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Old 11-02-2017, 10:41 PM   #4
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Join Date: Jan 2006
Location: NYC
Posts: 41,953

2003 L-Series 3.0L Sedan
Default Re: Belt tensioner and pully assembly

If you search threads about belt tensioners, you'll read equal votes for and against aftermarket parts. This includes Dayco as members have found them just as reliable as stock parts. What may make this belt tensioner issue more difficult in determining whether or not tensioners are interchangeable is the added fact of possibly using the wrong length drive belt. Some mechanical understanding is needed to appreciate what's involved with replacing drive belt and belt tensioners.

Most may overlook parts replacement if not aware of comparing the old one to the replacement. Tensioners may or may not have re-engineered the large clock spring (providing long term belt tension to keep a belt tight) and re-sized the pulley. Spring tension may vary and a different sized pulley affects belt tension. Now add an incorrect belt size to throw off a simple repair. An incorrect spring tension, incorrect pulley diameter and incorrect drive belt length can contribute to a thoroughly ruined repair. Either the belt is too tight or too loose and both create new problems. As a rule, most specs state belt tension around 50-65 pounds - that means if anyone knows how to use a spring gauge to measure belt tension and where to measure it, it would require 50 pounds to deflect a belt that's correctly mounted when new. Another way to measure tension is pulling the tensioner as if releasing the drive belt to measure how much pull is required to move the tensioner. 50 lbs is not light and either a long breaker bar is used or greater arm strength is required to move a tensioner out of the way to remove and replace a belt. That's tight and if the belt tensioner is not worn, makes belt removal difficult unless leverage is used on the tensioner pulley to tighten the tensioner before the belt can be removed. The large clock spring is designed with several windings to provide the large tension against the drive belt to keep it tight. This keeps the accessories from slipping; alternator, water pump, ac compressor and power steering pump. All accessories try to resist the pull of the drive belt and the belt tensioner tightens the belt against accessories to ensure every one is being turned. Some call this tension 'banjo tight' as an appropriate description for belt tension. Any less than that and accessories refuse to turn from a loose belt. The tensioner is only one part of this belt drive system. The other part is the correct belt and length.

The serpentine belt provides the connection from the engine drive pulley to turn the accessories, If too short, the tensioner may be too tight and prematurely wear out the belt, bearings and create other issues. A belt that's too long won't allow the belt to turn the accessories. The belt tensioner has a sweet spot where the belt tensioner applies correct tension in the middle third of its arc, neither too loose/low belt tension nor too tight/higher belt tension. The middle arc is where the belt resides, the ideal place for correct tension.

Unfortunately, there's a science to drive belts and some of it is described here. The trick is to use the correct parts and not deviate from original dimensions. Dimensions may be different diameter tensioner pulley, longer/shorter belt, and incorrect tensioner.

Last edited by fdryer; 11-02-2017 at 10:48 PM..


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