Why do I hear a squealing noise from the serpentine belt when I start the engine?

Steve Mowat

Steve Mowat

Steve Mowat, Owner of Avantgarde Automotive, began as a BMW apprentice in 1997, then excelled at McLaren and Mercedes-Benz. In 2009, he founded his business.

The Anatomy of the Serpentine Belt System

As a proud owner of a Mercedes Benz, it’s essential to understand the intricacies of the serpentine belt system in your vehicle. The serpentine belt is a single, continuous belt that drives multiple peripheral devices in your engine, such as:

  • Alternator
  • Power steering pump
  • Water pump
  • Air conditioning compressor

The belt is subject to wear and tear, and its condition directly impacts the performance of these components.

Common Causes of Squealing

The dreaded squealing noise you hear when starting your Mercedes may have several causes. Here are the main culprits:

Misaligned Pulleys

Misaligned pulleys can cause the belt to squeal, as it doesn’t travel in a straight path. For example, the M271 engines (found in the C-Class and E-Class) often suffer from this issue. To resolve the problem, you’ll need:

  • A straight edge or laser alignment tool
  • A spanner or socket set for adjusting pulleys

Align the pulleys by loosening the mounting bolts and adjusting them until the belt runs true. Don’t forget to tighten the bolts afterward.

Worn or Damaged Belt

A worn or damaged belt might produce a squealing noise. Check the belt for cracks, fraying, or glazing. Replace it with the correct part number:

  • M271 engine: Mercedes Part No. 0069976992
  • M274 engine: Mercedes Part No. 0009931076

Incorrect Belt Tension

Inadequate tension can cause the belt to slip and squeal. Newer Mercedes models have an automatic tensioner, while older models might have a manual tensioner. Ensure the tensioner is in good condition and functioning correctly.


Oil, coolant, or other fluids can cause the belt to slip and create noise. Clean the belt and pulleys with a degreaser, and ensure there are no leaks from nearby components.

Worn or Faulty Pulleys and Tensioners

A worn or seized pulley or tensioner may cause the belt to squeal. Check for play or roughness in the pulley and tensioner bearings. Replace any faulty parts with genuine Mercedes components.

Preventative Maintenance Tips

Proactive care can help prevent squealing noises and ensure your serpentine belt system performs optimally. Consider these tips:

  • Regularly inspect the belt for wear and damage
  • Keep the belt and pulleys clean and free from contaminants
  • Check pulleys and tensioners for signs of wear or damage

Schedule regular servicing appointments with a trusted Mercedes specialist to address potential issues before they become more significant problems.

When to Seek Professional Help

While some of these issues can be resolved by a confident DIY enthusiast, it’s essential to know when to consult an expert. If you’re unsure of your ability or the problem persists, consult a Mercedes specialist to diagnose and resolve the issue correctly.

Unleash Your Vehicle’s Full Potential

Don’t let a squealing serpentine belt dampen your driving experience. Follow these tips, and remember that professional advice is invaluable. Keep your Mercedes Benz in top condition and enjoy the open road.

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