It’s all down to torque!

Helpful hints for alternator installation

In our last article, we explained how—in the worst case—oil residue in the alternator can cause a short circuit. However, it’s not only oily liquids that are harmful to the alternator but also careless installation. Here, we explain how to do it correctly.

Be gentle: don’t use an impact wrench!

If the shaft fractures and separates during an alternator replacement, this can usually be attributed to improper mounting. When replacing an alternator (type MG), you should therefore pay very close attention to the following points.
Sometimes, it is necessary to fit the belt pulley from the old alternator onto the new alternator. If that is the case, on no account tighten the nut with an impact driver—otherwise, the shaft may fracture. However, if there is not enough tightening torque applied to the nut, this may cause serious damage to the alternator, since the inside ring of the roller bearing is braced against the shaft by means of the pulley nut.

  • Separated thread
  • Only use an impact driver to loosen the belt pulley, NEVER to tighten it!
  • Ground-down shaft in the area of the inner ring of the roller bearing

No stress: no contact between rotor and stator

The right way to do it: use a hold-down tool and torque spanner to tighten the pulley

Without the correct prestressing, the shaft wears quickly and ultimately begins to wobble. The rotor and stator can thus come into contact, eventually causing a short circuit and therefore the complete failure of the alternator.

Therefore: Please always use an appropriate hold-down tool (hexagon socket or multipoint) to secure the nut to the belt pulley, and use a torque spanner to tighten the alternator shaft according to specifications.

Tightening torque:
M16 x 1.5: 95 Nm (±5 Nm)
M27 x 1.5: 152 Nm (±17.5 Nm)

Align correctly to keep the right spacing

If reusing the old belt pulley, first check it thoroughly for damage and wear. When mounting the belt pulley, it is important to ensure it is in the correct position. You can customize the alignment using the washers.


We regularly provide technical tips relating to the powertrain, thermal management, and mechatronics in automobiles.


MPULSE - The magazine

The print issue of MPULSE will come out twice a year.