The alternator on the Audi A4 is responsible for powering the electrical components in your car. Most people think the power comes from the battery, but that's not the case. Your car requires the battery to start it, but after that, the alternator takes over. If the alternator goes bad, you will notice the lights going dim and eventually the car shutting down after the battery is dead. Not only does the alternator generate the power for distribution to your electrical components, but also it is responsible for keeping the battery charged so it is ready when you need to start the car.
Disconnect the battery terminal with a wrench. Label the wires on the alternator for proper placement on the new alternator using masking tape and a marker. Disconnect the wires using a wrench to remove the nuts.
Loosen the adjusting bolt on the alternator using a socket and ratchet. Push the alternator toward the engine and pull the alternator belt off.
Remove the bolts from the alternator using a socket and ratchet. Pull the alternator off of the engine. You may have to work it back and forth to get it out of the car.
Put the new alternator in place and loosely install the bolts. Put the alternator belt on and then pull the alternator toward the front of the engine to tighten the belt. Tighten the adjusting bolt with the socket and ratchet once you have the belt drawn tight. Tighten the bottom bolt with the socket and ratchet.
Reconnect the wires according to the labelling and tighten the nuts with a wrench. Reconnect the negative battery terminal.
The factory sound system is security coded so the set will not operate if someone steals it. Be sure to note the code before you disconnect the negative battery terminal so that you are able to reactivate the stereo when you are done.
Never attempt this procedure without disconnecting the battery as instructed. Failure to do so may result in electrical shock.