Automatic Belt Tensioner – Function – Failure Symptoms – Tensioner Testing

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

Above all, automatic belt tensioners, provide the proper tension all belts require.

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 inexpensive 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.

Belt Tensioner
Belt Tensioner

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

Tensioners And Belts Do Wear Out

Because, the tensioner drives the belt, wear on both will happen at some point. 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

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

Cracked Serpentine Belt
Cracked Serpentine Belt

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

Broken belt consequences:

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

Automatic Belt Tensioner Inspection:

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

Weak Tensioner
Weak Tensioner

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; that results in belt noise and misalignment. Over time, this will increase belt wear and lead to premature belt failure.
  • A worn bushing in the tensioner pulley can cause vibrations and noise. If the bushing seizes, 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. Springs lose tension over time from exposure to heat.
  • Cracks or damage to the tensioner housing or pulley arm; may prevent it from rotating smoothly and maintaining proper belt tension.

Belt And Tensioner Failure Symptoms

Visual Damage Signs:

There should be a gentle arm motion as accessories turn on and off; (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.

Damaged Tensioner
Damaged Tensioner

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

Tensioner Belt Damage:

  • 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. Any squealing, rumbling, growling or chirping noises should be investigated. Use a mechanic’s stethoscope to pinpoint the source of the noise. 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.

How To Test The Tensioner:

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 to rotate the tensioner. Consequently, you should be able to feel tension pulling it back. But, if it does not move it may be jammed or seized.

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

 

Spin By Hand
Spin By Hand

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

Conclusion

So, the problem with the drive belt tensioner is that once it goes; it usually means replacing the pulleys and sometimes the drive belt itself. Finally, it’s not usually a one-part replacement job.

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