The belt tensioner holds the serpentine belt under tension; allowing it to grip the pulleys firmly enough to transmit the engine’s rotating force to the accessories. Usually, the belt tensioner is bolted to the front of the engine block or timing cover, and the belt tension is maintained by a spring-loaded lever which has a pulley that engages the belt.
Most often, the tensioner assembly is secured by either one bolt, or two or three bolts and is fairly easy to remove once the belt is off. If an engine has more than one serpentine belt, if they are both conventional, there will be one belt tensioner for each belt.
Older engines (and some newer ones) still used sliding adjustments on accessories to hold the belt under tension. The disadvantage there is that tension may have to be adjusted periodically to accommodate belt wear and any irregularities in pulley operation (and temperature) can result in varying tension.
Cost Inclusions in Replacing a Belt Tensioner
For some examples of what a serpentine belt tensioner might cost to replace, assuming a labor rate of $100 an hour:
On the 2004 Nissan Pathfinder with the 5.6-liter engine, the labor time is 0.6 hours. An OE tensioner through dealer sources is $232, or an aftermarket tensioner from ACDelco comes to about $40. This makes the belt tensioner replacement cost about $292 through a dealership using factory parts or $100 using aftermarket parts.
On a 2009 Dodge Grand Caravan with a 3.8-liter engine, the labor time is 0.6 hours. An OE tensioner costs about $114 through dealer sources, while a Dayco belt tensioner costs about $33. The job will amount to about $174 with dealer parts, or about $93 going with aftermarket parts.
On a 2005 Honda Accord with a 2.4-liter engine, the labor time is 0.5 of an hour. A factory belt tensioner costs about $120 and a Gates belt tensioner costs about $48. The job cost would be about $170 with an OE part, or about $98 with an aftermarket tensioner.
On a 2007 BMW 328i with a 3.0-liter engine, the labor time is 1.4 hours. An OE tensioner through dealer sources is about $128, or a Continental tensioner is about $78. Total costs would be $268 through a dealership using factory parts, or $218 using aftermarket parts.
Of course, actual parts prices and labor rates vary from shop to shop and region to region and labor prices aren’t always necessarily charged at book time.
How Belt Tensioners Fail
Serpentine belt tensioners use a spring to apply tension. While they are made with metals resistant to fatigue and heat, they still can gradually weaken with age and use.
The signs of that would be a belt that isn’t tight enough and which squeaks, slips or both. It’s not always possible to tell whether a belt is squeaking because the rubber of the belt itself might have lost its gripping properties or because the tensioner has weakened.
So, very often, tensioners are replaced with the belt. The labor does almost entirely overlap; giving some savings to replacing both, as opposed to replacing them one at a time.
Belt tensioners apply tension through a pulley that bears against the belt. The pulley has a non-serviceable bearing and this can wear out.
It’s fairly common during belt replacement to find that the belt tensioner bearing feels rough or dry; even if it didn’t make any notable noise during operation. It is then usually recommended to replace the tensioner as a preventative measure.
Any small variance from perfect roundness in a driven pulley will change the size of the circuit of the running belt. This will be visible at the belt tensioner as a periodic oscillation following the variance and the engine’s rotation.
A small amount of this is normal but requires the tensioner to constantly work at its pivot point which can cause wear. With the belt off, wear can show up in the tensioner as looseness in the housing.
It can also cause the tensioner pulley to no longer sit square along the plane of the belt circuit. In which case, it should be replaced.
What Else Can Go Wrong?
The obvious thing is the serpentine belt which has to be removed to replace the serpentine belt tensioner. If it has signs of wear or is likely to cause any issue, the most economical time to replace it is when it is already off.
While the belt is off, it’s also a good idea to check the condition of the other accessory pulleys and bearings. If the pulleys need to be cleaned, that is a normal preventative procedure that protects the life of the belt.
The accessories and idler pulleys can also be checked for the condition of their bearings which should spin freely and smoothly without roughness or play. Bearings or accessories which show signs of bearing failure may be recommended to be replaced during a belt tensioner replacement.