If a car makes a squealing noise after the alternator has been replaced, it often needs the serpentine belt or belt tensioner replaced. Continuing to drive the car without repairing the problem often causes early failure of the replacement alternator due to a perpetually undercharged battery. Drivers should have a technician check both parts and replace what is necessary to correct the squeal.
The serpentine belt provides power to the vehicle's water pump, air conditioner, power steering and alternator. The tensioner is a spring-loaded device that holds the belt tightly against the pulleys of each of these accessories. The spring becomes weaker over time and allows the belt to slip when the demand for power for these accessories is high.
Many car owners complain of a squeal after starting that goes away as the car reaches normal operating temperature. Starting the vehicle draws power from the battery that the alternator must replenish, which increases the chances the belt will slip. Even if the squeal goes away, the alternator is working harder than necessary to charge the battery, and the battery remains undercharged.
Serpentine belts stretch less than older v-belts, but over time show signs of wear. When replacing the alternator, it is a good idea to replace the belt. The housing of the tensioner has marks that show the limits of its operating capability. If the indicator is outside of the marks, owners should have the belt replaced.