Faulty Fog Light - can't switch it off!

Tearing my hair out with this fault and getting quite close to getting professional help!
My rear fog light and fog dash light has suddenly come on and I can't switch them off! Even disconnecting the switch altogether and removing the fog light fuse (no. 6 from the left) results in the fog and dash lights still both on! Was told to check the fog light switch which is working and change the relay "if one is fitted". Not sure what a relay is and if one is fitted on my 300tdi. Have stripped my dashboard to expose the wires but can't see a relay - am I missing something? The pictures enclosed show the exposed wires and the relays (?). Removing the second fuse along from the left (20amp) shuts off the fog light and dash light but horn etc does not work - at least this stops the battery running down. How is my fog light getting a live feed? How do I find the problem and what should I do next? Dont't know where to turn next. Any advice much appreciated.

Drop me a PM with email address and I can send you a copy of the wiring diagram and also the Electrical library which has a good explanation on how the fog light circuitry works. Quick thought - do you have a towbar socket fitted, water / shorts in the socket can give strange faults

Or wiring chafed where it enters/ exits chassis

freelance said:

Or wiring chafed where it enters/ exits chassis

Click to expand...

Chafed wires will generally cause a short and blow a fuse, for a chafe to cause this type of fault there would have to be a chafe on a +12v wire and a chafe on the fog wiring and both these would need to be touching but without touching any of the chassis metalwork. Not impossible but highly unlikely

Could the fog light switch be goosed? ie broken internaly in the on position.

sorry, didn't read original post thoroughly!

Thanks Kwakerman. My email address is [email protected]

By the way, regarding the chaffed wires thought, I can see where the wires enter the chassis at the rear but can you please confirm where they exit at the front of the chassis and what do they do then please? I noticed that there are some black wire connecting blocks under the engine compartment picking up some wires coming from the rear - will one of these be connecting the rear lights to the fuse box etc? Thanks again, much appreciated.

Sorry, I forgot to mention Kwakerman that I do have a towbar socket fitted. Thanks.

I'm no electrical genius, but I would strongly suspect the earth rather than the fog light. You've got power earthing from another circuit 'exiting' via the fog light as that's the easiest route it can find.

Undo and clean up the earth connections from the chassis (follow any of the light wiring and you'll find them) reckon that will solve it. Fingers crossed. A

If the fog light is on with the ignition off, the number of possible sources is narrowed down considerably. There shouldn't be a permanent live feed to the rear of the vehicle, so the fault is likely behind the dash or even close to the fuse panel. Look for contact between the permanent feed wire colours and the fog wires.

What age is the vehicle? I was expecting to see the fog switch on the steering column. Your switch is where the internal light switch usually is - and this has a permanent feed. Interesting.

4crosses said:

Thanks Kwakerman. My email address is [email protected].

Click to expand...

On its way

Kwakerman has been very helpful and supplied lots of extra advice and information - thanks.

I have another question which really shows my ignorance. Have identified the fog light relay (behind the dash) and want to disconnect it as I may possibly wish to replace it, however (now comes the silly question), I can't disconnect it. It's in an awkward position in the dash but I don't know where to pull or apply pressure on the white connector! Tried a small flat scewdriver where I think but it won't budge. Can anyone please describe what bits in the white connecting block I need to be pushing/pulling/ or pointing my screwdriver into please to release the relay. Sounds simple but I can't work it out and don't want to destroy the connector in the process. Thanks. Thanks.

Looks like you need to grab it either side on the connector itself, finger and thumb, squeeze like a mad man and jiggle the fooker working it from side to side from finger to thumb and pull. You'll probably have to give it some for it to move.

be careful using a screwdriver as you don't want to short anything.