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How to Diagnose Belt or Pulley Squeal or Squeak – BMW and MINI

September 6, 2013

Is There A Hungry Mouse … Or A Cat Caught By The Tail …. Under Your Hood?

Plug your ears if your BMW has belt squeal

There’s a Serpent on my engine?

Most BMWs from the early ’90s-on and all MINIs employ the, now common, serpentine type accessory drive belt systems.  The serpentine belt system consists of a wide flat belt with multiple grooves (or ribs) on the inside running surface.  The drive pulley (on the engine’s crankshaft) will have the corresponding grooves to mate with the belt’s grooved/ribbed surface.  The driven pulleys (water pump, alternator, power steering and air conditioning compressor) may be grooved to mate with the belt’s ribs or smooth to run on the belt’s outer smooth surface.  Finally, the tensioner and idler pulleys may also be grooved or smooth.

BMW M54 serpentine drive belts and pulleys BAVauto

Ignoring what your BMW or MINI is trying to tell you, may cost you more than a slight inconvenience….

As the serpentine drive system wears, it is not uncommon to begin to hear squeaking, squealing or other belt and/or pulley related sounds.  Consider these noises as a polite warning from the belts and/or pulleys.  Ignoring these warnings will eventually result is a failure of the belt or pulley.  At best, it may be inconvenient and prevent you from continuing your drive.  At worst, it can cause much more damage (due to the belt flailing around and wrapping itself around other components) and even complete engine failure (due to engine overheating). Diagnosing the source of the sound can be a bit frustrating as it could be emanating from not only the serpentine drive system, but it could be from a worn alternator or other part of the driven accessories. In light of this, let’s take a look at some basic diagnostics that we can do to help us narrow down the source of the sound and ultimately replace the proper parts to cure the problem.


1)  With the engine running (and the noise present) use an atomizer bottle to spray the inner ribbed surface of the belt with a small amount of water.

  • If the noise stops until the water dries up, then returns, there is either a miss-aligned pulley (first suspects would be worn idler or tensioner pulley bearings) or the belt’s and/or pulleys’ ribbed/grooved surfaces are worn and need replacement.  This can be the opportunity to replace all of the idler and tensioner pulleys, as well as the belt, as a regular maintenance procedure or further diagnosis can be employed to attempt to narrow down to a specific part or assembly.  In this case, use a mechanic’s stethoscope (with an airborne sound, megaphone attachment) to determine if the sound is more pronounced at an individual pulley.
  • If the noise gets louder or more consistent, the belt is likely either worn out or may be loose.  We would recommend replacement of the belt and the tensioner.
  • If the water has no affect on the sound, there is either a worn bearing in one of the idler or tensioner pulleys, one of the driven accessories is at fault, the outer surface of the belt is worn (see step-2) or the sound is not coming from the serpentine accessory drive system at all.  Use a mechanic’s stethoscope to determine if one of the pulleys or driven accessories is creating the sound.  Replace the offending pulley or driven accessory.

Click HERE for DIY video on replacing typical BMW Serpentine belts and pulleys.

2)  Lightly spray water on the outside of the belt.  If the sound stops or changes and then returns as the water dries, the belt is likely worn.  Replace the belt.

3)  If all of the above steps are inconclusive, remove the serpentine belt(s), one at a time and start the engine after each belt is removed. When the main belt that drives the water pump and alternator is removed, do not run the engine for more than a minute.

  • If the sound stops when one of the belts is removed, we know that the sound is either coming from the belt, one of the idler or tensioner pulleys or one on the driven accessories.  Go back to re-test or just replace the pulleys and the belt.  If the sound remains, it is from one of the driven accessories (which you really should be able to determine through use of the mechanic’s stethoscope).
  • If the sound does not stop as the belts are removed, the sound is not coming from the belts, pulleys or the driven accessories.  Further diagnosis is required.

Click below for Mechanic’s Stethoscope:

Lisle mechanic's stethoscope 52750 from BAVauto

Click below for serpentine belts:

Serpentine accessory drve belts for BMW and MINI BAVauto

Click below for tensioners and pulleys:

Serpentine accessory drve tensioners and Pulleys for BMW and MINI BAVauto

Bavarian Autosport

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