The third generation Nissan Altima, model-years 2002-2006, uses a single serpentine belt to power the alternator, power steering pump, water pump, crankshaft and air conditioning. The belt tension is automatically adjusted by a pulley tensioner located near the center of the other components. Since this belt is so critical to the operation of your 2003 Altima 2.5, you should check it for wear frequently and replace it when it appears worn.
Locate the automatic tensioner. As you look down at the belt, the tensioner is the pulley most centered between the other pulleys, directly above the water pump.
Note the routing of the serpentine belt. There should be a diagram on the inside of the hood. If not, create your own diagram. Starting from the top-most pulley, the A/C, and moving down, the belt routes in this manner: A/C, crankshaft, tensioner, power steering, water pump and alternator.
Place a box-end wrench over the nut on the automatic tensioner. Move the wrench clockwise to take tension off of the belt. You can use a socket wrench to do this, but sockets often slip off the tensioner bolt. Insert a screwdriver into the holding boss from rear to front. This will prevent the tensioner from exerting tension on the belt. The holding boss is located slightly left and above the tensioner, just below the compressor pump pulley.
Remove the belt from the other pulleys by simply slipping it off them. Route the new belt into place following the correct routing, with the water pump being the final pulley.
Release tension on the screwdriver by turning the tensioner clockwise with the wrench. Remove the screwdriver and slowly release tension on the wrench. The tensioner will automatically apply the correct amount of tension to the belt.
Rotate the crankshaft pulley clockwise several times to equalize tension between all the pulleys.
