Stupid Fog Light

Hi all,
The 90 has its MOT this Friday - Buuuuut the fog light doesn't work....again....

I found an issue with the red cable coming up through the floor at the rear and decided to bypass the old wiring. So, running a new wire to the pos on the switch on the dash. but nothing - If I leave the old wire on, the dash illuminates with the fog light symbol - if i run the new wire - nothing. does not make sense. Either way, there is no power getting to the light - with or without the old wiring.

Ideas?

Thanks!

Is the earth ok? The power may be able to get there if it had an easy route to earth from the bulb unit. If you run a wire directly from the ignition, then you'll bypass the original switch circuitry and it won't light up on the dash.

(Written from my experience with 200tdi - other models may work differently)

The fog light it set to work only with the ignition on and lights. If you're getting a light on the dash, that suggests those are all ok. The fog light also has it's fuse before the dash light, so again if its on dash, then it should be fine.

My fog light switch was a bit messed up - I ended up using wd40 to clean it up, then it worked fine. Test voltage at the switch. This will help you narrrow down issues....

I'd suggest then finding the wire near the brake master cyllinder - unplug the rear loom and test it there. If there's no voltage there, then it's possible there's a break between there and the switch, if there is, then a break between there and the connector at the back of the vehicle is possible

The strange thing is, even with the pos wire disconnected from the back of the truck for the fog light, it will still illuminate from the dash..... before i could weld, someone welded a new cross member on the back of my truck and melted the rear wiring harness. I replaced / repaired it as best I could and all has been ok since the repairs. Fog light always been a pain every year. I need a way to rewire it really without to much fuss. Any ideas? Will probably start a rebuild on the truck soon anyway as wiring is a mess and all needs redoing.

Cheers

Where's Kwakerman when you need him?
So you're not getting 12v from the original red wire at the back? (mines red with a yellow tracer)
I presume you are getting 12v at the switch or you wouldn't be trying to connect another wire to it?
Kwakerman sent me a wiring diagram for my 300Tdi, but it'll be different to yours. If you had a wiring diagram, you could see where your red wire goes back to at the front of the truck and take a new feed from there. That way you would maintain the link to the dash lamp etc.
Another thought is that because the fog lamp works in conjunction with the headlights the fault may lie with the relay that connects the two or a broken feed wire from the relay to the fog lamp side. I'm not that fond of electronics and so these are just my amateur thoughts. Bumps the thread back up at least!

The way the light works(at least on my 200tdi) is it shows you that the light SHOULD be on, not necessarily that it is on. It basically shows you that the lights are on and the fuse is good. It's quite possible for the telltale to be on and the rear light not to be.

The two issues I had was the switch, and then also the light fitting was not keeping contact with the bulb. I ended up cleaning the switch and buying a new light unit for not very much on ebay.

hmmmm all this trouble for something is never used. LED tail lights are brighter anyway! I will have another look with the meter and see if switch is getting power. Guessing it is as the dash lights up but we will see.

Right had another look - I have had the switch apart and contacts look fine. When I turn the switch on and off I can hear the relay clicking. Power seems to be intermittent to the switch, 9-10V. However the dash light ALWAYS comes on regardless. Really am quite confused with this.

The dash light only tells you that the switch is working. If the relay is faulty it may still click. With the engine running and lights on, turning the fog light on should give you at least 11.5 volts at the fuse for the fog light. If you're getting it there, then there's most likely a wiring fault towards the back end. If you remove the cover in the rear tub (drivers side) quarter to expose where the loom comes in from the chassis, disconnect the fog lamp feed wire there and check the voltage again. If you're getting 10v or more, the problem is likely with the connections between there and the fog lamp itsself. There may be another connection point in the other corner of the tub. A test lamp is more useful than a meter for this, as if you can light it, there's probably enough power to light your fog lamp.