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How to check and adjust a drive belt

Updated July 19, 2017

A drive belt is a critical component of a well-functioning car engine and alternator. An over stretched belt can quickly wear and damage an alternator bearing; a loose belt may slip, become noisy, make the alternator vibrate and wear quickly. It is the alternator's job to recharge the battery, provide electricity when the engine is running, and adjust voltage output to meet the needs of electrical components around the engine. So you can see how a maladjusted or worn-out drive belt can stop your engine dead in a matter of seconds. You need a simple but effective method to check the drive belt and adjust it, if necessary.

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  1. Find the alternator drive belt at the front or right hand side of the engine compartment on the passenger side of the vehicle.

  2. Slide your fingers along the edges of the drive belt and hard to see areas, trying to feel for cracks and detached plies. Use a torch to make a visual inspection as well.

  3. Watch for fraying and shiny spots on the upper and underside of the drive belt which also indicate wear. Twist the belt to see underside areas. All these symptoms are signs of drive belt breakdown and a warning for a prompt replacement.

  4. Look for torn or missing rib sections on serpentine belts-also called V-ribbed belts. Cracks along the ribs are not cause for concern, but missing rib sections 1.3 cm (1/2 inch) or longer and cracks running parallel to ribs indicate serious wear. Replace the belt.

  5. Check the drive belt tension. Measure the distance -- from centre to centre -- between the two pulleys with the largest free span of drive belt. Write down your measurement in a small notepad.

  6. Place a straightedge atop the longest free span of the drive belt and using a ruler, push the drive belt down around the centre of the span.

  7. Write down the deflection reading indicated by the intersection of the straightedge against the ruler.

  8. Compare your numbers. If your measurement from Step 5 was 18 to 28 cm (7 to 11 inches), the belt deflection should be around 1/2 cm (1/4 inch). If your measurement from Step 5 was 30 to 41 cm (12 to 16 inches), the belt deflection should be around 1 cm (1/2 inch).

  9. Adjust the belt tension if your reading is out of specification. Loosen the mounting and adjusting bolts of the alternator with a spanner or ratchet. Using a steel bar, pry on the end frame of the alternator, holding the tension while tightening the adjustment bolt. Check the tension again and tight the mounting bolt when within specification.

  10. Try turning the alternator pulley with your hand. If the drive belt slips, replace it.

  11. Tip

    If you can afford it, buy a drive belt tension gauge which gives a more accurate tension reading. This tool is available at most car parts shops. If you are buying a new drive belt, make sure the new one has the same length, width and configuration as the old one.


    Some vehicles are equipped with an automatic tensioning system for the drive belt, while others require a different adjustment method from the one outlined in this article. Check the service manual for your particular vehicle to make sure you are following the correct procedure.

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Things You'll Need

  • Torch
  • Notepad and pencil
  • Straightedge
  • Ruler
  • Spanner or ratchet set
  • Pry bar

About the Author

Since 2003 Dan Ferrell has contributed general and consumer-oriented news to television and the Web. His work has appeared in Texas, New Mexico and Miami and on various websites. Ferrell is a certified automation and control technician from the Advanced Technology Center in El Paso, Texas.

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