of the relay. If fuse F3 under hood is cracked, won't have fog lights or the reminder light in the switch.