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Q: Electronic control units dysfunctional, probably because of water damage on 2003 Volkswagen Passat W8

I am located in Melbourne, Australia, and my W8 Passat has done 142,000km. It has just had the original battery replaced, but upon replacement, the electrical system no longer functioned properly. The rear vision mirrors, central locking, power windows, etc. no longer worked. The VW service people told me that the control unit under the passenger side floor had possibly been water affected. Can I access this myself, under the carpet, or is access possible only from under the car?

I have opened the control unit in the boot, which had water in it, cleaned it up with a toothbrush and dried it out. The central locking, air conditioning and rear vision mirrors are now working. The fuel gauge, however, is not, and the trip gauge is registering odd numbers (actually going backwards). Can you please tell me which functions are controlled by the unit under the floor and which are controlled by the unit in the boot?

The car is due to be returned to the VW service centre soon, so that they can continue to try and remedy the problems, but in view of my success with drying out the control unit in the boot, I would like to have a go at resolving the other problems myself.
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I can tell you that if there's a control unit under the seat, you'll find it above the floorpan, not beneath it. I'm glad to hear that you've had some success with drying the unit in the rear, but once there's water intrusion, all bets are off concerning electronics. Good luck with it. give it a try, you could get lucky. I've heard of an odometer running backwards before but I didn't quite get how that could occur. Electronics can be a mystery. For sure, get the water intrusion leak fixed or there'll be no end to your trouble.
In the spare wheel well of your W8 there is a sender unit for the fuel tank. Due to the unusual shape of the tank it has 4 level sensors in it which all feed back to this sender. My own car had water in the wheel well which killed the unit, and had to be replaced. Its encased in polystyrene under the spare wheel and is very easy to remove. I took mine out popped the cover off and had a look inside, it was beyond cleaning wth a toothbrush. Think its worth a look.
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