Question about 2008 Volkswagen Passat Sedan

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The battery on my 2008 Volkswagen passat keeps draining. I have replaced the alternator and replaced two batteries so far but the problem is still there.

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  • Donald DCruz
    Donald DCruz Jul 13, 2010

    It was important to find the cause of the batterry drain before you replaced the battery. Since being a confirmed fault the elimination process check is required. First check the leads of battery disconnect the positive and connect an ammeter. check for the current when there is no device switched on. the culprits to look for could be the audio /video system or the auto lock,remote switching circuits. Disconnect and check if the idle current is higher. once you have confirmed that the drain is Ok, then you will need to check the charging current, connect and check if the alternator is not over charging/ undercharging even with your accessories/ lights/wipers/ heaters switched on. test on idle and higher RPM. If it shows low readings then check for the voltage convertor/ regulator electronic unit. Also make sure that you are not leaving the park lamps on, the engien is cranking too much to start specially in the morning . if so retune your engine for quick start, this must solve your issue. Good day



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I've had a golf with a faulty brake light switch which came on and drain battery down, turn your ignition off and remove key placed your foot on brake and get someone to check if the brake lights turn on if they do this might be the problem, replace the switch really easy and cheap, goodluck Dom

Posted on Feb 14, 2013

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You need to find the source of the drain on the battery. To do this you will need a multimeter set up to read DC current (ammeter from zero to 1amp range). Switch engine off, switch radio off and disconnect the negative lead to the battery terminal. Connect negative lead of ammeter to negative battery lead and the positive lead of the ammeter to the negative battery terminal. With all electrical items switched off the current reading should be very low, no more than just a few milliamps to keep the dashboard clock running etc. If there is an appreciable current flow locate the fuse box and remove and replace each fuse in turn, note any changes in current and note down the fuse(s) that result in the greatest drop when the fuse is removed. Now switch on an interior light (open a door) just to show you how much extra current is drawn from the battery as a result of its coming on. Is this the sort of current being drawn from the battery when the door is closed? Check out that the circuit with the greatest current drain has a legitimate reason for being on (car alarm maybe). If there is no reason for the current draw then find the item left on when it should be off. A classic case is the trunk (boot) light or the glove box light being left on when the lid is closed. These lights are disconnected by contact switches that open (break the electrical circuit)) when when the lid is closed. Checking the trunk light is easy, just climb in and ask a friend to open and close the lid. The car doors too have contact switches by the hinges and these determine when the interior lights come on or off etc. Check all contact switches by pressing them open by hand and registering the current changes with their opening and closing.

Posted on Jul 13, 2010


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