The alternator is run off the engine belts and charges the battery. The battery provides the car with direct current electricity. Symptoms of a bad alternator would be the battery warning light coming on, less than about 14V at the battery when the engine is running, dimming headlights when the electrical load is higher than normal (and you know the battery is good), immobilizer false triggers, or clicking when you try to start the car (and the engine doesn't turn over). If the battery is weak, engine rpm when starting can be too slow. If the engine computer sees this it prevents injecting fuel and engine start. Luckily, the TDI has a relatively low electrical load compared to many cars since there are no sparkplugs.
If you are having a charging problem, it's either the alternator, battery, wiring, or bad grounds. The battery voltage when the car is off should about 12.6V. The battery voltage when the engine is running should be at least 14V even at idle. 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. A common problem is a bad alternator pulley. With the engine not running and the alternator belt removed, use a wood (or another soft material) dowel to jam the fan inside the alternator. The alternator pulley should turn one way and not the other due to an internal clutch. If it spins both ways, replace the alternator pulley.
Batteries usually last around 5 years, 3 in extreme heat and up to 7 years in optimal conditions.
For more alternator troubleshooting and replacement, see 1000q: alternator and voltage regulator replacement
If you have any questions on testing the battery or removing it, please post your question in the VW and Audi TDI forum
Remove the battery cover.
Remove the battery terminals by loosening the 1x 10mm nut on each terminal
and gently wiggling them off. Always remove the negative side first!
Don't let a wrench or other metal touch both terminals at the same time.
Here you can see the dark green "magic eye" hydrometer port that is on OEM
batteries. It lets you see if there's enough water and charge. If it's
black it's low. If it's yellow or colorless it needs distilled water
immediately. The service manuals says this is only for batteries younger
than 5 years old so I assume the magic eye may not work correctly after that.
If you already removed the air intake box, look beneath where the accordion
hose was and remove the 13mm bolt (yellow arrow below) holding the battery tie down clamp.
Remove them both. If the air intake box is in the way, use a magnet during
removal so that the bolt or tie down won't fall into the engine bay.
Remove the battery by wiggling it and then pulling straight up. There may be straps or flip out handles like the ones shown in the first picture. Don't bother putting it on a block of wood because putting the battery on the ground won't make it go bad.
NOTE - Every time you disconnect the battery some settings are reset. This includes the power seat memory, one touch windows, and steering adaptation. Click the links for more info on how to reset them.
Remove the air intake box. See 1000q: air intake box removal for more details.
Remove the 3x 10mm bolts (yellow arrows) holding the battery shelf. You can pull the
front of the battery box (the plastic part) out but the shelf will come out just fine with
it on too.
You should be able to wiggle the shelf out. If you can't, you can
remove the alternator wire at the fusebox for clearance. I suggest wiggling it out since it's one less thing to remember and mess up. Remove the fuse box cover by sliding the locks towards the rear of the cover
and lifting the cover off. Remove the 10mm alternator fuse nut pictured below (yellow arrow).
It's holding down the corner of the battery shelf. There's also a plastic
hook that holds the cable in place (yellow arrow). Place it to the side
In case you're curious what the fuses are, here is a diagram. The car
has electric power steering assist to save energy (fuel) by only giving you
assist when needed vs. constantly running an engine driven hydraulic power
steering pump. The aux heater is an interior cabin heater that is
activated when it's cold and econ isn't on. It won't blow hot air but it's
enough to not cause your breath to freeze on the windshield on icy mornings.
Installation is the reverse of removal. The tie down clamp is torqued to 15 ft-lb and the battery terminal nuts are torqued to 80 inch lbs (about 6 ft-lb). Make sure to replace the 10mm nut on the fuse box for the alternator if you didn't wiggle the shelf out. If you have any questions on how to remove the battery or fusebox on the Jetta or Audi A3 TDI, feel free to ask in the myturbodiesel Audi A3 TDI forum or search below: