My rear de-mist switch is playing up. The led intermittently doesn't light but the demise still works. I have replaced the switch but the problem persists. Anyone had this issue ?

Replaced the switch with a new or used one?

 

If it's not the switch then it suggests a bad connection somewhere.

The front comes away and you can change the bulb, but as long as it still works I wouldn't worry to much about it. As it's pushed in you can tell if it's on.

If I switch on and tap the console beside the switch it comes on every time ! It was a used switch. Another on the way. Wiring looks good. Flexing loom makes no difference.

Was same with new switch. Needed tapping. Now light doesn't come on at all ! Fuse is good. Loom looks ok ????

Does it matter?

If it operates as it should, the why split hairs!

OCD ! It lit up before so I want it to light up again !

If the screen is heating ? and the backlighting is working? it can only be an internal contact/bulb/LED in the switch

In 3 separate switches though ? Only started since the nav went in.

Screen was heating will need to check again. Backlighting does work.

I had a similar issue with strange behaviour of the light on the demist button on a Golf but before I got to the bottom of it I had a few other power issues and ended up inspecting the brushes on the alternator and the commutator was worn really bad. Once a new alternator was fitted the problem went away.

 

At the time I thought it could have possibly been the heated window loading up the alternator too much for its poor output and warning me by not powering the light, although everything did still work fine but It may have also just been a coincidence.

 

Just food for thought I suppose. If I was you I would check the alternator output and maybe give the battery a good charge and see if it helps

Edited 13 January, 20179 yr by SuperbTWM

As SuperbTWM suggested, do your Voltage checks on the alternator and battery to rule out the obvious stuff, failing that, you will have to pop the switch, and gently move the loom while it is switched on to check for broken wires at the connector block.

 

I suppose if the units were used, the possibility exists that you may have a freak load of defective units  possibly why they were put up for sale, but it does seem less likely.

Here's the circuit diagram

 

 

As you can see the ON light (K10) is completely internal switched, if there is power going to the HRW & Mirrors, the ON (K10) bulb should light 

So battery Volts - 12.10V , with Engine Running - no load - 13.95V , Nav, Lights, rear demist on - 13.5V.

I read somewhere alternator op should be 14.2V to 14.7V ? and battery should be 12.6V ?

https://m.flickr.com/#/photos/145997016@N08/31617497284/

I've no idea why you're considering changing the alternator because one small LED light is only coming on when you tap it. You said it's only since you fitted the sat navigation in, which heavily hints at a wire being disturbed upon the fitting of that.

So battery Volts - 12.10V , with Engine Running - no load - 13.95V , Nav, Lights, rear demist on - 13.5V.

I read somewhere alternator op should be 14.2V to 14.7V ? and battery should be 12.6V ?

 

I said earlier on that you can rule out a power issue straight away by connecting another running car via jump leads to get the voltage up. Have you tried this?

That's a pic of it working now. Another new switch and it's good. Won't be changing the alternator , earth cable has signs of overheating at the fuse box so I will change those when I get them.

Cheers for all the help.

The cable at the fusebox is not an earth, if you mean the cable sleeved in black, it is the positive feed to the fusebox from the alternator itself. Don't know why VAG chose to do that as it is confusing.