Function and assembly of the alternator freewheel clutch

Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their work. The information provided on this website is intended for use by suitably qualified personnel only.


During an engine's combustion cycle, the rotary movement of the crankshaft is sped up and slowed down This rotational irregularity is transferred to the accessory drive as a result of the alternator's moment of inertia As a result, extreme forces and high torque fluctuations act on the belt drive.

Function and structure of the alternator freewheel clutch: Function

The overrunning alternator pulley, or alternator freewheel clutch, is mounted directly on the drive shaft of the alternator instead of the rigid pulley and has the task of decoupling the three-phase alternator from the rotational irregularities of the crankshaft. This reduces the influence of the moment of inertia on the alternator and the load on the belt drive.


The overrunning alternator pulley is structured as follows:

  1. Inner race with serration
  2. Outer race with profiled raceway
  3. Freewheel unit
  4. Radial support bearings
  5. Overrunning alternator pulley

Advantages of the alternator freewheel clutch: Benefits

By mounting an overrunning alternator pulley, only the driving force of one direction of rotation is transmitted to the alternator, thus reducing friction and wear.


Advantages of an overrunning alternator pulley:

  • Reduced impact on the belt drive
  • Reduced fuel consumption
  • Longer lifetime for all components
  • Greater driving comfort and improved noise levels
  • Higher alternator idle speed


Effects with and without overrunning alternator pulley:

  • Accessory drive with rigid pulley (A): Severe vibrations and resultant thrashing noise from the drive belt
  • Accessory drive with alternator freewheel clutch (B): Reduction in belt vibrations

Installation instructions for alternator freewheel clutch: Mounting instructions

Article numbers: 9XU 358 038-xxx and 9XU 358 039-xxx


We recommend the following procedure:

  • Switch off engine
  • Remove ignition key
  • Remove the V-ribbed belts
  • Remove the cover cap on the alternator freewheel clutch
  • Fix the overrunning alternator pulley in place
  • Insert installation tool
  • Rotate installation tool
  • Loosen and remove the alternator freewheel clutch
  • Mount and tighten the new alternator freewheel clutch (tightening torque 80 +/- 5 Nm)
  • Fit cover cap and V-ribbed belts
  • Perform functional check

Refer to the removal and installation instructions from the relevant
vehicle manufacturer.

  • Installation may only be carried out by a qualified specialist
  • Note tightening torque
  • Do not use pneumatic tools