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B5 VW Passat TDI battery replacement and upgrade-94r size
How to test your VW battery and battery removal
back to 1000q: B5.5 VW Passat TDI "how to" index
This article shows how to test and remove the battery on a VW Passat and upgrade it to 94R size.
WARNING: The battery contains electricity and acid and can spontaneously explode! Lead acid batteries make hydrogen gas and they can explode, sometimes with no warning, sending sulfuric acid spraying everywhere. Always wear eye and skin protection when handling a battery. Remove or insulate all rings, necklaces, watches, etc., before working near the battery since accidental contact with the positibe terminal and a ground can cause electrocution or serious injury.
The alternator charges the battery which provides the electrical needs of your car. Symptoms of a bad alternator would be the battery warning light coming on, less than 14V at the battery when the engine is running, or dimming headlights when the electrical load is higher than normal (and you know the battery is good). If the battery is weak, engine rpm when starting can be too slow. The engine computer sees this and does not inject fuel. Luckily, the TDI has relatively low electrical load since there are no sparkplugs.
If you are having a charging problem, it's either the alternator, battery, or grounds. The battery voltage when the car is off should be over 12V. The battery voltage when the engine is running should be at least 14V even at idle. It could also be a loose battery terminal or ground that prevents proper charging of the battery. If you are not sure, take the alternator to an auto parts store because many places will check it for free. Many places can even check the charging system on the car.
The stock VW Passat TDI battery is a 72ah but it also accepts the 90ah battery in the 94r size, same as the Audi A4. I bought this because it was cheaper - to secure it you have to change the tie down bracket (VW or Audi# 8d1 803 123 d).
If you have any questions, please post it in the VW Passat TDI forum in the myturbodiesel bulletin board!
Testing the battery
Battery voltage should be about 13.5-14V with the engine running. The system works on 12V and the battery acts like a voltage damper. Do not run the car without the battery. With the engine off, battery voltage should be about 12.5V and no load. With electrical load, voltage should steadily drop to about 9V.
Testing the alternator
See 1000q: alternator testing and repair
Removing the battery
Remove the rubber engine gasket and this piece of plastic trim.
Remove the 1x 13mm bolt holding the battery clamp.
Remove the 1x 10mm nut holding the negative (on the left as you're looking at it) terminal cable first. Set it aside and insulate it so that it can't accidentally touch anything. Do the same with the positive terminal.
I suggest removing the spring clip above the battery holding the windshield trim to the metal. This will let the windshield trim move a little and make it easier to pull the battery out.
Look for a built-in handle on the battery to help you pull it out. You have to tilt the left side (closest to the air filter) up and slide it out right against the air filter. Watch out for the black knob on the AC line!
Installation is the reverse of removal. While you're here, clean out all the leaves in this area and flush the drains. If the drains clog, it will cause water to back up into the interior. One of the drains is under the battery.
Note the small lip on the battery tray - the new battery has a lip to rest against it.
Replacement with 94r battery
Simply use the new tiedown with the old 13mm bolt.
Here is what the tiedown looks like.
Here is a few from the brake booster side showing how it sits.