Rear Window Heater \ Demister not working

[ChuckMountain]

Anybody else had issues in the past with their rear windscreen heater\demister not working on an A6 Saloon (2015). It doesn't appear to work at all.

The light on the dashboard and goes off after the usual amount of time, so that appears to be ok.

The 40A fuse on in the back right of the boot is ok.

VCDS does not report anything related to this or any fault codes.

On another post somebody suggested it could be a relay, however, I have three different ones, none of which match the one suggested.

I will have a look properly in the daylight and can test with a multimeter if I can trace the wiring to find but just wondered if anybody had come across it before. Or have a wiring diagram to try to work out if I can identify the relay (and possibly swap with the one to work out if it is just that).

Car is 3 and a half years old so just out of warranty

[A6S]

There’s a connector at the bottom of the window I would check that, it can come loose. Can’t remember how to get it on the saloon but you should see it looking up from the boot towards the parcel shelf.
Had this on my first a6 and when it went in for warranty fix they showed me that it just came loose.

[ChuckMountain]

Thanks,

Will have a look just need to figure out how to get to the wiring as it all disappears behind the boot padding...

[A6S]

window replacement; adhesives and sealing compound Audi A6/Avant (A6) 2016 year Audi EUROPA 845000
Maybe this might help.
Doesn’t look like it’s a long cable

[ChuckMountain]

Thanks for that site, that's really useful.

Have done some more digging but still not resolved.

The relay in the boot is the top right one with a black connector if anybody needs to do it. This appears to be switching ok and I can hear it clicking when switched on. The 40A fuse is ok and I can measure around 14V or so with the fuse out across the terminals when the switch is on. Normally if you can measure across the fuse I thought the circuit would be ok but happy to have peoples' thoughts.

If I take out either the fuse or the relay than the car seems to know about it and turns the light off on the dash as well as logging a fault in VCDS.

Now need to find the cable as the boot "lining" seems to be in quite a big piece

[A6S]

If you look at that site you should see an exploded diagram of the parcel shelf and see how it comes apart. If you lie in boot and look up at the metal parcel shelf you should see holes that big enough for your fingers go go through. Normally a plug behind the bigger holes.
Other option is it might come down the side of the car so behind the side panels in the boot. They should pop off easier than shelf and see if you can see the cable and get a multimeter across it to see if it’s open circuit.

[ChuckMountain]

If you look at that site you should see an exploded diagram of the parcel shelf and see how it comes apart. If you lie in boot and look up at the metal parcel shelf you should see holes that big enough for your fingers go go through. Normally a plug behind the bigger holes.
Other option is it might come down the side of the car so behind the side panels in the boot. They should pop off easier than shelf and see if you can see the cable and get a multimeter across it to see if it’s open circuit.

Yep will have to have another look as couldn't see anything obvious through the holes. The side panel is one complete unit so its going to be a royal pita to remove I think....

[ChuckMountain]

Ok well, sort of some progress.

The connection to the rear windscreen demister is on the offside rear 45-degree pillar. This goes into a suppressor to stop unwanted radio interference being picked up. After that the wire just drops to the 40A fuse.

The actual aerial is the top 3 (or so) bars of the rear window in the saloon.

On the nearside the is a connection to the car ground. On both sides of the cable there is something wrapped in material to stop it rattling. There is also one wire that is coming off the offside to some sort of device possibly a monitor?

Connecting a multimeter across the window I am getting a good low resistance as would be expected, also from the fuse to the demister I am getting a low resistance (<1 ohm) so I am ok that part too.

I am also getting ~12V across the demister when turning it on.

As I sat in the back I did happen to steam it up but then running the engine again cleared it and had a drive around and the multimeter still said ~12V across it.

So just wondering if there was an intermittently loose connection that I have inadvertently sorted.

As it is a saloon, I mainly use the demister for clearing the rain as if you have it on then it generally heats up till the rain clears better. What I have noticed is it seems to clear a little bit either side of the bars but then stop and the rain return.

I haven't had it particularly steamed up yet or ice again since.

Anybody got any other ideas or do I hope for the best ?

[A6S]

If you think it’s working then bring a cup of tea in the car and let it just steam up the window and see if it’s working.
The device that you think could be a timer/control of some sort to stop you overheating the screen. It could be this device that’s faulty so your multimeter reading is showing power but screen isn’t getting it. Test both side of this device and see if it power getting through.
For the rain on the rear window use rainx or something similar so water just rubs off.