Tips for replacing the alternator

Replacing the alternator might sound easy, but in practice it is often more complicated than you think. If the alternator fails, there are some points to consider. We’ll tell you about them in this article.

Make a diagnosis

The DIAGNOSTICS are carried out prior to any replacement. First, always check whether the alternator is really faulty or if a fuse may have blown, the battery is flat, or the drive belt is simply loose.

To do this, you need to review the following points:

  • Is the battery charged and in good condition?
  • Are the battery leads correctly connected and undamaged?
  • Are the electrical connections properly tightened and free of corrosion?
  • Are all screws and nuts properly tightened?
  • Is the drive belt undamaged and properly tensioned?
  • What does the area around the alternator look like? Is the engine compartment very dirty, or is there any leakage of oil or fuel?
  • Dust and other contaminants reduce heat dissipation and can result in thermal overload of the alternator.
  • Clogged rear side
  • Cracked drive belt

Procedure for the disassembly of the generator

Always remove the earth cable first

If, after inspection, it turns out that the alternator really needs to be replaced, then there are a couple of important points to bear in mind for the DISASSEMBLY:

  • Only work on the alternator when the power is switched off. Therefore, always disconnect the battery.
  • Before disconnecting the battery, it is important to know, for example, the radio code and other system settings so they can be reset afterwards.
  • Always follow the sequence of steps given in the vehicle manufacturer’s instructions when disconnecting the battery.
  • Remove the drive belt.
Label connections

IMPORTANT: Note the running direction of the drive belt when it is reinstalled.

  • Remove any other connecting cables on the alternator. Label connections and lines if necessary, to avoid confusion and reverse polarity.
  • IMPORTANT: If the alternator is replaced without the electrical circuit being interrupted, this can result in expensive short circuits and damage to the electrical system.
    >> see Technical Messenger 04/2018

The following points must be checked before the new alternator is installed:

  • The new part must match the old alternator that was removed. Check the part number using the catalog. Also check the direction of rotation, the rated voltage, the rated output current, the mounting brackets, the type of belt pulley, and the electrical connections.
  • The battery terminals and cables must be free of damage or corrosion.
  • The drive belt and tension rollers must not show any signs of damage.

Last but not least:

During INSTALLATION there are again a few more points to check:

  • The cables must be connected according to the labels. Cables must not come into contact with any hot or moving parts.
  • All mounting bolts must be tightened with the correct torque.
  • If the belt pulley/overrunning clutch has been retrofitted, this also has to be tightened with the correct torque. Otherwise there is again a risk of severe consequential damage. >> see Technical Messenger 08/2016
  • The drive belt must be correctly aligned. If the old drive belt is reinstalled, the running direction must be correct.
  • Only reconnect the earth cable to the battery at the very end.
  • The correct torque is important
  • Check the alignment of the drive belt

Does it work?

After successful installation, test the FUNCTION:

  • Is the battery charging?
  • Is the charging voltage correct?
  • Is the battery charge indicator in the electronic instrument panel off?
  • Is the drive belt fitted correctly?

Please note: Alternators and engine oil do not go well together—
especially if there’s a spill during the oil change and oil ends up on or in the alternator.

You want more?

More MPULSE articles about generators can be found with just one click via the direct links below.

The MAHLE effect to counter alternator failure

Always powered up

Details of how an alternator functions can be found in this article "Always powered up".


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


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