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Discussion Starter · #1 ·
Hello everyone. Recently been battling some issues with my car - electrical corrosion has been causing havoc for me for all of 2020 it would seem. Some of you may have seen my post about my cooling fans running non-stop. I created a somewhat detailed write-up on how to trace some wires throughout this car: 2009 S5 Cooling Fan Running Non-stop

Anyways, I'm back with a new post. This time my car went radio silent on me. Couldn't run a VCDS scan, couldn't roll the window up or down. Couldn't move the seat. Nothing... so I started taking the car apart trying to resolve my issue.

Now, some of you may be familiar with the problems that arise with the sunroof drains on these cars. Despite my best efforts to keep them clean, I still fell victim to water getting into the back right corner of the trunk (I hadn't realized this was happening until you see what happens next). But the problem doesn't end there.

Despite the incredible amount of engineering and consideration I've seen within the manufacturing of this car (wild suspension components, incredible detail on the strain relief and wrapping of the wires), Audi has a fatal flaw. GASP! Here it is, and I believe anyone with a sunroof should pay attention.

Again, we're in the RH side of the trunk (er, boot?) where all your electronics are. Take out whatever interior carpets you can to get a better exposure from the '+' terminal on your battery. Follow this lead up into that corner of the trunk. If you have an older car like me that's not afraid of rain, you'll likely see something similar to this:


Yes, that green stain is corrosion. We'll want to open that harness up to see what's underneath. I pried up some of the loom anchors in the area to bring this part of the harness out into the light a little bit and to help me work around the harness while I opened it carefully with a new razor blade. Here's what I found:


So, the junction that carries those angry little pixies from the battery to the rest of the car is absolutely corroded into oblivion. But, there was no standing water in my trunk. In fact, the only evidence I had that indicated moisture was this corrosion. So what's the flaw? The flaw is the little "cover" that goes over this junction to protect it:


This is slid over the junction. BUT... it is not filled with any sealant AND the orientation in which it sits in the car encourages water to pool into it (like a cup) and the water becomes trapped. This all-inclusive stay provides the water with an infinite buffet of wires to eat and, over time, will render you a car that cannot move, or speak, or do anything really.

I'd recommend everyone take a peek at the corners of their cars to make sure they're not the next victim! To clean it up, I simply cut back the wires to good clean wire, soldered them together, and used a heat shrink with adhesive lining. This creates a seal that should help protect the wires from water intrusion. Lastly, I attempted to route the wires with the use of additional zip ties a little bit higher off the floor. This should help you out for many years to come.

I hope this helps some people out. Let us know in the comments if you see similar problems. I'm genuinely curious how many people this may affect. Would be a shame to see people lose their love for these cars from a silly condition such as this. Good luck everyone!
 

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Blimey, amazing!!! Thanks for the excellent explanation and photos, my old S4 has a sunroof so I will have a good look tomorrow, she lived for many years in Switzerland so has been exposed to plenty of rain and snow I am sure.....
 

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Yes, thanks for this useful info.
Mine has a sunroof and on close inspection (thankfully) does not have this issue, so I also wonder if this could have been caused another way- which could therefore affect cars without a sunroof as it's in the very back underside corner- exactly where water ingress either through the wheel arch, filler cap drain, rubber trunk seal (a known issue on the opposite side affecting the amplifier) or even a panel weld or seal could occur?
I would say it's worth checking anyway, even if you don't have a sunroof..
 

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That first photo appears to have little stones/grit (its the area by the rear light cluster - is that right?). That might suggest that the vents are leaking for example when you go through a deep puddle. In the UK I get that on the left hand side as that's the side of the road with the gutter, drains and of course most of the pot holes. I don't have a sun roof (mines a cab).
 

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Discussion Starter · #5 ·
You all have valid points. I made the assumption this was due to the sunroof drain but I suppose water could ingress via other means. As far as the debris is concerned, I can't say for certain that I didn't bring that grit in on my own through the daily use. I've had mountain bike wheels, sandy towels, etc. tossed in the back of the car throughout it's life. This would be the first time I've torn the interior out far enough to get a proper clean. I know the vent flap works, but at the same time anything that splashes up could theoretically push past it. Worth a check and at the very least a wire re-route would provide extra peace of mind.
 
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