Possible Reason for Passenger Side Water Ingress

[ChrisG]

I have had 3 A8's now over the last decade, and for reasons I could never get to the bottom of, all of them had water coming in to the passenger side.

I suffered this with the last two, however on my current one, I decided to get quite invasive with it and try to find out what the problem was.
I asked my garage to investigate this, and the obvious culprit was the drain holes and tubes being blocked.
They checked everything - including the sunroof drain - nothing. All of the drain tubes were clean and clear.
Sadly at that point we couldnt come up with anything else.
Last year, the Indicator relay blew, so I had to have it replaced, and during this investigation they found the reason it blew out was, it was full of water. The leak seemed to be coming in around the passenger side windscreen area and was dripping on to the electrics.
We replaced the Relay and sealed it before refitting so it didnt fill with water again however it didnt explain why or what was causing the leak.

I went to get my daughter from school earlier this year and while waiting, a guy was finishing work and got chatting with me as he was a A8 enthusiast.
About half way through the conversation he asked what kind of issues I had with it, and I mentioned the soggy passenger side.
He simply said "I know what the issue is there. Its the windscreen"

He explained he had a friend who worked in the trade who adored his A8's and the first thing he did when he got one in was have the windscreen replaced.
Doesnt explain why its always the passenger side though, but and I dont know how true this is, but it seems very logical...
At the factory, they use Robots to put the mastic on the windscreen, and then the same robot fits it.
When applying the mastic, it starts in the bottom left corner, goes around the outside of the window to end up in the bottom left corner.
That bottom left corner is where there is a slight joint, which is actually sealed when the windscreen goes in, but because its so accurate, there is a weak spot there.
Over a period of years, the mastic dries out and shrinks. Where you have a constant line, it remains sealed but where the joint is, it cracks and starts to let water in.
Hence why all the A8's I have had, have that issue on the passenger side.

Now, I will admit, its a very aggressive way to fix the problem in having the windscreen replaced and there are known issues with the drain points blocking up and that is certainly worth checking first, but if you get to the point where you have confirmed all the drain holes are clean and the problems still there, its is most likely the windscreen thats letting the water in.
If you do get it replaced you need to urge the glass fitters to go to town though, as I have previously had a windscreen replaced and they didnt seal it correctly and water came in even worse.

I hope this helps