SteveoH

Good morning, everyone;

I replaced the battery in my '05 A8L (4.2L) yesterday. When I pulled out the old battery, I found a pool of water(!) in the cut-out area directly below the battery! It didn't quite cover the module in there, but it was measurably deep. Obviously, I dried it out before replacing the battery.

Any ideas what might have caused this? I'm frankly not sure if the vent hose was installed on the old battery. This is my initial suspicion, but if it is the culprit, and is letting in rain water, I don't think it'd be an effective vent for the battery anyway. I did attach the hose to the vent hole in the new battery, but am concerned that I might be creating a problem if water is finding it's way in through the hose.

Also, out of curiosity, can anyone tell me what the module is that is located in the void directly under the battery? Since it was essentially sitting in water, I'd like to know what it controls so as to look out for problems.

Thanks in advance!

MP4.2+6.0

You should look at the situation again--until you deduce what the issue really is. Possibles off top of my head/from various experiences:

• Yes, the battery drain is there, but it probably was not that. Mostly that functions as a vent for hydrogen gas. Only if charging system/alternator gets really whack that it overcharges the battery and then essentially boils over. Had that happen once 30 years ago; basically repair error and not nearly enough volume to fill that area. It is acid of course so you would probably have felt the burn.
• I forget if the sunroof drain comes down through there (on each rear side), but that is another possible. Could (carefully...) run water down through passenger rear corner of roof opening to ensure it comes out onto the ground--or if it fills that area again and needs fixing.
• Something wrong with the rubber seal around the gas tank lid area--look at that carefully from outside.
• Leaky taillight seal--the outer one of course. BTDT on some prior Audis. Taillight is not very hard to remove, and you may want to do that.
• Flow through ventilation air vents on each side of the the trunk at the back corners. They are low down under the tail lights behind the bumper cover. You really dont see them as a result. They have delicate plastic flaps on them that are designed to open when air pushes rearward, but stay closed at rest. If there was ever any body work, bumper change, etc. back there, that vent grill could have been messed up. I came across it when I put the hitch on mine and had bumper cover off. More delicate than I would have guessed.

The electronic box under the battery could be a number of things. On a W12 it is for the rear seat video that was W12 standard--basically an overdone relay. When you put in a trailer hitch with factory wiring, the same location is used for the same box by part number, but now it becomes a trailer lighting relay/controller. If you have neither feature/option on your 2005, I couldn't tell you what it then does. In 2005 I'm guessing you had neither unless a one off retrofit. But if it is the same form factor box, logically a controller relay type switch for something. On my W12, box had two electrical connectors on the same edge, one with more terminals than the other. It was about an inch thick and something like 4"x6" in the other dimensions, held in with two 10mm little bolts into one face of the case underneath.

SteveoH

Wow; thanks for all the suggestions! I'll check these out, and if I find something interesting, I'll report back.

haggisuk

Some great Info to go at there from MP.

my two cents too add would be the rear screen bonding failing as has happened on my car and a few others that I know of.

I found mine first by tell tale condensation on the bottom of rear window then one day while it was pouring down with rain the left side of the boot where all the amps etc live had water dripping in and shorted the navigation drive.

I proved the source of the leak by having the wife running water from hose over bootlid then up the sides and finally over the top of the window while I lay inside the boot with the lid closed and a flashlight.

the module you mention could be the handbrake control module but once you go investigating again a quick Google of the part number will confirm it's purpose.

good luck

CamFix

Hi SteveoH.
A friend of me had the same problem on a -06 and to him it was the tail-light seal. The light had fogged up also.
Good luck on this one.

03_AR_CO

+1, under the battery is the EPB (Electronic Parking Brake) Module, replaced mine in my '04 D3

SteveoH

Good morning;

Again, thanks for the replies and suggestions, guys. I'll update when I find and correct the problem. Hopefully, it's the tail light seal; seems like the easiest to access!

Regards,