Automatic Belt Tensioner – Function – Failure Symptoms – Testing

Automatic Belt Tensioner - Function - Failure Symptoms - Testing
Automatic Belt Tensioner - Function - Failure Symptoms - Testing


Because, the automatic belt tensioner is spring loaded, it will naturally apply the proper amount of tension.

Most people already know that all belts, are a maintenance item, and will eventually have to be replaced.

Consequently, an automatic belt tensioner is a relatively cheap part to replace. So, every time you replace a belt, be sure to replace, the automatic belt tensioner at the same time.

The automatic belt tensioner, also provides a little “give”, so it can absorb and cushion shock loads. An example would be like, when the A/C compressor clutch cycles on and off.

What’s more, the tensioner automatically compensates for wear, and keeps the belt under constant tension.


Because, the tensioner drives the belt, wear on both will happen at the same time. So, having either one fail, could cause a multitude of problems. Finally, nothing automotive lasts forever, not belts and not automatic belt tensioners.

Belt Replacement

So, the typical service life of a serpentine belt, is about 60,000 miles or five years. Consequently, when the belt nears the end of its life, it may become cracked, glazed and noisy. But, cracks on the underside of a serpentine belt are normal.

However, if you see more than three cracks, in a three inch section on any one rib. Then, the belt has already used up, 80% of its intended life.

The Consequences Of A Broken Belt:

  • If the water pump stops turning. Then, the flow of coolant stops, and the engine begins to overheat.
  • And, if the alternator stops turning. Then, there is no charging output, and the battery starts to run down.
  • Also, if the power steering pump stops turning. Then, the steering suddenly gets very stiff, and hard to steer.

Automatic Belt Tensioner Inspection

So, the automatic belt tensioner, prevents the belt from slipping. However, a weak tensioner can also allow, the belt to slip and squeal.

Inspect The Tensioner For:

  • Rust or corrosion, can jam the tensioner housing, and prevent it from rotating freely. A seized tensioner, cannot maintain proper belt tension.
  • Dirt or mud, can also jam the tensioner housing.
  • A loose or worn pivot arm, can allow unwanted movement. And, that results, in belt noise and misalignment. Over time, this will increase belt wear, and lead to early belt failure.
  • A worn bushing in the tensioner pulley, can cause vibrations and noise. Also, if the bushings seize, it may cause the belt to snap.
  • A weak or broken spring inside the tensioner, can’t maintain proper tension, and the belt will slip. Consequently, springs lose tension over time, from exposure to heat.
  • Cracks or damage to the automatic belt tensioner housing or pulley arm, may prevent it from rotating smoothly. As a result, not maintaining proper belt tension.


Visual Damage Signs

So, there should be a gentle arm motion, as accessories turn on and off. For example, (such as the A/C compressor clutch engaging and disengaging). If the tensioner pulley does not move at all, the tensioner may be stuck. Also, watch for excessive chattering or tensioner arm oscillation, while the engine is idling.

Excessive arm oscillation is a warning sign, that the damper has possibly failed or is failing. As a result, the tensioner needs to be replaced now.

 Damaged Belt Tensioner:

  • Battery runs down (because belt is slipping and not turning the alternator fast enough)
  • Belt glazing (caused by slipping). Any glazing (shine) on the underside of the belt, indicates that the belt has been slipping.
  • Excessive movement or rocking of the tensioner pulley, or “belt flutter” when the engine is running.
  • Wobble in the tensioner pulley (or idler pulley).

Tensioner Noise

The tensioner should be quiet, when the engine is running. So, any squealing, rumbling, growling or chirping noises, should be checked. Use a mechanic’s stethoscope to pinpoint the source of the noise. And, the same goes for all the engine driven accessories (water pump, alternator, PS pump and A/C compressor).

Tensioner Pulley Damage

Physical damage of any kind on the tensioner pulley, may indicate excessive tension or physical interference. Always replace both, the pulley and the tensioner at the same time.


Check the movement of the tensioner arm, with the engine off. Use a socket with a long handle ratchet or breaker bar, on the tensioner pulley center bolt. Then, try to rotate the tensioner. Consequently, you should be able to feel tension pulling it back. So, if it does not move, it may be jammed or seized.

Also, note the position of the arm, on the automatic tensioner. Because, many units have marks on the housing, that show the normal range the arm can pivot. So, if the position of the arm is outside these marks, it indicates a problem. As a result, (the belt may be too long or too short, or the tensioner may be jammed).

Also, with the belt removed, try spinning the pulleys by hand. So, all the pulleys should turn freely, with no binding, roughness or wobble. Finally, any binding, roughness or wobble, means these parts are bad and need to be replaced.


So, the problem with the automatic belt tensioner is that, once it goes. It usually means that you have to replace the pulleys, and sometimes the drive belt itself. Finally, this is usually not, just a one part replacement job.