The pulley and Free Wheel Pulley

An alternator pulley transfers the mechanical energy from the crankshaft to the alternator shaft through a grooved belt.


Operation of the pulley

The alternator pulley is a key part for the alternator operation, it recharging the battery and supply the consumers in the car when it is running. The pulley is subjected to acyclic rotation of the crankshaft, these acyclic result in a variation of the alternator torque.

Design and technology

  • The rigid pulley in monobloc steel does not absorb acyclic. It may differ depending on the number of "V" grooves which allows you to change the amplitude of the torque ratio received by the alternator.


  • Clutch-free pulleys are designed to aid the absorption of rotary and axial loads created by certain direct fuel injection engines. Its function is to prevent jump/whip of the accessory drive belt created by excessive engine vibrations and rapid changes in engine speed. This type of pulley is fitted as original equipment by the vehicle manufacturer, but is not a Valeo manufactured pulley. The pulley is constructed from rubber and sealed around a metal central core. These pulleys are known to fail after approx 20 000 miles on average, the common cause being deterioration in the bonding of the rubber to the metal core. Failure of the pulley can cause the drive belt to jump off the pulley. This could lead to the belt becoming entangled with the engine pulley, which may result in engine damage. In addition, because the assembly of the clutch-free pulley onto the alternator requires specific tools belonging to the car manufacturer, the inconsistency in the tightening torque of the pulley increases the risk of further vehicle breakdown.


  • The Overrunning Alternator Pulley (OAP) have an internal clutch allows the rotor of the alternator to coast to a stop when the engine is shut down.  This “overrunning” feature eliminates “chirp” sounds that occur when the engine decelerates quickly, causing the belt to slip (engine shut down or transmission shifting). 


  • (Overrunning Alternator Decoupler (OAD) it also incorporates a torsion spring to absorb energy reduced noise and reduce vibration.




  • In the belt drive clockwise CW direction, the outer ring must NOT rotate. IF NOT Change the Freewheel pulley
  • In the belt drive counter clockwise CCW direction, the outer ring must rotate freely. IF NOT Change the Freewheel pulley

Valeo recommends changing the freewheel pulley every 90,000 Kms.

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