1 Answer

This is for an  S420 but the instructions are the same for pretty much any car. 

Instructions
1
Disconnect the cable running to the negative battery terminal. Loosen the nut on the cable clamp running to the negative battery terminal with a socket wrench and slide the clamp off the terminal.

2
Open the hood and look straight at the front of the engine. You will see a belt that "snakes" through a system of pulleys. This is the the serpentine belt, also called a "drive belt." It needs to be removed in order to remove the alternator. Draw a diagram of the belt path and orientation. You will need this diagram when reinstalling the belt.

3
Remove tension on the drive belt with a socket wrench and a 14mm socket. The tensioner is on the driver's side of the engine, near the front of the engine. The tensioner bolt protrudes out of the engine's pulley system at a 45 degree angle, with a little bit of a hex shape at the top of the bolt and a small hole in the middle of the bolt. Turn the bolt counterclockwise to remove tension on the belt.

4
Pull the belt off the pulley system.

5
Remove the electrical connector running to the alternator.

6
Remove the alternator. Remove the upper and lower alternator mounting bolts. The alternator is mounted on the front of the engine.

7
Install the new alternator. Installation is the reverse of removal. Make sure that you orient the drive belt properly when installing it.

8
Attempt to start the engine. As long as the battery was not drained entirely when the alternator failed, you should be able to start the engine. If the battery is dead, you may need to install a new battery. To do this, remove the cable clamps on the battery terminals following the procedure in Step 1. This time, you'll be removing both the positive and negative cables. Always remove the negative cable first and attach it last. Once the cable clamps have been removed from the terminal, remove the battery from the battery holder and drop in a new battery. Then, reconnect the battery terminals and tighten the cable clamps.

9
Check to make sure that the battery light does not stay on once the Mercedes is running.

Read more: How to Replace an Alternator in an S420 Mercedes | eHow.com http://www.ehow.com/how_5502439_replace-alternator-mercedes.html#ixzz2ErPAY5Cv