The Solid Alternator Pulley used to be the industry standard and has been around for many years. It has evolved from the V-type to the V-ribbed type (serpentine belt). Its only purpose is to drive the alternator via the belt.
If your alternator has a solid pulley, only replace if damaged, rusted or worn.
The Overrunning Alternator Pulley has a simple one-way-clutch inside the pulley. This internal clutch allows the rotor of the alternator to coast to a stop when the engine is shut down. This "overrunning" feature eliminates "chirp" sounds that happen when engine decelerates quickly causing the belt to slip at the alternator pulley (engine shut down or transmission shifting).
What to look for...
OAPs should spin freely in one direction and immediately lock in the other direction. Never replace an OAP with a solid pulley. The manufacture has designed the vehicle with it and damage to other components may occur if a solid pulley is substituted. Manufactures of simple OAPs include INA, NSK, Koyo etc.
The Overrunning Alternator Decoupler™ not only has a one-way clutch inside, it also incorporates a torsion spring to absorb energy. The effects of the internal clutch are the same as mentioned above; however, the patented internal torsion spring design is the key to the much higher level of function associated with the OAD™. The internal spring is tuned (engine specific) to absorb base engine vibrations (cylinder firing pulses) before they reach the alternator rotor and therefore negatively affect the accessory drive. With the OAD™ Pulley installed you will see much less tensioner motion, reduced NVH and an all around more robust accessory drive. Many manufactures are now including OAD™ Pulleys when designing their accessory belt drives. This allows them to use narrower belts with lower output tensioners. The lower system tension means the alternator, water pump, and other accessories bearings will last much longer.
What to look for...
OAD™ Pulleys should rotate freely in one direction and have a "spring feel" in the other direction. Since duty cycle and/or mileage are usually unknown when rebuilding the alternator, you should always install a new OAD™ Pulley at this time. This will ensure a quality rebuilt alternator and help eliminate field failures.
Never substitute a solid alternator pulley or a simple OAP in place of an OAD™ Pulley. The vehicle manufacture has designed the vehicle with this higher level of function for a reason! Substituting a different alternator pulley may cause damage to accessory drive components. The only manufacturer of the OAD™ Pulley is Litens Automotive Group®.