Problem:

These pictures show fan belt material on the front of the drive end plates of the alternators. This is caused by fan belt slippage.

Slippage of the belt can be caused by a loose or glazed belt, worn or misaligned pulley. Often this can be detected by a shiny or discolored pulley (Blue) or this black belt material. Pulleys can be misaligned with the other pulleys on the engine when the installer changes them from one alternator to another. The proper spacers must be used or removed as required.

Solution:

Often fan belts that appear to be tight may not start to slip until alternator is under full load. It is extremely important to inspect the old alternator and pulley before replacing them. If the belt is worn, glazed or cracked, REPLACE IT.

Make sure the belt tension is correct. Applications without automatic belt tensioners must be adjusted manually.
Applications with automatic belt tensioners must be inspected to see if they operate correctly, if not, REPLACE THEM.