VW Touran rear fog light

"Tim Downie" gurgled happily, sounding much like they were saying:

Wild guess - the working one's on the driver's side?

The other side probably isn't wired up.

provided that the working one is on the driver's side then it would not fail the mot. if it has worked in the past then the first check is that you have actually changed the correct bulb and replaced it with a working one of the correct type. I would check this by having a powered fly lead that I can poke in the bulb live connector or wire and see that it actually lights in situ. Many UK vehicles only actually have the wiring for one brake light yet have bulb holders for two, so if the RH one works I would not worry too much.

Alas it is the driver side one that's not working.

I assume you meant fog light rather than brake light there. I've swapped bulbs around with known working ones and the socket is definitely dead.

Tim

"Tim Downie" gurgled happily, sounding much like they were saying:

Ah.

Then it's time to have a prod about with a multimeter.

This is likely to be a multiplexed vehicle, so prod carefully...

Chris

yes, I intended fog !

If the left one works then it may be possible to temporarily just take a lead from the wiring on the left to power the right hand one, I realise this will leave the bulb warning light on, but it would pass an MoT. My guess is that it is likely to be something silly such as bulb holder or wiring connector fault close to the lamp.

Yep. It's beginning to look like one of those small problems that's going to be expensive to fix.

I don't really know how the multiplexing works but it does odd things if you remove a bulb "live" and then replace it. The replaced bulb won't work until the lights are turned off and then on again.

I'm wondering if it might be a computer fault. Does that sound possible? Certainly the wiring looks sound and the plug to the light is well seated (and re-seated).

Next question, main dealer or auto-electrician? I live in Ayr, SW Scotland if anyone has any recommendations.

Tim

That has crossed my mind!

Doesn't seem to be bulb holder or wiring connector fault and the wiring

Tim

"Tim Downie" gurgled happily, sounding much like they were saying:

Thought... Does it have a towbar?

Yes it does have a detachable bar (which is current detached) and standard trailer single electrics (7 pin socket). If it was a fault with the wiring of that I would expect both fog lights to be knocked out though if the fault was mimicking trailer electrics being plugged in.

"Tim Downie" gurgled happily, sounding much like they were saying:

Depends how the towbar's been wired...

You sure that its gone

I think there is one fog and the other reversing.??

Multiplexing uses a single electrical feed to a local electronic module, which then distributes out to a number of items. A signal wire from an ECU tells the electronics which output to turn on. (The signal wire can carry two-way traffic, so can report back if a circuit isn't drawing current, for example. This is how bulb failure alarm works.)

I've had a quick look at Autodata, but it has no wiring diagrams or relevant component locations for your vehicle - sorry.

In your case, if you can check back for a voltage (using a meter or test lamp) from the bulb holder contacts, through any connectors, to the electronic module, then you will have done all an amateur can do. If there is no output from the module, then module failure is a strong possibility.

I personally would not entrust this type of work to an auto-electrician unless they came recommended; if no one has any other suggestions, I can only sympathise with the need to use a main dealer.

Chris

Chris Whelan gurgled happily, sounding much like they were saying:

Another thought...

Could the ECU have had a brain-fart and gone into "LHD tail-lights" mode?

It is possible, yes, as you say, the lights can do weird things with CAN cars like yours (or my Leon). I recently had my front bumper off, which means disconnecting the foglamp, and the warning light was stubborn until the DTC was cleared.

Broken wire at the plug, if you've tugged the wire a bit? Cables in current VAG looms are quite thin, and the plugs very securely held on.

Get VCDS on it, if you can. I know it sounds OTT, but the Central Electronics will give you a clue.

I wondered that- but I would it still throw the DTC for a failed bulb?

Hmm.. good point. Does it have the pukka VAG harness and CAN controller?

Would disconnecting the battery clear it or would that open up a whole new can of worms? ;-)

Well I do try not to be too cack-handed when changing bulbs but it's not impossible. The thing is, the feed to the fog light is the 3rd one (of six) in a line in the plug and I would have thought that it would be the end wires that would be the most likely to get strained and potentially broken.

Tim

Um, as it normally has two rear fog lights, I'm not sure why RHD or LHD is relevant.

Tim