Overrunning Alternator Decoupler
An alternator pulley with an internal spring and clutch that dampens belt vibrations and absorbs rotational shocks.
An Overrunning Alternator Decoupler (OAD) is a highly engineered pulley mounted on the input shaft of an alternator. Unlike a standard solid pulley, an OAD contains an internal one-way clutch mechanism and a heavy-duty torsion spring. This allows the alternator rotor to spin freely (overrun) when engine speeds suddenly decrease, and dampens belt vibrations.
Modern engines experience micro-fluctuations in speed with each cylinder firing stroke, especially four-cylinder and diesel engines. Because the alternator has significant rotational inertia, a solid pulley forces the drive belt to absorb these speed changes, leading to belt squeal, tensioner wear, and vibration. The OAD decouples the alternator from these pulsations, smoothing out belt operation and reducing stress on the accessory drive system.
When an OAD pulley fails, the internal clutch or spring can seize, turning it into a solid pulley, or it can strip completely, preventing the alternator from charging. Symptoms of a failed OAD pulley include a growling or rattling noise at idle, severe belt flutter, and premature serpentine belt failure. Technicians test the pulley by spinning it by hand to verify it locks in one direction and spins freely in the other.