Tensioner for the serpentine/accessory drive belt for the Powerstroke 6.4L engine used in the Ford SuperDuty. Not for the A/C compressor belt, which requires a different tensioner, available. Fits both trucks with both single or dual alternator packages.
A failed tensioner will allow the serpentine drive belt to slip, resulting in noisey and weak power steering, low alternator output among other problems.